Browse Subject Areas

Click through the PLOS taxonomy to find articles in your field.

For more information about PLOS Subject Areas, click here.

  • Loading metrics

Genome Analysis and Expression Patterns of Odorant-Binding Proteins from the Southern House Mosquito Culex pipiens quinquefasciatus

  • Julien Pelletier,

    Affiliation Honorary Maeda-Duffey Laboratory, Department of Entomology, University of California Davis, Davis, California, United States of America

  • Walter S. Leal

    Affiliation Honorary Maeda-Duffey Laboratory, Department of Entomology, University of California Davis, Davis, California, United States of America

Genome Analysis and Expression Patterns of Odorant-Binding Proteins from the Southern House Mosquito Culex pipiens quinquefasciatus

  • Julien Pelletier, 
  • Walter S. Leal


Olfactory-based behaviors in mosquitoes are mediated by odorant-binding proteins (OBPs). They form a multigenic family involved in the peripheral events in insect olfaction, specifically the transport of odorants to membrane-bound odorant receptors. OBPs contribute to the remarkable sensitivity of the insect's olfactory system and may be involved in the selective transport of odorants.

We have employed a combination of bioinformatics and molecular approaches to identify and characterize members of the “classic” OBP family in the Southern House mosquito Culex pipiens quinquefasciatus ( = Cx. quinquefasciatus), a vector of pathogens causing several human diseases. By taking advantage of the recently released genome sequences, we have identified fifty-three putative Cx. quinquefasciatus OBP genes by Blast searches. As a first step towards their molecular characterization, expression patterns by RT-PCR revealed thirteen genes that were detected exclusively and abundantly in chemosensory tissues. No clear differences were observed in the transcripts levels of olfactory-specific OBPs between antennae of both sexes using semi-quantitative RT-PCR. Phylogenetic and comparative analysis revealed orthologous of Cx. quinquefasciatus OBPs in Anopheles gambiae and Aedes aegypti. The identification of fifty-three putative OBP genes in Cx. quinquefasciatus highlights the diversity of this family. Tissue-specificity study suggests the existence of different functional classes within the mosquito OBP family. Most genes were detected in chemosensory as well as non chemosensory tissues indicating that they might be encapsulins, but not necessarily olfactory proteins. On the other hand, thirteen “true” OBP genes were detected exclusively in olfactory tissues and might be involved specifically in the detection of “key” semiochemicals. Interestingly, in Cx. quinquefasciatus olfactory-specific OBPs belong exclusively to four distinct phylogenetic groups which are particularly well conserved among three mosquito species.


In insects, odorants (aka semiochemicals) are detected by specialized sensory structures, the olfactory sensilla, present on different chemosensory tissues such as antennae, maxillary palps and proboscis. Hydrophobic odorant molecules have to pass through an aqueous medium, the sensillar lymph, separating the port of entry on the sensilla (the pore tubules) and receptors neurons. There is now increasing evidence that a multigenic family of small soluble proteins first identified in moths, the odorant-binding proteins (OBPs) [1], is involved in this important process leading to the delivery of odorants to the odorant receptors [2], [3].

A detailed mechanism has been proposed for a pheromone binding protein of the silkmoth, BmorPBP1, suggesting that a pH-dependent conformational change is involved in pheromone binding and release [4], [5], [6], [7]. Indeed, structural biology studies showed that the C-terminal part of the protein forms an additional α-helix at low pH capable to compete with pheromone for the binding pocket [8], [9], [10], thus enabling the delivery of the pheromone in acidic environment similar to that formed by the negatively charged dendrite surfaces of the olfactory receptor neurons [11]. Functional study also showed that BmorPBP1, when co-expressed with pheromone receptor BmorOR1 in the empty neuron system of Drosophila, enhanced the response to the pheromone, indicating that OBPs contribute to the inordinate sensitivity of the insect's olfactory system [12].

In mosquitoes, the first OBP (CquiOBP1) was isolated from antennae of female Culex quinquefasciatus by native gel electrophoresis and further cloned from cDNA to obtain a full-length sequence [13]. Recently this protein was shown to bind to a mosquito oviposition pheromone [14] in a pH-dependent manner and to be expressed in a subset of sensilla including one type responding to this pheromone [15]. Taken together, these experiments suggest that CquiOBP1 in involved in the detection of semiochemicals involved in mosquito oviposition behavior.

The release of the genome sequences of several insects including three dipteran species has allowed the identification of large multigenic families of OBPs in Drosophila melanogaster [16], [17], [18], [19], Anopheles gambiae [19], [20], [21], [22] and Aedes aegypti [23]. In mosquitoes, different subgroups of OBPs have been identified, each possessing its own characteristic features. The “classic” group includes the majority of OBPs characterized so far and is structurally similar with other insect OBPs. “Classic” OBP genes are predicted to encode small secreted proteins which display a characteristic pattern of six conserved cysteine residues called the “classic motif” [23], as well as a N-terminal signal peptide sequence. Several members of “classic” OBPs have been determined as important components of the insect's chemosensory system, as suggested by their specific association with functionally distinct classes of olfactory sensilla in D. melanogaster [24], [25], [26], [27] or by their high expression levels in A. gambiae antennae [28], [29]. On the other hand, studies performed on other OBP classes in the malaria mosquito A. gambiae revealed that “atypical” OBPs, which possess an extended C-terminal segment, were mostly expressed in early aquatic stages or at very low levels in adult tissues [21], [22], [29], whereas “plus-C” OBPs, which possess at least two additional conserved cysteines, showed no evidence of being olfactory-specific [22], with a few exceptions detected at relatively high levels in antennae [29].

The southern house mosquito Cx. quinquefasciatus is an important human health pest as a vector of several pathogens including agents of lymphatic filariasis, West Nile encephalitis and St. Louis encephalitis. In this species only two OBPs have been identified at the molecular level, CquiOBP1 [13] and CquiOBP7 [30], raising the question of how many genes encoding putative OBPs are present. In this study, we have mined the yet to be published genome sequence of Cx. p. quinquefasciatus (The genome sequence of Culex pipiens quinquefasciatus; Culex Genome Consortium), examined the diversity of this multigenic family, and focused on the “classic” OBP genes. Taking advantage of the genomic data, we have identified a total of fifty-three genes encoding putative OBPs in Cx. quinquefasciatus. Based on expression studies, we have identified two classes of OBPs, one being specifically expressed in olfactory tissues - and thus suggested to be involved in olfaction (“true” OBPs”) - and an ubiquitous group, encapsulins [2], which might play other physiological role(s).

Results and Discussion

Identification of putative “classic” OBP genes

To explore the diversity of the OBP family in the genome of Cx. quinquefasciatus (The genome sequence of Culex pipiens quinquefasciatus; Culex Genome Consortium), we have used the previously identified OBP sequences from other dipteran species (A. gambiae, A. aegypti and D. melanogaster) as probes to look for structurally similar proteins by Blast search [31]. Candidate sequences that displayed significant similarity were manually screened for characteristic features of the OBP family. Several criteria were used to assign a protein sequence as putative OBP: a small size (molecular weight around 14 kDa) and the presence of both a predicted N-terminal signal peptide sequence and highly conserved six cysteines spacing designated as the “classic motif”: C1-X15-39-C2-X3-C3-X21-44-C4-X7-12-C5-X8-C6 [23], which is now considered as a hallmark of the family. Candidate OBPs were further blasted in NCBI conserved domain database (CDD) to confirm the presence of characteristic motifs conserved in the OBP family.

Homology searches coupled with bioinformatics analysis allowed the identification of fifty-three putative OBP genes in Cx. quinquefasciatus, including CquiOBP1 the first ever mosquito OBP characterized [13] and CquiOBP7 recently described as an orthologue of AgamOBP7 [30]. Structural characteristics and GenBank accession numbers of CquiOBP1 to CquiOBP53 are compiled in Table 1. Six proteins had no predicted signal peptide (CquiOBP10, 29, 34, 40, 41, 42), possibly because they lack a full-length N-terminal as suggested by their overall shorter sizes. CquiOBP21 and CquiOBP46 did not fit the “classic motif” of cysteine spacing and CquiOBP45 and CquiOBP47–50 did not match with any conserved OBP domain when blasted in CDD. Yet, these proteins were further analyzed because of their similarity with other mosquito OBPs (see further phylogenetic analysis). CquiOBP45 and CquiOBP50 had been previously identified from salivary glands transcriptome and annotated as “putative salivary odorant-binding proteins” based on their similarity with the C-terminal region of an “atypical” OBP from A. gambiae [32]. Both proteins display a slight variation of the “classic motif” as they possess thirteen residues between C4 and C5, a feature they share with five other putative OBPs (CquiOBP44, 47, 48, 49 and 53).

Table 1. Structural characteristics of Cx. quinquefasciatus putative OBPs.

An amino acid alignment of mature Cx. quinquefasciatus putative OBPs highlights the very low average identity of this highly divergent multigenic family (Fig. 1). Only the six cysteine residues are fully conserved in each protein, the conservation of C4 being less visible on the alignment because of a more flexible number of residues between C3 and C4 and between C4 and C5.

Figure 1. Amino acids alignment of Cx. quinquefasciatus putative OBPs.

Residues conservation is indicated by different levels of shading: dark grey: 90% conservation; medium grey: 60% conservation; light gray: 40% conservation. The conserved cysteine residues are indicated by the letter C below the alignment. GenBank accession numbers are available in Table 1.

We have carried out cloning and sequencing of nine genes, CquiOBP3, 4, 5, 8, 9, 11, 12, 13 and 14 to add to four previously characterized OBP genes, CquiOBP1 [13], CquiOBP2 and CquiOBP6 (Ishida and Leal, unpublished data), and CquiOBP7 [30], and two putative salivary odorant-binding proteins CquiOBP45 and CquiOBP50 [32]. The other putative OBPs identified in this study originate from VectorBase automated annotations and were not confirmed by cDNA cloning. Most cloned sequences were similar to VectorBase annotations and only three genes (CquiOBP6, 9, 12) differed from corresponding predicted genes. All new sequences were deposited into GenBank (Table 1).

This bioinformatics-based approach likely gives a good estimation of the range of the OBP family in Cx. quinquefasciatus. Multigenic families of “classic” OBPs have now been identified in three different mosquito species with thirty-three genes in A. gambiae [20], [21], [22], [23], thirty-four genes in A. aegypti [23] and fifty-three genes in Cx. quinquefasciatus (this study). This diversity and high divergence of OBP encoding genes in mosquito might be correlated with the structural diversity of semiochemicals perceived by their olfactory system and thus suggest differential affinities for OBPs towards these odorant molecules. Of particular notice, three OBPs that we have already isolated and cloned from A. aegypti [33] have been renamed [23]. Thus, previously identified AaegOBP1, 2, and 3 have been renamed AaegOBP39, 27, 56, respectively [34].

Phylogenetic analysis of mosquito OBPs

In order to gain insight of the relationships among mosquito OBPs, we have carried out a phylogenetic analysis using putative amino acid sequences. A consensus sequence comparison tree was constructed by the neighbor joining method [35] with one thousand bootstrap replicates. The resulting tree suggests that based on their amino acid identity, most mosquito OBPs are clustered into different groups, each comprising related proteins of the three mosquito species (Fig. 2).

Figure 2. Phylogenetic relationships of mosquito “classic” OBPs.

The unrooted consensus tree was generated with 1000 bootstrap replicates using the neighbor joining method. Cx. quinquefasciatus OBPs are in black, A. gambiae OBPs are in blue and A. aegypti OBPs are in red. A. gambiae and A. aegypti OBPs follow the nomenclature established in [21] and [23]. Robust groupings identified by high bootstrap values at nodes are indicated in bold.

Among these groups, several OBPs of Cx. quinquefasciatus share high identity with other dipterans OBPs already described in previous works, as indicated by the amino acid identity percentages compiled in Table 2. These groups of orthologous proteins have been named OS-E/OS-F, LUSH/OBP19a, PBPRP1, and PBPRP4 based on their similarities to D. melanogaster OBPs [20], [21], [22], [23], [36]. In Cx. quinquefasciatus, five proteins (CquiOBP1 to CquiOBP5) cluster within the OS-E/OS-F group, one (CquiOBP7) within the PBPRP1 group, one (CquiOBP6) within the LUSH group, six (CquiOBP8 to CquiOBP13) within the OBP19a group, and one (CquiOBP14) within the PBPRP4 group. All these groups are strongly supported by high bootstrap values ranging from 97 to 100%. Amino acid alignments of mosquito OBPs from these groups are provided in Figure 3. Other Cx. quinquefasciatus OBPs, mostly in group B, also share high identity with OBPs from other mosquito species (Table 2). Group B is not as strongly supported as others (71% bootstrap support) and encloses nine different subgroups of orthologous OBPs (98 to 100% bootstrap supports). Group A (90% bootstrap support) provides an unexpected example of gene expansion in Cx. quinquefasciatus, enclosing eighteen OBPs of this species (CquiOBP25 to CquiOBP42) all related to AgamOBP13 and AaegOBP57. This expansion is a possible explanation for the highest number of putative OBPs identified in Cx. quinquefasciatus compared to those found in other mosquito species. The remaining OBPs share less amino acid identity and are not clustered together but rather dispersed at the bottom of the tree. Some of those are classified as putative “salivary” OBPs in NCBI database (Table 1). Among these proteins, CquiOBP53, 52, 51 50, 49 and 47 display some identity with AaegOBP17, 18, 19 and 64 considered so far as A. aegypti specific [23], but far less with A. gambiae OBPs (Table 2). Overall, Cx. quinquefasciatus OBPs are more closely related to A. aegypti than A. gambiae OBPs, reflecting the fact that both Culex and Aedes species belong to the same Culicidae subfamily.

Figure 3. Amino acids alignments of five groups of mosquito OBPs.

(A) OS-E/OS-F-like OBPs; (B) PBPRP1-like OBPs; (C) LUSH-like OBPs; (D) OBP19a-like OBPs; (E) PBPRP4-like OBPs. Residues conservation is indicated by different levels of shading: dark grey: 100% conservation; medium gray: 80% conservation; light gray: 60% conservation.

Table 2. Homology relationships of Cx. quinquefasciatus with other mosquito OBPs.

Comparative analysis highlights several highly related proteins in Culex, Anopheles and Aedes, as well as other proteins much less conserved among these three species. It is tempting to speculate that highly conserved OBPs should perform a common role within all species. However conservation of sequences does not necessarily imply conservation of functions, and only further functional experiments could shed light on common roles of mosquito highly “homologous” OBPs. Likewise, divergent OBPs will have to be investigated to support their potential implication in species-specific roles.

Genomic organization of putative OBP genes

Genomic organization was studied according to the relative positions of genes on genomic supercontigs and revealed that most OBP genes (thirty-six of fifty-three) are not distributed randomly in the genome but organized in clusters of genes (Table 3). Eight different clusters ranging from two to eight genes were identified. The most important in term of number of genes are cluster #8 on contig 3.315 regrouping eight genes (CquiOBP46 to CquiOBP53) within 16 kb, cluster #3 on contig 3.424 regrouping eight genes (CquiOBP15 to CquiOBP22) within 69 kb, cluster #5 on contig 3.181 regrouping six genes (CquiOBP37 to CquiOBP42) within 33 kb, and cluster #4 on contig 3.1894 regrouping four genes (CquiOBP33 to CquiOBP36) within 26 kb. Two OS-E/OS-F-like genes (CquiOBP3, 5) are also located at close range on supercontig 3.150 (cluster #1), as well as three OBP19a-like genes (CquiOBP9, 12, 13) on supercontig 3.865 (cluster #2).

OBPs of one cluster always belong to the same phylogenetic group, indicating that they share more identity among them than with other OBPs (Fig. 2) (Table 3). From an evolutionary point of view, close localization and sequence conservation inside a cluster suggests that Cx. quinquefasciatus OBP gene family might have evolved by multiple gene duplication events followed by rapid diversifications, as already suggested for A. gambiae [21] and A. aegypti OBP families [23]. Most clustered adjacent genes are located at close range, but genomic data suggest that such events might also result into long range duplications. For example, two OS-E/OS-F-like genes, CquiOBP1 and CquiOBP2 that share 63% amino acid identity and are located on the same supercontig 3.150 are nevertheless separated by more than 342 kb. Another OS-E/OS-F-like gene, CquiOBP4, is not part of cluster #1 but we have found an almost identical partial OBP gene (XP_001848931, CPIJ007609) located between CquiOBP3 and CquiOBP5 on cluster #1, suggesting that CquiOBP4 might have arisen from duplication of this gene. Additionally, we have also found two triplets of adjacent genes located on two different clusters (clusters #4 and #5) sharing around 90% identity between each pair (CquiOBP34 and CquiOBP40, CquiOBP35 and CquiOBP39, CquiOBP36 and CquiOBP38), indicating that a large duplication event involving three genes might have occurred.

Interestingly, eight clustered OBPs (CquiOBP15 to CquiOBP22, cluster #3) share high identity with related proteins in A. gambiae (AgamOBP23 to AgamOBP28) and in A. aegypti (AaegOBP11 to AaegOBP15 and AaegOBP65, 66), which are also part of a cluster [21], [23] (Table 2). These data suggest that duplication events likely occurred in a common ancestor before the radiation of the three mosquito species. Detailed comparative genomic analysis is now needed to confirm the orthology relationships among mosquito OBPs, as recently demonstrated for PBPRP1-like genes; CquiOBP7, AgamOBP7, and AaegOBP2 [30]. (Note that the protein referred here as AaegOBP2 [23] is not the previously isolated AaegOBP2 [33], which has been renamed AaegOBP27 [34]).

Expression patterns in different tissues

Tissue-specificity of forty-seven OBP genes was studied by non-quantitative RT-PCR to determine expression profile of the OBP family members in Cx. quinquefasciatus. Expression studies represent an important step to determine if putative OBPs are potentially involved in odorant reception. This assumption is supported by the fact that hitherto all OBPs with identified function have been demonstrated to be expressed only in olfactory tissues. There are a number of OBP-like proteins expressed in non-olfactory tissues, but their olfactory functions have never been demonstrated or even examined [2]. Our assumption is that a gene abundantly and exclusively detected in chemosensory tissues likely encodes an olfactory protein. Gene-specific primers of forty-seven OBPs were used in PCR reactions using cDNA templates prepared from adult antennae, maxillary palps, proboscis, legs and bodies of both sexes. Four genes (CquiOBP34, 40, 41, 42) were not included in the experiment and two pairs of highly similar genes (CquiOBP35/39 and CquiOBP36/38) were considered as single genes. Two distinct cDNA pools were tested, one-day-old and one-to-seven-days old adults. No bands corresponding to genomic DNA amplification were observed, confirming the quality of cDNA samples. In order to examine the transcripts levels between olfactory and non-olfactory tissues, specific primers of a “housekeeping” gene encoding ribosomal protein L8 (CquiRpL8) were used as control to check the integrity of each cDNA preparation.

Non-quantitative RT-PCR experiments showed a high variability in the expression profiles of putative OBP genes, with considerable variations both in tissue distributions and also in term of expression levels. Comparison between sexes did not show a single sex-specific gene, and no differences were observed between one-day-old and one-to-seven-days-old adults. Results are compiled in Table 4 which lists the presence or absence of the expected PCR product for each gene in different tissues.

Distribution of Cx. quinquefasciatus OBP transcripts highlights heterogeneous expression profiles in olfactory as well as non-olfactory tissues. Thirty-two genes were consistently detected in antennae (68%), twenty-six in maxillary palps (55%) and twenty-three in proboscis (49%) but also twenty-two in legs (47%) and eighteen in bodies (38%). The high proportion of genes detected in the main olfactory organ, the antennae, is consistent with the presence of multiple functional classes of sensilla recently described in Cx. quinquefasciatus [37]. Contrary to antennae, maxillary palps harbor a single type of olfactory sensillum that has been shown to respond to a broad spectrum of odorants in Cx. quinquefasciatus [38]. Even if co-expression of several OBPs can occur in the same sensillum type [25], [27], the unexpected high number of genes detected in this organ remains to be elucidated. A similar proportion (thirteen of twenty-five genes, 52%) of OBPs was detected in A. gambiae maxillary palps by RT-PCR [29]. Proboscis, the main gustatory organ in mosquito, was demonstrated to be an accessory olfactory organ in A. gambiae, which expresses at least twenty-four odorant receptor genes and responds to a small set of volatile compounds [39]. Consequently, it is reasonable to assume that such olfactory function might also exist in Cx. quinquefasciatus proboscis thus requiring the presence of the diverse group of OBPs observed in this study. Alternatively, OBPs expressed in proboscis may be involved in gustatory reception.

We have classified Cx. quinquefasciatus OBPs into different categories according to their expression patterns (Fig. 4). For simplicity, we grouped antennae, maxillary palps and proboscis as olfactory tissues, whereas legs and bodies were considered as non-olfactory tissues. Only thirteen genes (28%) were detected exclusively in olfactory tissues, whereas twenty-five (53%) were detected in olfactory as well as non-olfactory tissues, and nine (19%) were not detected at all. These genes which have not been detected in any adult tissues might represent pseudogenes, may be expressed in earlier stages (which are not the focus of this study), or could be expressed in adults at so low levels that were not detected under the conditions employed in this study. With four independent replications, non-quantitative RT-PCR sufficed to clearly demonstrate differences in bands intensities showing that the most abundant transcripts detected in antennae, maxillary palps and proboscis, belong mainly to the olfactory-specific gene class (data not shown). Among those, CquiOBP1 displayed the highest transcript level in antennae, which is consistent with a previous study showing that CquiOBP1 was the most abundant protein detected in female antennae extracts on a native gel [13]. Based on their high expression levels restricted to chemosensory tissues, we suggest that these thirteen olfactory-specific genes in Cx. quinquefasciatus are “true” OBPs, which may be involved specifically in the reception of important olfactory cues.

Figure 4. Expression patterns of OBP genes in various tissues of adults Cx. Quinquefasciatus.

Specific primers of forty-seven putative OBP genes have been used in non quantitative RT-PCR experiments using thirty-four cycles of amplification. (A) OBP genes can be subdivided into three main categories. Olfactory-specific genes were detected exclusively in antennae, maxillary palps or proboscis. (B) Distribution profiles of olfactory-specific genes in olfactory tissues. Details are available in Table 4.

Among the twenty-five genes detected in both olfactory and non-olfactory tissues, some transcripts were detected at very high levels in legs and/or in bodies indicating that the encoded proteins probably perform some important but non-olfactory functions in these tissues. Interestingly, CquiOBP29 was detected in every tissue but at very high levels in antennae, maxillary palps and proboscis, comparable with some olfactory-specific OBPs. Without any functional evidence, we cannot exclude that genes expressed in olfactory tissues but also in legs and/or in bodies are involved in olfaction, but it is reasonable to consider that proteins involved in the sensitivity and selectivity of the insect's olfactory system are restricted to the sensillar lymph. Some OBPs have been shown to be expressed in broad areas including regions without chemosensory functions, for example in D. melanogaster [16] and A. gambiae [22], [29]. In A. aegypti, AaegOBP22 (close to CquiOBP43 and AgamOBP9) has recently been proposed as a “multi-functions” protein performing different roles in distinct tissues, including non-olfactory functions as suggested by its expression in male reproductive apparatus and in spiracles [40], which are part of the insect's respiratory system. We suggest that this class of broadly expressed OBPs in Cx. quinquefasciatus might be encapsulins [2], probably involved in other physiological functions most likely unrelated to odorant reception. On the other hand, the roles of “true” OBPs might be restricted to transport, protection, and delivery of odorants. Test of these hypotheses must await functional studies.

Correlation between expression patterns and phylogeny

Comparison between expression and phylogenetic data could lead to a better understanding of the role(s) of OBP family in mosquitoes. In Cx. quinquefasciatus, olfactory-specific genes (CquiOBP1 to 9, CquiOBP11 to 14) are not distributed randomly in the tree, but along with other mosquitoes related OBPs, belong exclusively to four strongly supported phylogenetic groups: OS-E/OS-F, LUSH/OBP19a, PBPRP1 and PBPRP4 (Fig. 2) (Table 2). These groups, with the exception of one member, CquiOBP10 (an OBP19a-like, which is also detected in legs), constitute groups of exclusively olfactory-specific OBPs in Cx. quinquefasciatus. Orthologous proteins in D. melanogaster were also shown to be exclusively expressed in chemosensory tissues [16]. In order to study this correlation in another mosquito species and in the absence of expression data for A. aegypti OBPs, we have compared our data with other expression studies performed on A. gambiae OBPs. Interestingly, all but one of the eleven OBPs characterized in [22] as the most likely to play a role in olfaction (AgamOBP1, 2, 3, 4, 7, 15, 18, 19, 20, 66) belong to the same groups. This comparison was done by semi-quantitative RT-PCR to determine expression levels of A. gambiae OBPs in heads, legs and bodies. Results showed that these eleven genes were expressed exclusively or mainly in head tissues. In another study [28], A. gambiae antennal cDNA libraries have been characterized by filter array hybridization. Seven OBPs (AgamOBP1, 2, 3, 4, 5, 6, 7) were shown to be the most abundant transcripts in antennal cDNA populations. Additionally, RT-PCR experiment revealed that these genes were exclusively expressed in heads but not in bodies without heads. These OBPs belong also to the same groups (AgamOBP66, the PBPRP4-like was not tested in this study). In a third study [29], the expression patterns and relative abundances of twenty-five “classic” A. gambiae OBP genes have been characterized using microarray hybridization, non-quantitative and quantitative RT-PCR. Results notably showed that eight genes (AgamOBP1, 2, 3, 4, 5, 7, 17, 20) belonging to the same groups were among the ten most expressed OBPs in female antennae (AgamOBP66, the PBPRP4-like was not tested in this study). Expression studies are not yet available for A. aegypti OBPs.

This comparison suggests the existence of four distinct groups of “true” OBPs in mosquitoes which consistently display high and/or exclusive expression in chemosensory tissues, both in Cx. quinquefasciatus (this study) and A. gambiae. OBPs from these groups are, therefore, potentially involved in peripheral reception of “key” semiochemicals for mosquito behaviors. Further experiments are now needed to establish their precise localization in chemosensory tissues, to determine in which functional sensilla types they are expressed, and especially to understand which role they play in the olfactory behavior of mosquitoes. Characterization of their binding to relevant ligands and unveiling their structural features may open the door for the identification of novel attractant and/or repellent compounds. Previously, CquiOBP1 (an OS-E/OS-F-like protein) was demonstrated to be an olfactory protein and subsequently used as a molecular target to identify an oviposition attractant, which was then tested in field tests and is currently employed as lure for trapping gravid female mosquitoes [15].

Comparison of OBPs expression levels between female and male antennae

Non-quantitative RT-PCR screening allowed the identification of thirteen olfactory-specific OBP genes in Cx. quinquefasciatus (CquiOBP1 to 9 and CquiOBP11 to 14). To identify which of these genes are more likely involved in sex-specific behavior, we have carried out semi-quantitative RT-PCR experiments and determined more accurately the expression ratios between antennae of both sexes. For such comparison, the choice of a suitable control gene is of paramount importance. We have decided to use two different alternatives, an ubiquitous ribosomal protein encoding gene (CquiRpL8) and the atypical odorant receptor 7 gene (CquiOR7) [41] to normalize the expression levels of antennal cDNA samples. After normalization, specific primers for each OBP and for both control genes were used in standardized PCR reactions. Quantifications of PCR products intensities (reflecting the transcripts levels) were used to calculate the female antennae/male antennae (FA/MA) expression ratio for each OBP as well as for both control genes.

Semi-quantitative RT-PCR data revealed clear differences in OBPs expression ratios in RpL8 compared to OR7 normalized cDNAs (Fig. 5). FA/MA ratios were consistently higher when RpL8 was used as control (OBPs ratios from 1.45 to 1.81, average 1.65) than when OR7 was used as control (OBPs ratios from 1.07 to 1.35, average 1.17). These values likely reflect the difference in the antennal structures in male and female adults. Indeed, in Culex mosquitoes, female antennae harbor about three and a half times more olfactory sensilla than male antennae, which harbor sensilla only on the two last distal segments [42]. Thus, the average higher FA/MA value for OBPs in RpL8 normalized cDNAs (1.65) compared to OR7 normalized cDNAs (1.17) might represent an artifact due to a much lower level of OR7 transcript in corresponding male sample. This discrepancy becomes obvious when looking at the transcripts levels of RpL8 and OR7 between sexes. In RpL8 normalized cDNAs, the average FA/MA ratio of OR7 was 2.25, indicating a clear enrichment of OR7 transcript in females. Similarly, in OR7 normalized cDNAs, the average FA/MA ratio of RpL8 was 0.565, indicating a clear enrichment of RpL8 transcript in males. This difference is highlighted in Figure 6 which compares the PCR amplification products of OBPs and control genes in both RpL8 (Fig. 6A) and OR7 (Fig. 6B) normalized cDNAs on agarose gels.

Figure 5. Expression of OBP genes in female and male antennae.

Expression ratios (FA/MA) of thirteen olfactory-specific OBP genes and two control genes (RpL8, OR7) were calculated after quantification of bands intensities in semi-quantitative RT-PCR experiments. Antennal CDNAs of both sexes were normalized to the expression levels of CquiRpL8 (purple) and CquiOR7 (blue). Bars represent standard deviations.

Figure 6. PCR amplification in female and male antennae.

Amplification of thirteen olfactory-specific OBP genes and two control genes (RpL8, OR7) in female antennae (FA) and male antennae (MA) cDNAs. (A) cDNAs normalized to the expression levels of CquiRpL8; (B) cDNAs normalized to the expression levels of CquiOR7.

Whereas the “housekeeping” RpL8 gene represents basically per-cell transcripts comparison, OR7 gene might represent a more suitable control to quantify olfactory-specific transcripts ratios considering the structure of Cx. quinquefasciatus antennae. This atypical receptor, orthologue of D. melanogaster OR83b, is co-expressed with conventional odorant receptors in almost every sensilla type, with the exception of basiconica (grooved pegs) sensilla [41], [43], [44], [45]. Thus, equivalent levels of OR7 transcripts in male and female antennae cDNAs might reflect more accurately equivalent levels of sensilla-specific transcripts, if we assume that both sexes do express the same amount of OR7 transcript in their respective sensilla, which has never been determined in this mosquito species. In A. gambiae, a mosquito species which display a similar discrepancy in the number of sensilla between male and female antennae, OR7 has been shown to be expressed about twelve times more in female antennae than in male antennae by quantitative RT-PCR, after normalization by a ribosomal protein (RpS7) [46]. As one would expect about three times higher expressions in female antennae for equally expressed olfactory genes (due to difference in antennal structures), the authors have suggested that a greater proportion of sensilla on female than male antennae might express OR7.

Based only on OR7 normalization, our data show that transcripts levels of olfactory-specific OBPs in Cx. quinquefasciatus are relatively similar between antennae of both sexes (OBPs ratios between 1.07 and 1.35) suggesting that none of these genes might be involved directly in sex-specific olfactory behavior in this mosquito species. In A. gambiae, mRNA levels of twenty “classic” OBPs have been compared in antennae (or heads) of male and female by microarray hybridization and quantitative RT-PCR after normalization by a ribosomal protein (RpS7), and several transcripts displayed significant enrichment in one or the other sex [29]. It is not clear whether this difference is due to real species-specific variation in OBP expression between Culex and Anopheles, or to the different control genes used (ribosomal protein VS OR7), or because only a relatively small set of genes (thirteen of thirty-two genes detected in antennae) was tested in our study.

Materials and Methods

Identification of putative OBP sequences in Culex quinquefasciatus

Predicted peptide sequences database (CpipJ1.2 geneset) of the whole genome of Cx. quinquefasciatus (The genome sequence of Culex pipiens quinquefasciatus; Culex Genome Consortium) was downloaded from VectorBase ( and entered into BioEdit v7.0.9.0 [47] to perform homology searches using Blastp algorithm [31]. A. gambiae (thirty-five sequences), A. aegypti (thirty-four sequences) and D. melanogaster (thirty-five sequences) “classic” OBP amino-acid sequences were retrieved from GenBank (NCBI) and used as queries in Blast searches. Conservation of the six cysteines spacing pattern and sequence identities with other dipterans OBPs were assessed from multiple alignments using GeneDoc software ( and BioEdit. N-terminal signal peptide sequences were predicted using SignalP v3.0 server ( [48]. Molecular weights and isoelectric points were computed using ExPASy proteomics server ( Blast in NCBI conserved domains database (CDD) was used to identify PBP_GOBP (pfam01395) or PhBP (smart00708) motifs. Relative positions of putative OBP genes on genomic supercontigs were studied following VectorBase genome annotations. Cx quinquefasciatus OBP names (CquiOBP1 to CquiOBP53) were assigned, when possible, based on their phylogenetic relationships and positions on genomic clusters.

Phylogenetic analysis of mosquito OBPs

Amino acid sequences of putative “classic” OBPs identified in three mosquito species (fifty-three in Cx. quinquefasciatus (this study), thirty-three in A. gambiae and thirty-four in A. aegypti) were used to create an entry file for phylogenetic analysis in MEGA 4.0.2 [49]. An unrooted consensus neighbor joining tree [35] was calculated at default settings with pairwise gaps deletions. Branch support was assessed by bootstrap analysis based on 1000 replicates. Nomenclature of A. gambiae and A. aegypti OBPs used in phylogenetic analysis was the same as described in [21] and [23].

Determination of expression patterns by non-quantitative RT-PCR

Cx. quinquefasciatus mosquitoes used in this study were from a laboratory colony originating from adult mosquitoes collected in Merced, CA in the 1950s and maintained under laboratory conditions at the Kearney Agricultural Center, University of California, as previously described [38]. Tissues (antennae, maxillary palps, proboscis, legs and bodies) from adults of both sexes were dissected on ice under a light microscope. Total RNA was extracted using TRIzol Reagent (Invitrogen, Carlsbad, CA) and first-strand cDNAs were synthesized from 0.5 µg RNA using SuperScript II Reverse Transcriptase (Invitrogen) and an oligo (dT) primer, following manufacturer's instructions. Integrity of each cDNA template was confirmed by amplification of a “housekeeping” gene encoding ribosomal protein L8 (CquiRpL8, GenBank accession XP_001841927). Gene-specific primers for forty-seven putative Cx. quinquefasciatus OBPs were designed manually according to three criteria: spanning at least one predicted intron in order to be able to distinguish between genomic DNA and cDNA amplifications, an annealing temperature around 60°C in order to prevent non-specific amplifications and an expected size around 250–350 bp. PCR reactions were carried out in a GeneAmp PCR System 9700 (Applied Biosystems, Carlsbad, CA) using equivalent amount of cDNA and one unit of Titanium Taq DNA polymerase (Clontech, Palo Alto, CA) in a final volume of 25 µl. After thirty-four cycles of amplification (95°C for 30s, 56°C for 30s, 72°C for 30s), PCR products were loaded onto ethidium-bromide stained agarose gels (1,5% (w/v)) and visualized using a Gel DOC XR Molecular Imager (BioRad, Hercules, CA). Two replicates were performed on two different cDNA samples, one-day-old and one-to-seven-days-old adults. All primers used in RT-PCR experiments are listed in Table 5.

Comparison of OBPs expression levels in male and female antennae by semi-quantitative RT-PCR

To compare transcripts levels between antennae of both sexes, antennal cDNA samples (same preparation as described above) were normalized to the expression levels of two different control genes, RpL8 (CquiRpL8, GenBank accession XP_001841927) and OR7 (CquiOR7, GenBank accession ABB29301) [41]. Gradual dilutions and cycle-controlled PCR reactions were used until amplifying equivalent amounts of RpL8 and OR7 in corresponding samples of both sexes. RpL8 and OR7 normalized cDNAs were used in standardized PCR reactions (25 µl, with one unit of Titanium Taq DNA polymerase) with gene-specific primers for thirteen olfactory-specific OBP and for both control genes. All reactions were carried out in the linear range of PCR amplification, as determined for each gene, to prevent saturation bias. PCR products (15 µl) were loaded onto ethidium-bromide stained agarose gels (1.5% (w/v)) and visualized using Gel DOC XR Molecular Imager (BioRad). Quantification of bands intensities was done using Quantity One software (BioRad). Intensity value of each OBP band was divided by those of corresponding control band prepared from the same reaction mix, after background removal. Resulting values were used to calculate the expression ratios between female and male antennae (FA/MA). Three replicates were performed on two different cDNA samples (one-to-seven-days-old adults) for both RpL8 and OR7 normalized samples.

Cloning and sequencing

Full-length sequences of CquiOBP2 and CquiOBP6 were amplified from female antennal cDNA using Smart Race cDNA amplification kit (Clontech) with specific primers designed from Culex pipiens OBP2 and OBP6 genes (unpublished) and universal primers, according to the manufacturer's instructions. Full-length sequences of nine putative OBP genes (CquiOBP3, 4, 5, 8, 9, 11, 12, 13, 14) were amplified from female antennal cDNA using Pfu Ultra II polymerase (Stratagene, La Jolla, CA) with specific primers designed in 5′ and 3′ ends of predicted genes (see below). PCR products were gel purified using QIAquick Gel Extraction Kit (Qiagen, Valencia, CA) and ligated into pBluescript SK (Stratagene). Ligation products were used to transform One Shot OmniMAX competent cells (Invitrogen) and positive clones were grown in LB medium containing ampicilline. Plasmids were purified using QIAprep Spin Miniprep Kit (Qiagen) and sent to Davis Sequencing Inc (Davis, CA). Sequences of all these genes were deposited into GenBank. Accession numbers are available in Table 1.

  9. fl-CquiOBP5 forward: 5′-ATGACGGTGGCCACCTGGTTATCT-3′
  10. fl-CquiOBP5 reverse: 5′-TCAAAACAGGTAATAGTGGACCGG-3′
  11. fl-CquiOBP8 forward: 5′-ATGATCTGGCGAAGGTTTGCGATT-3′
  13. fl-CquiOBP9 forward: 5′-ATGAGTGTTCGCGCATTTCTTCCG-3′
  14. fl-CquiOBP9 reverse: 5′-TTACGCAAAGAAAAACTTGGGATTA-3′
  15. fl-CquiOBP11 forward: 5′-ATGGCCACTCGGGTGGAGCTGGCT-3′
  16. fl-CquiOBP11 reverse: 5′-CTAGGGAAACACAAACTTGGGGTTG-3′
  17. fl-CquiOBP12 forward: 5′-ATGAAGTGCGACAGTTGGGCCACC-3′
  18. fl-CquiOBP12 reverse: 5′-CTAGGGGAAAATAAACTTTGGATTGT-3′
  20. fl-CquiOBP13 reverse: 5′-CTACGGGAAAAAGAACTTGGGCGT-3′
  21. fl-CquiOBP14 forward: 5′-ATGGGTGTCAAAACGGTGATCTTC-3′
  22. fl-CquiOBP14 reverse: 5′-TTATCGCCTTTTGCTGTCCTTGCT-3′

Author Contributions

Conceived and designed the experiments: JP. Performed the experiments: JP. Analyzed the data: JP. Contributed reagents/materials/analysis tools: WSL. Wrote the paper: JP WSL. Conceived the experiments: WSL.


  1. 1. Vogt RG, Riddiford LM (1981) Pheromone binding and inactivation by moth antennae. Nature 293: 161–163.
  2. 2. Leal WS (2005) Pheromone reception. Top Curr Chem 240: 1–36.
  3. 3. Pelosi P, Zhou JJ, Ban LP, Calvello M (2006) Soluble proteins in insect chemical communication. Cell Mol Life Sci 63: 1658–1676.
  4. 4. Wojtasek H, Leal WS (1999) Conformational change in the pheromone-binding protein from Bombyx mori induced by pH and by interaction with membranes. J Biol Chem 274: 30950–30956.
  5. 5. Damberger F, Nikonova L, Horst R, Peng G, Leal WS, et al. (2000) NMR characterization of a pH-dependent equilibrium between two folded solution conformations of the pheromone-binding protein from Bombyx mori. Protein Sci 9: 1038–1041.
  6. 6. Leal WS, Chen AM, Ishida Y, Chiang VP, Erickson ML, et al. (2005) Kinetics and molecular properties of pheromone binding and release. Proc Natl Acad Sci U S A 102: 5386–5391.
  7. 7. Xu W, Leal WS (2008) Molecular switches for pheromone release from a moth pheromone-binding protein. Biochem Biophys Res Commun 372: 559–564.
  8. 8. Sandler BH, Nikonova L, Leal WS, Clardy J (2000) Sexual attraction in the silkworm moth: structure of the pheromone-binding-protein-bombykol complex. Chem Biol 7: 143–151.
  9. 9. Horst R, Damberger F, Luginbuhl P, Guntert P, Peng G, et al. (2001) NMR structure reveals intramolecular regulation mechanism for pheromone binding and release. Proc Natl Acad Sci U S A 98: 14374–14379.
  10. 10. Lautenschlager C, Leal WS, Clardy J (2005) Coil-to-helix transition and ligand release of Bombyx mori pheromone-binding protein. Biochem Biophys Res Commun 335: 1044–1050.
  11. 11. Keil TA (1984) Surface coats of pore tubules and olfactory sensory dendrites of a silkmoth revealed by cationic markers. Tissue Cell 16: 705–717.
  12. 12. Syed Z, Ishida Y, Taylor K, Kimbrell DA, Leal WS (2006) Pheromone reception in fruit flies expressing a moth's odorant receptor. Proc Natl Acad Sci U S A 103: 16538–16543.
  13. 13. Ishida Y, Cornel AJ, Leal WS (2002) Identification and cloning of a female antenna-specific odorant-binding protein in the mosquito Culex quinquefasciatus. J Chem Ecol 28: 867–871.
  14. 14. Laurence BR, Pickett JA (1982) Erythro-6-acetoxy-5-hexadecanolide, the major component of a mosquito oviposition attractant pheromone. J Chem Soc, Chem Commun 59–60.
  15. 15. Leal WS, Barbosa RM, Xu W, Ishida Y, Syed Z, et al. (2008) Reverse and conventional chemical ecology approaches for the development of oviposition attractants for Culex mosquitoes. PLoS ONE 3: e3045.
  16. 16. Galindo K, Smith DP (2001) A large family of divergent Drosophila odorant-binding proteins expressed in gustatory and olfactory sensilla. Genetics 159: 1059–1072.
  17. 17. Graham LA, Davies PL (2002) The odorant-binding proteins of Drosophila melanogaster: annotation and characterization of a divergent gene family. Gene 292: 43–55.
  18. 18. Hekmat-Scafe DS, Scafe CR, McKinney AJ, Tanouye MA (2002) Genome-wide analysis of the odorant-binding protein gene family in Drosophila melanogaster. Genome Res 12: 1357–1369.
  19. 19. Zhou JJ, Huang W, Zhang GA, Pickett JA, Field LM (2004) “Plus-C” odorant-binding protein genes in two Drosophila species and the malaria mosquito Anopheles gambiae. Gene 327: 117–129.
  20. 20. Vogt RG (2002) Odorant binding protein homologues of the malaria mosquito Anopheles gambiae; possible orthologues of the OS-E and OS-F OBPs of Drosophila melanogaster. J Chem Ecol 28: 2371–2376.
  21. 21. Xu PX, Zwiebel LJ, Smith DP (2003) Identification of a distinct family of genes encoding atypical odorant-binding proteins in the malaria vector mosquito, Anopheles gambiae. Insect Mol Biol 12: 549–560.
  22. 22. Li ZX, Pickett JA, Field LM, Zhou JJ (2005) Identification and expression of odorant-binding proteins of the malaria-carrying mosquitoes Anopheles gambiae and Anopheles arabiensis. Arch Insect Biochem Physiol 58: 175–189.
  23. 23. Zhou JJ, He XL, Pickett JA, Field LM (2008) Identification of odorant-binding proteins of the yellow fever mosquito Aedes aegypti: genome annotation and comparative analyses. Insect Mol Biol 17: 147–163.
  24. 24. Pikielny CW, Hasan G, Rouyer F, Rosbash M (1994) Members of a family of Drosophila putative odorant-binding proteins are expressed in different subsets of olfactory hairs. Neuron 12: 35–49.
  25. 25. Hekmat-Scafe DS, Steinbrecht RA, Carlson JR (1997) Coexpression of two odorant-binding protein homologs in Drosophila: implications for olfactory coding. J Neurosci 17: 1616–1624.
  26. 26. Park SK, Shanbhag SR, Wang Q, Hasan G, Steinbrecht RA, et al. (2000) Expression patterns of two putative odorant-binding proteins in the olfactory organs of Drosophila melanogaster have different implications for their functions. Cell Tissue Res 300: 181–192.
  27. 27. Shanbhag SR, Hekmat-Scafe D, Kim MS, Park SK, Carlson JR, et al. (2001) Expression mosaic of odorant-binding proteins in Drosophila olfactory organs. Microsc Res Tech 55: 297–306.
  28. 28. Justice RW, Dimitratos S, Walter MF, Woods DF, Biessmann H (2003) Sexual dimorphic expression of putative antennal carrier protein genes in the malaria vector Anopheles gambiae. Insect Mol Biol 12: 581–594.
  29. 29. Biessmann H, Nguyen QK, Le D, Walter MF (2005) Microarray-based survey of a subset of putative olfactory genes in the mosquito Anopheles gambiae. Insect Mol Biol 14: 575–589.
  30. 30. Sengul MS, Tu Z (2008) Characterization and expression of the odorant-binding protein 7 gene in Anopheles stephensi and comparative analysis among five mosquito species. Insect Mol Biol 17: 631–645.
  31. 31. Altschul SF, Madden TL, Schaffer AA, Zhang J, Zhang Z, et al. (1997) Gapped BLAST and PSI-BLAST: a new generation of protein database search programs. Nucleic Acids Res 25: 3389–3402.
  32. 32. Ribeiro JM, Charlab R, Pham VM, Garfield M, Valenzuela JG (2004) An insight into the salivary transcriptome and proteome of the adult female mosquito Culex pipiens quinquefasciatus. Insect Biochem Mol Biol 34: 543–563.
  33. 33. Ishida Y, Chen AM, Tsuruda JM, Cornel AJ, Debboun M, et al. (2004) Intriguing olfactory proteins from the yellow fever mosquito, Aedes aegypti. Naturwissenschaften 91: 426–431.
  34. 34. Zhou JJ, He XL, Pickett JA, Field LM (2008) Addendum to Zhou et al., IMB, 17, 147–163. Insect Mol Biol 17: 445.
  35. 35. Saitou N, Nei M (1987) The neighbor-joining method: a new method for reconstructing phylogenetic trees. Mol Biol Evol 4: 406–425.
  36. 36. Biessmann H, Walter MF, Dimitratos S, Woods D (2002) Isolation of cDNA clones encoding putative odourant binding proteins from the antennae of the malaria-transmitting mosquito, Anopheles gambiae. Insect Mol Biol 11: 123–132.
  37. 37. Hill SR, Hansson BS, Ignell R (2009) Characterization of antennal trichoid sensilla from female southern house mosquito, Culex quinquefasciatus Say. Chem Senses 34: 231–252.
  38. 38. Syed Z, Leal WS (2007) Maxillary palps are broad spectrum odorant detectors in Culex quinquefasciatus. Chem Senses 32: 727–738.
  39. 39. Kwon HW, Lu T, Rutzler M, Zwiebel LJ (2006) Olfactory responses in a gustatory organ of the malaria vector mosquito Anopheles gambiae. Proc Natl Acad Sci U S A 103: 13526–13531.
  40. 40. Li S, Picimbon JF, Ji S, Kan Y, Chuanling Q, et al. (2008) Multiple functions of an odorant-binding protein in the mosquito Aedes aegypti. Biochem Biophys Res Commun 372: 464–468.
  41. 41. Xia Y, Zwiebel LJ (2006) Identification and characterization of an odorant receptor from the West Nile virus mosquito, Culex quinquefasciatus. Insect Biochem Mol Biol 36: 169–176.
  42. 42. McIver SB (1982) Sensilla mosquitoes (Diptera: Culicidae). J Med Entomol 19: 489–535.
  43. 43. Larsson MC, Domingos AI, Jones WD, Chiappe ME, Amrein H, et al. (2004) Or83b encodes a broadly expressed odorant receptor essential for Drosophila olfaction. Neuron 43: 703–714.
  44. 44. Pitts RJ, Fox AN, Zwiebel LJ (2004) A highly conserved candidate chemoreceptor expressed in both olfactory and gustatory tissues in the malaria vector Anopheles gambiae. Proc Natl Acad Sci U S A 101: 5058–5063.
  45. 45. Melo AC, Rutzler M, Pitts RJ, Zwiebel LJ (2004) Identification of a chemosensory receptor from the yellow fever mosquito, Aedes aegypti, that is highly conserved and expressed in olfactory and gustatory organs. Chem Senses 29: 403–410.
  46. 46. Iatrou K, Biessmann H (2008) Sex-biased expression of odorant receptors in antennae and palps of the African malaria vector Anopheles gambiae. Insect Biochem Mol Biol 38: 268–274.
  47. 47. Hall TA (1999) BioEdit: a user-friendly biological sequence alignment editor and analysis program for Windows 95/98/NT. Nucl Acids Symp Ser 41: 95–98.
  48. 48. Bendtsen JD, Nielsen H, von Heijne G, Brunak S (2004) Improved prediction of signal peptides: SignalP 3.0. J Mol Biol 340: 783–795.
  49. 49. Tamura K, Dudley J, Nei M, Kumar S (2007) MEGA4: Molecular Evolutionary Genetics Analysis (MEGA) software version 4.0. Mol Biol Evol 24: 1596–1599.