Introduction

An estimated 25% of all plant species and 10% of animal species are involved in hybridization in nature [1,2]. Nonetheless, the role of hybridization in speciation has historically attracted little interest among zoologists, as hybridization has often been assumed to constitute a “reproductive mistake” [1,3], rather than a mechanism of speciation. In teleost fishes, hybridization among closely related species is relatively frequent, leading to increased genetic diversity and speciation [4,5]. Hybridization often compromises or even prevents further sexual reproduction, especially when two divergent parental homologs need to be paired during meiosis [6–9]. Such complications are circumvented, if hybrids shift to asexual reproduction. Most gynogenetic species originate from hybridization of bisexual ancestors [10,11]. Gynogenesis (also called sperm-dependent parthenogenesis or pseudogamy) is a peculiar mode of unisexual reproduction in which the sperm from a heterospecific donor is required for activating the development of the unfertilized diploid egg, yet the paternal genetic information does not contribute to the next generation [12]. Gynogenesis is enigmatic, as it seems to combine disadvantages of both asexual and sexual reproduction (i.e., no benefits from genetic recombination, but costs associated with mating).

The Amazon molly (Poecilia formosa) is a hybrid of the Atlantic molly (Poecilia mexicana, maternal) and the sailfin molly (Poecilia latipinna, paternal) originating from a single hybridization [13–20]. Mitochondrial DNA evidence suggests that the Amazon molly is at least 120,000 years old [21]. As an all-female and gynogenetic species, P. formosa occurs in a limited geographical range located along the coastal versant of northern Mexico, overlapping in distribution range with its sperm donor species [20]. Embryogenesis of diploid P. formosa eggs is triggered by sperm from closely related species (P. latipinna, P. mexicana, P.latipunctata) [13,22,23]. This reproductive mode involves copulation with heterospecific males, as egg development is initiated internally in P. formosa. In rare cases only, genomic material from the sperm enters the egg, leading to paternal introgression of either a complete haploid genome (causing triploids) or parts of a genome (creating microchomosomes) (for more details see ref [24,25]).

Sex hormones are crucially involved in a wide range of biological processes, including mating behavior, species recognition, regulation of reproductive behavior, and sexual development. Androgens, as a group of sex hormones, play a crucial role in stimulating the development of masculine traits, sex differentiation, mating behavior, and spermatogenesis in male vertebrates. Androgens are also indispensable for females [26,27]. A recent study reported that an increased production of 11-ketotestosterone was observed both in male and female P. latipinna after mating with conspecifics [28,29]. Interestingly, this was not detected in heterospecific matings between male P. latipinna and P. formosa, indicating a potential role of androgen bidirectional interactions in the recognition of conspecifics [28,29]. Males of both P. mexicana and P. latipinna have a preference to mate with their conspecific females, relative to mating with P. formosa [30,31]. However, the strength of this preference is weaker in P. mexicana than in P. latipinna.

Studies on other species have shown that the effect of androgens (i.e., testosterone (T) and 5α-dihydrotestosterone (DHT) in mammals, 11-ketotestosterone (11KT) and testosterone (T) in fish [27]) is mediated by androgen receptors (Ars), which belong to the nuclear receptor super-family [32]. In mollies, the genomic basis and the expression of androgen receptors has not been studied so far. Androgen precursors can be converted to estrogen, which plays a key role in reproduction and development by binding with the estrogen receptors (Ers). This conversion is catalyzed by cytochrome p450, family 19, subfamily A (Cyp19a) aromatase [33]. For Cyp19a and Ar, both of which are involved in high-affinity interactions in the steroid-signaling pathway, a coevolution has been reported in both vertebrates and invertebrates [34,35]. In teleost fish, coevolution of genes can be more complex, as almost all genes occur in duplication, due to the well-known whole genome duplication in fish.

In the present study, we investigate expression levels for six candidate genes among P. formosa, P. mexicana, and P. latipinna in different tissues, i.e., ovary, gill, and brain. Given the hybrid nature of P. formosa and its unisexual inheritance without meiosis, we aimed at evaluating gene-wise three alternative hypotheses, i.e., 1) genes in P. formosa maintain conserved absolute expression levels in different organs; 2) genes, which are involved in gonad differentiation and reproduction could be expressed differently in the unisexual hybrid, as compared to its bisexual ancestors. 3) gene expression levels in P. formosa reflect its hybrid nature, by either intermediate expression, relative to the ancestors or expression as one of the two ancestors, when ancestors differ in their expression pattern. We selected the target tissues for the following reasons: The brain, as an integration center, not only produces sex hormones needed for sex differentiation and reproduction, but also transmits the signals for appropriate behaviors to the whole organism [36,37]. The gills are considered to be a portal route between endogenous and exogenous sex hormones, as both males and females can release pheromonal compounds via the gills to communicate and stimulate each other [38–40]. Finally, the ovary is not only the organ for female gamete production, but also a major site of synthesis of sex hormones in females. It is also involved in hormonal regulation and endogenous interaction via the brain-ovary axis.

Our candidate genes comprise 3 pairs of gene duplicates, which have emerged in the course of the fish-specific whole-genome duplication (FSGD, [41]), i.e., two duplicates each of ar, er, andcyp19a. To our knowledge, for none of these genes, expression levels have been previously examined in these three species. We complement our study by sequence analysis of all genes. We investigate sequence evolution by comparison of inferred amino acid sequences with other vertebrate species.

Materials and Methods

Results

Genetic variation and sequence evolution

As the consequence of a fish-specific, whole-genome duplication (FSGD [39]), ar, er, cyp19a have two isoforms in most teleost fish, i.e., arα and arβ, erα and erβ1, cyp19a1 (also referred to as cyp19a1a) and cyp19a2 (also referred to as cyp19a1b), respectively. In some teleost fish species, an additional isoform named erβ2 has also been reported [59–62]. We initially cloned the entire coding region of both ar isoforms cDNAs, which contain different putative functional domains, namely the N-terminal domain (NTD), the DNA binding domain (DBD), and the ligand binding domain (LBD), as well as the entire coding region of ers and cyp19as in our 3 focus species. All coding sequences are available from NCBI (for GenBank accession numbers see S1 Table). The Ar amino acids position of each putative domain is shown in Fig 1. The alignment of Poecilia Ars with those of other teleost fish and other vertebrate species illustrates that the important functional domains (e.g., DBD and LBD) are highly conserved, not only among the three species analyzed here, but also among Poeciliids in general (e.g., Gambusia and Xiphophorus; Fig 1, see S1 File for all pairwise comparisons).

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Fig 1. Structural comparison of P. formosa Ars with those of other species.

The numbers within each box indicate the percent amino acid identity of each putative domain (NTD: N-terminal domain, DBD: DNA binding domain (blue color), LBD: ligand binding domain (green color)), relative to P. mexicana Arα. The numbers above each box indicate the amino acids position of each domain. P. formosa had two alleles of both Arα and Arβ, pointing to its hybrid origin.

https://doi.org/10.1371/journal.pone.0156209.g001

Due to its hybrid origin and subsequent unisexual inheritance, P. formosa has been previously reported to be fixed in a heterozygous state (frozen hybrid) at many loci [19,63]. Indeed, the alignment of the CDS of P. formosa ars with its ancestor species also shows a heterozygous state of ar in P. formosa (Fig 2). Based on our arα and arβ sequences, P. formosa inherited one allele per locus from its maternal ancestor (P. mexicana), the other from its paternal ancestor (P. latipinna). In the coding region of arα, there are nine polymorphisms distinguishing between the two parental lineages, including five polymorphisms in NTD, four in LBD, but none in DBD. In the alignment of arβ, the two parental lineages are distinguished by ten differences distributed in all three domains and a 6bp insertion/deletion. Non-synonymous differences among the parental lineages were only observed in the NTD of ars, but not in the functional domains. This implies that there is no diversity in the functional domain amino acid sequences, which suggests that proteins of all species maintain a similar biological function (e.g., downstream binding). We conducted the same analysis on ers and cyp19as CDS (S1 Fig). Interestingly, we observed only one allele of erβ1 in P. formosa transcripts, which originated from P. latipinna. A prediction of potential functional implications of non-synonymous substitutions is shown in Table 1, using two different prediction methods, i.e., Polyphen2 and PROVEAN. Consistently inferred by both methods, there are two sites (positions 919 and 982, corresponding to amino acid positions 307 and 328) inferred to exhibit a potential functional change in Arα. At both these sites, P. formosa shared a nucleotide (and inferred AA) with P. latipinna (putatively ancestral), while the character state in P. mexicana was different (putatively derived). The non-synonymous substitutions specific to P. formosa in Ars are generally inferred not to cause a functional change, except for the AA position at 454 predicted to have functional implications by the PROVEAN method. The non-synonymous single nucleotide polymorphisms (SNPs) among the ancestral bisexual species (interpreted as ancestral substitutions; cf. Table 1) in all genes were predicted not to alter protein function. This suggests that the heterozygous state of P. formosa at these sites is not harmful to the biological function of the respective proteins. For AA substitutions inferred to deviate from neutral expectations (i.e., inferred to be probably damaging/deleterious), variability, major AA frequency, and frequency of the specific AAs found in our alleles is provided in S2 Table. All but two of these substitutions occured at highly conserved positions (major allele frequencies of 74%-100%; S2 Table).

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Fig 2.

Polymorphic nucleotide positions in the coding region of arα (A) and arβ (B) among the 3 analyzed Poecilia species. Red triangles indicate non-synonymous substitutions among the ancestral alleles of P. mexicana and P. latipinna origin. The blue round point represents non-synonymous substitutions caused by mutation within a single lineage. The green capital letters below the alignment in NTD of arβ represent additional amino acids at a position with an insertion/deletion event. Green stars indicate positions, at which P. mexicana was heterozygous (C/T). Dots in the alignment indicate identity with P. mexicana. Colored boxes indicate different functional domains within the protein (Green: NTD, Blue: LBD, Orange: DBD). Abbreviation in figure, Me: P. mexicana, Fo: P. formosa, La: P. latipinna.

https://doi.org/10.1371/journal.pone.0156209.g002

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Table 1. Prediction of impact of non-synonymous inferred amino acid substitutions (cf. Fig 2).

https://doi.org/10.1371/journal.pone.0156209.t001

To obtain a more detailed understanding of ars and related pathway genes sequence evolution in Poeciliidae, we constructed a phylogenetic tree based on the CDS of candidate genes using MrBayes (Fig 3). The phylogenetic tree clearly illustrates that the ars of fish fall into two clusters, arα and arβ. Furthermore, the arβ of fish is–on average—quite similar to the ar of other vertebrates or mammals. Our phylogenetic tree clearly uncovers a heterozygous state for P. formosa with allele1 inherited from P. mexicana and allele2 from P. latipinna on both the arα and arβ gene, confirming its hybrid origin once again. The high similarity of ars across Poeciliidae (i.e., genera Poecilia, Gambusia, and Xiphophorus) is also illustrated in our phylogenetic tree. The other duplicated candidate genes exhibited a similar pattern and clustered consistently with a previous study [37]. Note that for erβ1, only one allele was revealed in P. formosa transcripts, i.e., that originating from P. latipinna (S1 Fig). For ARβ, the gene tree does not fully resemble the species tree of the respective Poecilia species. Inspection of the data revealed that this pattern is supported by the AA polymorphism at position 239 where P. latipinna and P. formosa allele2 show Histidine, but other related species (including P. mexicana) Glutamine.

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Fig 3.

Phylogenetic tree of ars (A), ers (B) and cyp19as(C) in Poeciliidae and other vertebrates. Amino acid sequences were used for phylogenetic analysis with MrBayes. Labels provide Bayesian posterior probabilities. Teleost fish typically possess two copies of all genes. Note that Danio rerio has lost arα. This phylogenetic tree clearly demonstrates that the two gene copies emerged in the course of the fish-specific whole-genome duplication (FSGD, Meyer and Schartl 1999). It also confirms the hybrid origin of P. formosa, as they are heterozygous and possess one allele per locus from each of its ancestors. See S1 Table for accession numbers.

https://doi.org/10.1371/journal.pone.0156209.g003

Expression profiles of candidate genes

In order to identify potential differences in gene expression between the gynogenetic hybrid species P. formosa and its bisexual ancestors, P. mexicana and P. latipinna, we performed quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) on six candidate genes which are involved in the sex hormone-related pathways on RNA isolated from ovary, gill, and brain tissue in 3 species. Differences in expression patterns across tissues are shown in Fig 4 (for relative expression value and statistical analysis results see S2 File). Generally, expression levels were the lowest in gill. The highest values were obtained in the ovary, except for cyp19a2 (in P. mexicana and P. latipinna) and erα (in P. mexicana) where the highest expression was measured in brain. Fig 4 also provides detailed information on expression variation among genes, tissues, and species, as well as among biological replicates.

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Fig 4. Gene expression profiling.

This profiling presents each single gene expression distribution through all 3 tissues (ovary, gill, brain) among P. formosa and its ancestors. Expression levels are normalized relative to P. mexicana ovary expression data of 6 different pathway genes, arα (A), arβ (B), cyp19a1 (C), cyp19a2 (D), erα (E), erβ1 (F). They are shown on a logarithmic scale. Red stars indicate that expression has not been detected in our study. * 0.01≤P≤0.05; ** P< 0.01.

https://doi.org/10.1371/journal.pone.0156209.g004

The general expression pattern of ar genes (Fig 4A and 4B) implies that the gynogenetic P. formosa maintains a conserved expression similar to its bisexual ancestors for the crucial genes involved in the sex hormone function pathways. No specifically reduced or absent expression in any gene or tissue was observed in P. formosa, the all-female species.

In two cases, arβ in gill and cyp19a2 in ovary, P. mexicana had a significantly lower expression than the other two species. Likewise, in two other cases, cyp19a1 in brain and erα in gill, P. latipinna had a significantly lower expression than the other two species. In summary, (1) there were only four genes with a significant difference among the two bisexual ancestral species in one of the analyzed tissues and (2) for these genes and tissues, the hybrid species P. formosa always exhibited an expression level alike the higher expression level among the two bisexual species. In three cases, cyp19a1 in gill and erβ1 in gill and brain, no expression was detected in any of the analyzed species.

Discussion

In this study, we investigated sequence evolution in the genes of the androgen receptor and related pathways. Both bisexual molly species were found to be fixed for a single allele in our laboratory strains in all candidate genes (except for the arα gene in P. mexicana where 3 Single Nucleotide Polymorphisms (SNPs) occured; cf. Fig 2), while the hybrid Amazon molly was heterozygous with one allele per gene clearly deriving from P. mexicana, and the other from P. latipinna (except for the erβ1 gene). Consequently, we tentatively suggest that a polymorphism predates the formation of P. formosa (called “ancestral” in Table 1), if we distinguish between (1) P. latipinna and the P. latipinna-derived allele of P. formosa and (2) P. mexicana and the P. mexicana-derived allele of P. formosa. If a substitution occurs only in one of the three species, it may have occurred after the formation of P. formosa (called “derived” in Table 1). For the erβ1 gene, only one allele was found in P. formosa transcripts, despite of the analysis of 26 clones from that cDNA amplificate in our study. The likelihood for having overlooked a second allele is very low (P = 3.414E-07, Χ² test). This might indicate that only one allele is present in P. formosa (e.g., because of gene conversion, cf. [19]). Alternatively, as our study is based on cDNA, it may have been caused by different allelic specific expression (ASE). Although the single allele in P. formosa likely derived from P. latipinna, its expression pattern did not show a larger deviation from the expression in P. mexicana. Expression of erβ1 was very low in the ovary and not detected in gill and brain in all species (more intensely discussed below). Among the inferred amino acid substitutions, those with a higher predicted impact on protein function (i.e., predicted as “probably damaging” by Polyphen2 and/or “deleterious” by PROVEAN; cf. Table 1) were all found deviant (“derived” in Table 1) in the ancestral species P. mexicana, while those substitutions distinguishing between the two bisexual species (“ancestral” in Table 1) and specific to P. formosa (except AA position 454 by PROVEAN method) were mostly predicted to be benign or neutral. This suggests that the heterozygous state of P. formosa is not deleterious to the function of proteins as compared to both of its bisexual ancestors and that the two alleles which P. formosa received from its ancestors are likely to be equally well suited to serve the biological function of this gene. In most occasions, AA substitutions inferred to deviate from neutral expectations were at highly conserved positions (see S2 Table), which is compatible with an assumed biological impact of the respective substitution. Nonetheless, such computationally inferred impacts only comprise a first hint on a functional impact. Ultimately, sophisticated molecular modelling and/or biochemical/genetic experiments (e.g., mutagenesis) would be necessary to verify the impact of a substitution. Future experimental protein function studies could further investigate, how this is related to the gynogenetic reproductive mode of P. formosa. In this context, our study provides also SNP markers for allele specific expression research. Furthermore, the P. mexicana allele shows slightly less similarity in the NTD domain of Arα (cf. Fig 1), which is caused by the substitutions of positions 747, 919, 982 (cf. Fig 2). At those sites, P. mexicana was heterozygous. One of the nucleotides is the same as in the other species, the other one is different. If these are real SNPs within P. mexicana, they could constitute “derived” substitutions in the P. mexicana lineage. Alternatively, they may comprise ancestral lineage sorting in this bisexual species, i.e., an ancestral polymorphism that has been divergently sorted into two lineages (1) the sampled P. mexicana population and (2) the P. formosa lineage. Furthermore, we cannot fully exclude the possibility of an Taq amplification error here.

After the FSGD, the arα is secondarily lost in some teleost fish, e.g., zebra fish, Otophysi, and Salmoniformes, whereas the basal teleosts and percomorphs kept two copies of ar[64]. In our study, we observed a similar expression trend of arα and arβ only between ovary and brain, while expression in gill is different. This coincides with the gills not being involved in the brain-pituitary-gonadal axis, which is necessary for the reproduction and regulation of sex hormone [38], especially androgens. This phenomenon suggests the expression of ar genes to correlate with the amount and the location of androgen production.

Divergent evolution between arα and arβ is clearly illustrated by the large numbers of AA polymorphisms (cf. Fig 1). This could suggest that arα plays a more crucial role in the estradiol synthesis pathway, whereas the evolutionary pattern of arβ may indicate relaxed selection or even positive selection towards an alternate function, i.e., a neofunctionalization, as has been suggested for arβ in other teleost fish [64].

In the context of the evolution of genes interacting with the androgen pathway and involved in estrogen synthesis pathway in teleost fish, the aromatase has two conserved tissue-preferential isoforms, gonadal aromatase, preferentially expressed in ovary and encoded by the cyp19a1 gene, and brain aromatase, preferentially expressed in the brain and encoded by the cyp19a2gene. Aromatase plays a pivotal role in sex determination and sex differentiation by controlling not only ovarian differentiation, but also testicular differentiation and brain sexual development in teleost fish by their up-regulation or down-regulation [65,66]. The strict ovary-specific cyp19a1 and brain-specific cyp19a2 expression reflects a common expression trend in teleost found in previous studies [67,68]. However, some studies demonstrate cyp19a expression not to be strictly limited to a specific tissue in some teleosts. A novel expression pattern of cyp19a1 and cyp19a2, where both genes shifted the expression to testis tissue, was observed in Ectodine and Haplochromine cichlid fish [37]. Our expression data also show a deviation from a simple tissue-specific expression pattern. We found higher expression of cyp19a1 in ovary as compared to brain (P< 0.05), and slightly higher expression of cyp19a2 in brain as compared to ovary (except in P. formosa) (Fig 4). Taken together, cyp19a1 and cyp19a2 show deviation from strict tissue expression, i.e., cyp19a1 is not only expressed in ovary and cyp19a2 not only in brain, in agreement with other studies.

Estrogens have also been extensively studied as important hormones for ovarian differentiation in vertebrates [69–71], including teleost fish [72–75]. Interestingly, aromatase is also involved in estrogen synthesis by converting androgen to estrogen. The highly expressed cyp19a2 is regulated by its promoter on EREs (estrogen response element) through the estrogen and aromatase feedback loop [33,67]. The expression pattern of arα and cyp19a1 in ovary is similar among all three species. Hence, these two genes may be considered to be involved in the same pathway, which could be estrogen synthesis or other functions in the ovary. In a previous study, overexpressed cyp19a2 of zebra fish embryo was observed in an estradiol treatment experiment and its induction was blocked by treatment with estradiol and ICI-182, 780 (ICI or Fulvestrant), an estrogen receptor (Er) antagonist. This suggests that Ers are involved in estradiol-dependent induction of the Cyp19a2 [76]. In our study, erβ1 expression was not detected in gill and brain and extremely low in ovary across all three species. Consequently, we suggest that erβ1 is not involved in the estradiol synthesis pathway at least in brain, where the aromatizable androgens could be aromatized to estrogen via Cyp19a. According to our expression data, erα may be more important in this context. In vivo and in vitro experimental approaches could confirm this interpretation.

The co-evolutionary relationship between Ar and aromatase was investigated in invertebrate and vertebrates including teleost fish [34,35]. Reitzel [34] demanded a clarification of gene orthology and appropriate background control genes for the analysis of co-evolutionary relationships among duplicated ar and pathway-related genes (for details see [34]). They also suggested alternative molecular or experimental approaches which could be combined with sequence divergence data to unravel the complicated evolutionary relationship between Ars and aromatases, both of which existing in duplicate in most teleost. We consider our combined sequence analysis and gene expression data to be potentially meaningful in this context, as the two paralogs cyp19a1 and cyp19a2 exhibit tissue-preferential expression Thus, we suggest that experimental approaches for illustrating the co-evolutionary relationship between ar and cyp19as need to be specific with regard to both gene duplicates and tissues, as the different paralogs may be involved in different pathways within different tissue.

In conclusion, the heterozygous state of ars gene and related pathway genes in P. formosa and its evolutionary relationship with its bisexual ancestors were confirmed in this study. The evolution of two paralogs of each gene, arα and arβ, erα and erβ1, cyp19a1 and cyp19a2, were also investigated. Furthermore, we studied the distinct gene expression pattern found in various tissues. Taking into account the higher expression in ovary and brain than in gill, gene expression level of ars may correlate with the amount and the location of androgen production. The expression pattern in P. formosa was conserved in those genes where expression did not differ between the two bisexual ancestors. In those cases, however, where gene expression differed between the two bisexual species, the P. formosa expression was always comparable to the higher expression of the two bisexual ancestors. This pattern does not indicate any significant deviation in gene expression between unisexual and bisexual mollies. Instead, it may reflect the hybrid nature of P. formosa: P. formosa has–for most genes–two different alleles, one of each of the two ancestors. As our expression analysis was so far not allele-specific, we could not confirm, whether similar expression levels between P. formosa and one of the two bisexual ancestors was caused by allele-specific expression differences among the two different alleles in the hybrid species P. formosa. Here, we suggest to utilize our allele-specific SNPs to establish a qPCR assay for the investigation of allele-specific expression in P. formosa.

Such studies should also involve specimens from different wild populations of the three species, in order to contrast among species effects with the within-species variability occurring in the wild.

Supporting Information

Acknowledgments

We are thankful to Prof. Dr. Manfred Schartl for kindly providing P. formosa and P. latipinna fishes. We thank Dr. Luis Valente for his help on statistical analysis in R. We also thank Dr. Francesco Lamanna for his help on phylogenetic analysis and comments. We thank Ina-Maria Schedina for tissue collecting and Dr. Sandra Schwarte for discussion. Two reviewers provided very valuable comments on a previous version of the manuscript. Financial support was provided by the University of Potsdam.