Figure 1.
Schematic of a cross species test of allelism to determine homology at co-localized hybrid sterility loci.
For illustration, only the chromosome containing the target locus is shown for each genotype. Introgression lines (ILs) show locations of the heterospecific chromosomal region containing an isolation QTL in red (light) shaded and blue (dark) shaded from species H and P, respectively, on a species SL (white) genetic background. A) Homozygous ILs are crossed together to generate heterointrogression lines ILHP or ILPH. B) Fertility of hybrid heterospecific ILs are compared with homozygous parental ILs and the pure (fertile) parent to determine evidence for homology. Complementation indicates that alleles are not homologous; lack of complementation (low fertility in the heterointrogression line(s)) is consistent with a shared underlying mutation.
Figure 2.
Test of allelism at pollen sterility locus pf7.2.
A) Chromosomal location (shaded) of SP and SH introgressions represented in ILPP and ILHH lines, respectively, used in cross-species tests. Marker IDs (solgenomics.org) are shown to the right (SP) or left (SH) of chromosome 7. To resolve the chromosomal position of each introgression boundary more finely than in the original mapping studies, we performed additional fine-scale genotyping of ILPP and ILHH lines (Text S1); marker genotypes are from these analyses. B) Pollen fertility (percent fertile pollen) in 5 genotypes (SL = S. lycopersicum; ILHH = introgression line with homozygous SH alleles; ILPP = introgression line with homozygous SP alleles; ILHP = heterointrogression line (from ILHH maternal parent); ILPH = heterointrogression line (from ILPP maternal parent)). C) Inferred placement of mutational change underlying pf7.2.
Figure 3.
Test of allelism at seed sterility locus sss1.2.
A) Chromosomal location (shaded) of SP and SH introgressions represented in ILPP and ILHH lines, respectively, used in cross-species tests. Marker IDs (solgenomics.org) are shown to the right (SP) or left (SH) of chromosome 7. B) Seed fertility (number of viable seed per fruit) in 4 genotypes (SL = S. lycopersicum; ILHH = introgression line with homozygous SH alleles; ILPP = introgression line with homozygous SP alleles; ILHP = heterointrogression line (from ILHH maternal parent)). C) Inferred placement of two mutational changes (sss1.2.1, sss1.2.2) underlying sss1.2. Note that introgression regions corresponding to sss1.2 in the SP versus SH lines are incompletely overlapping (Fig. 2A), with an additional distal heterospecific region represented in line ILPP but not in ILHH; therefore sss1.2.2 could be located within the introgressed region unique to ILPP. Regardless, because no ILHH line exhibited reduced seed fecundity at this genomic location in the original SH×SL mapping experiment [7], our results are consistent with the inference that sss1.2.2 is SP-specific.
Figure 4.
Test of allelism at seed sterility locus sss2.1.
A) Chromosomal location (shaded) of SP and SH introgressions represented in ILPP and ILHH lines, respectively, used in cross-species tests. Marker IDs (solgenomics.org) are shown to the right (SP) or left (SH) of chromosome 7. B) Seed fertility (number of viable seed per fruit) in 4 genotypes (SL = S. lycopersicum; ILHH = introgression line with homozygous SH alleles; ILPP = introgression line with homozygous SP alleles; ILHP = heterointrogression line (from ILHH maternal parent)). C) Inferred placement of mutational change underlying sss2.1.
Table 1.
Co-localized QTL evaluated with interspecific tests of allelism.
Table 2.
Genotype fertilities (least squares means) from tests of allelism at the three co-localized QTL.
Figure 5.
Inferred placement of all detected isolation loci acting on (A) pollen fertility, and (B) seed fertility, between SL and each of SH and SP.
Details and effect size data for each locus are in Table S3. Loci that are unique to one species pair must be due to mutations that occurred on a branch that is unique to that species pair. Placement of the shared isolation mutations on branch b is based on the inference that they are derived in the common ancestor of SH/SP (see main text, Figures 2 and 3, Text S2, and Figure S3).
Figure 6.
The number of potential deleterious interactions among derived alleles only (left panels), or including ancestral alleles (right panels), that involve a specific derived fixation in lineage I, under a combinatorial epistatic model of hybrid incompatibility accumulation [15].
The number of potential interactions is evaluated at the most recent (top) time point. Loci that have undergone substitutions in lineage I and II are labeled with unique letters. Derived alleles, denoted with subscript ‘d’, are depicted at their time of origin along each lineage using open circles. For alleles that are derived in lineage I, ancestral alleles at the homologous locus in lineage II are depicted with closed circles (and subscript ‘a’) in the right panels. Potential interactions are depicted with arrows between alleles for each relevant pair (panels A and B) or trio (panels C and D) of alleles. A) Pairwise interactions involving an early mutation in lineage I (at locus C). B) Pairwise interactions involving a later mutation in lineage I (at locus I). In general, for pairwise interactions, at time t the possible number of derived-derived interactions (left panels) involving the ith derived allele in one lineage is simply to total number of derived alleles in the other lineage, regardless of when the ith allele arose during divergence (A and B, left panels). In contrast, the possible number of derived-ancestral interactions involving the ith derived allele in one lineage is [i-1] (A and B, right panels). C) Complex (3-locus) interactions involving a lineage I fixation that occurred early in the history of divergence between I and II (at locus C). D) Complex (3-locus) interactions involving a lineage I fixation that occurred late in the history of divergence between I and II (at locus I). For both C) and D), the second derived mutation in lineage I is at locus G. In general, for complex (3-locus) interactions, except for the first derived allele, at time t the possible number of complex derived interactions involving the ith and jth derived alleles in one lineage is simply the total number of derived alleles in the other lineage, regardless of when the ith allele arose during divergence (C and D, left panels). In contrast, except for the first derived allele, the possible number of derived-derived-ancestral interactions involving the ith and jth derived alleles in one lineage is [i-1] or [j-1], whichever is smaller (C and D, right panels).