Mapping Genotype Distributions in the Unisexual Ambystoma Complex
Mapping species' distributions is a primary challenge when managing cryptic lineages of conservation concern. In the case of unisexual Ambystoma salamanders, mapping geographic distribution of genotypes can also help us understand the evolutionary dynamics of one of the most intriguing vertebrate reproductive systems. We combined a mitochondrial sequencing technique with existing nuclear microsatellite methods to map genotypes in 15 Ambystoma populations throughout Massachusetts. We found that the host species A. jeffersonianum and A. laterale have disjunct east/west distributions, whereas unisexuals are distributed widely in Massachusetts. We did not find both host species at any single locality. In our samples, unisexuals outnumbered either host species in 11 of 15 populations. Ambystoma jeffersonianum nuclear genomes were present in at least 97% of unisexual salamanders in regions where A. laterale and unisexuals exist but A. jeffersonianum mitochondria were absent. If previous studies of the unisexual reproductive mode are correct, our observations suggest that natural selection favors hybrid nuclei in these populations.Abstract
(A) Distributions of nuclear genomotypes in 151 salamanders at 14 breeding sites in Massachusetts. Microsatellites were used to determine whether nuclei contained only Ambystoma jeffersonianum (J) genomes, only A. laterale (L) genomes, or both. Pie chart sizes represent the total numbers of salamanders scored for the nuclear analysis. Site 11 is excluded from this map because only one of the microsatellites successfully amplified. The individual with a unisexual mitochondria from site 5 is not represented in the top map because only one microsatellite locus amplified successfully. (B) Distributions of mitochondrial haplotypes of A. laterale, A. jeffersonianum, and unisexual salamanders at 15 breeding sites in Massachusetts. A portion of the mitochondrial D-loop was sequenced and compared with published sequences to determine haplotype presence at each pond. At populations where unisexuals were found (sites 1–14), we successfully amplified mitochondrial sequences from between 1 and 11 individuals (Appendix 1). At the population where no unisexuals were found (site 15), we successfully amplified mitochondrial sequences from 24 individuals. The south-flowing Connecticut River is drawn as a dark blue line.
Snout–vent lengths and masses of salamanders with mitochondrial D-loop sequences that matched Amybsomta jeffersonianum (“JJ”), A. laterale (“LL”), or unisexual hybrids. In this plot, unisexual classifications are not based on direct nuclear genotype observations, but instead are inferred on the basis of whether they occur within the range of A. jeffersonianum (“LJJ”) or A. laterale (“LLJ”). “a”, Tukey's post hoc tests vs. L significant at P < 0.001. “b”, Tukey's post hoc tests vs. J significant at P ≤ 0.001. “c”, Tukey's post hoc test vs. LLJ significant at P = 0.002.
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