Data from: Cryptic sex? estimates of genome exchange in asexual mole salamanders (Ambystoma sp.)

Cryptic sex has been argued to explain the exceptional longevity of certain parthenogenetic vertebrate lineages, yet direct measurements of genetic exchange between sexual and apparently parthenogenetic forms are rare. Female unisexual mole salamanders (Ambystoma sp.) are the oldest known unisexual vertebrate lineage (~5 million years), and one hypothesis for their persistence is that allopolyploid female unisexuals periodically exchange haploid genomes (‘genome exchange’) during gynogenetic reproduction with males from sympatric sexual species. We test this hypothesis by using genome-specific microsatellite DNA markers to estimate rates of genome exchange between sexual males and unisexual females in two ponds in NE Ohio. We also test the prediction that levels of gene flow should be higher for “sympatric” (sexual males present) genomes in unisexuals compared to “allopatric” (sexual males absent) unisexual genomes. We used a model testing framework in the coalescent-based program MIGRATE-N to compare models where unidirectional gene flow is present and absent between sexual species and unisexuals. As predicted, our results show higher levels of gene flow between sexuals and sympatric unisexual genomes compared to lower (likely artifactual) levels of gene flow between sexuals and allopatric unisexual genomes. Our results provide direct evidence that genome exchange between sexual and unisexual Ambystoma occurs and demonstrate that the magnitude depends on which sexual species are present. The relatively high levels of gene flow suggest that unisexuals must be at a selective advantage over sexual forms so as to avoid extinction due to genetic swamping through genome exchange.

学界曾提出，隐蔽性生殖（cryptic sex）可解释某些孤雌生殖（parthenogenetic）脊椎动物支系非凡的存续时长，但目前针对有性生殖类群与表观孤雌生殖类群间的基因交流的直接定量研究仍较为匮乏。雌性单性钝口螈（Ambystoma sp.）是目前已知最古老的单性脊椎动物支系，其存续时长约500万年；针对其长期存续的假说之一认为，异源多倍体（allopolyploid）雌性单性个体在雌核发育（gynogenetic）生殖过程中，会定期与同域分布（sympatric）的有性雄性物种交换单倍体基因组，即所谓的"基因组交换（genome exchange）"。本研究通过基因组特异性微卫星DNA（microsatellite DNA）标记，对俄亥俄州东北部两处池塘内的有性雄性与单性雌性个体间的基因组交换速率进行估算，以此检验上述假说。同时我们验证了如下预测：与异域分布（allopatric，即无同域有性雄性存在）的单性基因组相比，单性类群中携带同域分布雄性基因组的基因流水平应更高。本研究采用基于溯祖理论（coalescent-based）的程序MIGRATE-N中的模型检验框架，对比了有性物种与单性类群间存在或不存在单向基因流的两类模型。结果与预测一致：观测到有性类群与同域单性基因组间的基因流水平更高，而有性类群与异域单性基因组间的基因流水平则较低，该低水平结果大概率为人为假阳性产物。本研究结果为钝口螈属有性类群与单性类群间的基因组交换提供了直接证据，并证实基因交流的程度取决于共存的有性物种种类。较高的基因流水平提示，单性类群必须相较于有性类群具备选择优势，才能避免因基因组交换带来的基因淹没（genetic swamping）而走向灭绝。