Mole Salamander - Hybrid All-female Populations

Hybrid All-female Populations

Unisexual (all-female) populations of ambystomatid salamanders are widely distributed across the Great Lakes region and northeastern North America. The females require sperm from a co-occurring, related species to fertilize their eggs and initiate development. Usually the eggs then discard the sperm genome and develop asexually (i.e., gynogenesis, with premeiotic doubling); however, they may incorporate the genome from the sperm into the resulting offspring. Sperm incorporation commonly takes the form of genome addition (resulting in ploidy elevation in the offspring), or genome replacement, wherein one of the maternal genomes is discarded. This unique mode of reproduction has been termed kleptogenesis by Bogart and his colleagues. This is in contrast to hybridogenesis, where the maternal genome(s) are passed hemiclonally and paternal genome is discarded every generation before the egg matures and reacquired from the sperm of another species.

The nuclear DNA of the unisexuals generally comprises genomes from up to five species: the Blue-spotted Salamander (Ambystoma laterale), Jefferson Salamander (A. jeffersonianum), Smallmouth Salamander (A. texanum), Streamside Salamander (A. barbouri), and Tiger Salamander (A. tigrinum), denoted respectively as L, J, T, B, and Ti. This flexibility results in a large number of possible nuclear biotypes (genome combinations) in the unisexuals. For example, an LJJ individual would be a triploid with one laterale genome and two jeffersonianum genomes, while an LTJTi individual would be a tetraploid with genomes from four species. Because they have hybrid genomes, unisexual salamanders are a cryptic species with morphology similar to coexisting species. For example LLJs look like Blue-spotted Salamanders and LJJs look like Jefferson Salamanders. Silvery Salamanders LJJ (A. platineum), Tremblay's Salamanders LLJ (A. tremblayi), and Kelly's Island Salamanders LTT and LTTi (A. nothagenes) were initially described as species. Species names were later dropped for all unisexual salamanders because of the complexity of their genomes. The offspring of a single mother may have different genome complements, for example a single egg mass may have both LLJJ and LJJ larvae.

Despite the complexity of the nuclear genome, all unisexuals form a monophyletic group based on their mitochondrial DNA. The maternal ancestor of the unisexual ambystomatids was most closely related to the Streamside Salamander(A.barbouri), with the original hybridization likely occurring 2.4-3.9 million years ago making it the oldest known lineage of all-female vertebrates. The hybridization was most probably with an A.laterale. All known unisexuals have at least one A.laterale genome and it is hypothesized that this is essential for unisexuality. However, the laterale genome has been replaced several times, independently, in each of the lineages by matings with A.laterale.