For most organisms it can be pretty straightforward to link a DNA sequence of a known gene to the species it came from, like a genetic barcode. These genetic barcodes are effective due to the slow accumulation of changes over evolutionary time. Every time two individuals reproduce, there is a chance that changes in their genes will be passed on to the next generation. Over time, these changes accumulate, leading to genetic differentiation between species and populations. However, for some species like the unisexual Ambystoma lineage, that logic completely falls apart.
Ambystoma salamanders are a group of large “mole” salamanders that spend most of their lives underground. They briefly emerge in the spring and migrate to breeding pools where they reproduce before heading back to their underground homes. For most of these salamanders, reproduction requires a female of one species to contribute eggs and a male of that same species to contribute sperm. The sperm fertilizes the eggs, and the resulting male and female offspring carry a combination of genes from both parents.
Unisexual Ambystoma salamanders are an all-female lineage that uses a different reproductive strategy called kleptogenesis, where female salamanders “steal” the sperm from male salamanders of closely related species. The sperm is used to trigger egg development, but genetic material from the sperm is not necessarily incorporated by the offspring. When genetic material from the sperm is incorporated, it’s usually via genome addition or replacement. This means that instead of the usual two genome copies, one from the male parent and one from the female parent, unisexual Ambystoma often have three to five genome copies. Collectively, this means that it can be complicated to genetically differentiate a unisexual Ambystoma from the other species they reproduce with including the Jefferson salamander, the tiger salamander, the blue-spotted salamander, the smallmouth salamander, and the streamside salamander.
In collaboration with researchers at the University of Maine, we are developing molecular tools to tease apart the presence of members of the Ambystoma species complex using environmental DNA (eDNA) detection methods. The development of these methods will increase our ability to apply the most sensitive and non-invasive methods for studying an extremely cryptic species complex.
