The evolutionary and ecological consequences of reproducing sexually
The maintenance of sexual reproduction is one of the fundamental questions in evolutionary biology. Sexual taxa are expected to be more robust to extinction in the evolutionary long-term due to increased genetic variability in offspring. In contrast, asexual taxa are released from the burden of producing two sexes, one of which does not directly reproduce, and so are expected to outcompete related sexual taxa in the evolutionary short-term but suffer a cost in terms of reduced genetic variation limiting long-term persistence. This appears to be the case, as asexual taxa are well-recognized for their young lineage ages compared to related sexual species. Nonetheless, there are some “asexual” lineages that are unexpectedly old, which raises the question: what are the evolutionary mechanisms that allow these “anomalous” asexual lineages to persist over evolutionary time?
I study the wonderfully strange unisexual Ambystoma salamanders, an all-female lineage of salamanders that are widespread across Eastern North America. These salamanders are proposed to reproduce uniquely among vertebrates by “stealing” sperm from the males of other sexual species. This has resulted in a single line of females with multiple genomes from other species, all without a male in sight! I’ve sought to explain how these unisexuals get away with this mode of reproduction without driving sexual species, or themselves, extinct. We’ve used molecular, ecological, and physiological data to make new discoveries about the ecological and evolutionary relationships between sexual and unisexual salamanders. These projects include finding vast differences in dispersal ability between sexual species and unisexuals (Denton et al. 2017), faster tissue regeneration in unisexual salamanders (Saccucci et al. 2016), and gene expression balance between unisexual subgenomes (McElroy, Denton et al. 2017).
The discordance between mitochondrial and nuclear genomes
Hybridization between species is a well-studied phenomenon that has vast evolutionary and ecological implications. However, an underappreciated consequence of hybridization is the one-way infiltration of mitochondrial genomes from one species into another. When these genomes spread, a species can have a mismatch between the nuclear genomes of their own species and the mitochondrial genomes of another.
We’ve identified this pattern of mitonuclear discordance in two species of Ambystoma salamander, the Smallmouth Salamander (A. texanum) and the Streamside Salamander (A. barbouri) and showed that ancestral hybridization has led to an infiltration of Streamside Salamander mitochondria into Smallmouth Salamander populations (Denton et al. 2014). These same mitochondria are also the closest relatives of the all-female lineage of Ambystoma, raising interesting questions about why this mitochondrial genomes appears to be so “sticky” when mixed with other salamander species.
Amphibians have some of the largest and most complex genomes of vertebrates, challenges that have prevented the fast proliferation of whole genome assemblies compared to other taxanomic groups. I am now working on one of the few amphibian genomes nearing completion, that of the African Bullfrog (Pyxicephalus adspersus). This frog is of great interest to evolutionary biologists due to the important link it provides between anamniotes (like fish) and more recently evolved animals (like humans). African Bullfrogs are in the minority of amphibians in which males are larger than females, and their extreme phenotypes provide opportunities to link intense sexual selection to the evolution of sex chromosomes.