Celebrate diversity: Old female salamanders

Imagine, if you will, a line of ancient females, who trick males into having sex with them, so that the females can continue living indefinitely.

I know, you’ve seen it before in a dozens of movies, books, and television episodes. Who hasn’t seen a vain sorceress stealing youth, particularly from young men?

Something like this goes on in some salamanders, except that no one salamander is living unusually long. The evolutionary line of salamanders, though, is showing surprisingly longevity.

Some salamanders are all female... but they are not, strictly speaking, asexual or parthenogenetic. These unisexual female lineages engage in a little sperm “theft” from several sexual salamanders species (Ambystoma laterale is pictured), a process known as kleptogenesis. Genetically, these unisexual females are all over the map. Some unisexuals have a paired set of chromosomes like the more typical sexual species (i.e., they are diploid); others have three, four, or five sets of chromosomes.

This unusual mode of reproduction is interesting, because it might allow a unisexual species to avoid genetic stasis. In theory, sexual species have the edge in a changing environment because more genetic variations are possible through sexual reproduction. Having every individual be genetically identical is great as long as the environment never changes.

But environments do change, so it’s generally though that parthenogenetic species tend to go extinct at much higher rates than sexual ones.

This new paper by Bi and Bogart tries to settle how old this odd lineage of female salamanders is. One previous study suggested millions of years; another estimated tens of thousands of years. Bi and Bogart come down on the side of millions of years; a little over 5 million years, to be precise.

The reason for the discrepancy comes down to an issue concerning the wrong bit of DNA being amplified and analyzed. Most animal cells have DNA in two places: the cell nucleus (where most people think of it being), and in the mitochondria (the cell’s power plant). In normal sexual species, nuclear DNA comes equally from both parents, while mitochondrial come from mom.

Because these all-female salamanders get nuclear DNA from the sperm they “steal,” you can really only try to trace relationships using mitochondrial DNA in these animals.

The problem is that nuclear DNA sometimes has some bits in it that are extremely similar to mitochondrial DNA. These sequences, called numts, are probably derived from the mitochondria. Bi and Bogart argue that the previous research that suggested that these female salamanders split from the sexual salamanders about 25,00 years ago probably sequenced a numt instead of a mitochondrial gene.

That DNA. It’s tricky.

Parthenogenesis in Marmorkrebs is blessedly simple in comparison.

Reference

Bi, K., & Bogart, J. (2010). Time and time again: Unisexual salamanders (genus Ambystoma) are the oldest unisexual vertebrates BMC Evolutionary Biology, 10 (1) DOI: 10.1186/1471-2148-10-238

Photo by whiteoakart on Flickr, used under a Creative Commons license.