News from IBE (CSIC-UPF)
The very rare animals that reproduce asexually ―only about one thousand of all living vertebrate species― are thought to be at an evolutionary disadvantage compared to their sexual counterparts. One of the theories that spell this out is the idea that if no new DNA is introduced during reproduction, then harmful gene mutations can accumulate over successive generations, leading to eventual extinction. Another hypothesis states that because asexual reproduction limits genetic diversity within species, the animals eventually become unable to adapt to changes in the environment. These theories, however, do not hold true to the Amazon molly (Poecilia formosa), an all-female fish species that has thrived for millennia in the fresh waters along the Mexico-Texas border.
The Amazon molly was the first asexual vertebrate discovered in 1932. It reproduces by “mating” with a male fish of a related species. The male’s DNA is not incorporated, however, into the offspring. Instead, mating with the male fish triggers the replication of the entire maternal genome. In essence, mollies clone themselves. They do not lay eggs but give birth to large broods of live offspring instead.
The researchers found that the Amazon molly resulted from a sexual reproduction event involving two different species of fish, when an Atlantic molly first mated with a Sailfin molly 100,000 to 200,000 years ago. Since then, the resulting Amazon molly has been a hybrid species that remarkably has remained frozen in evolutionary time ―yet still continuing to thrive. The expectation is that many harmful mutations would accumulate in that time, but that is not what they found. “The Amazon molly shows few harmful mutations, low genetic decay levels and a unique and in constant evolution variability ―mainly in genes related to the immune system”, says Raquel García Perez, predoctoral researcher at IBE. “Such features could explain the evolutionary success of the Amazon molly”.
Warren W.C. et al. Clonal polymorphism and high heterozygosity in the celibate genome of the Amazon molly. Nature Ecology & Evolution, Feb. 12, 2018