Researchers studying the water flea, which features both sexual and asexual genotypes (the asexual ones reproduce when females simply lay unfertilized eggs that hatch, a process known as parthenogenesis) found:
Ground-breaking new research from a team of evolutionary biologists at Indiana University shows for the first time how asexual lineages of a species are doomed not necessarily from a long, slow accumulation of new mutations, but rather from fast-paced gene conversion processes that simply unmask pre-existing deleterious recessive mutations.
In another remarkable finding from the genome-wide survey for asexual markers, the team was also able to determine the age of the entire asexual radiation for D. pulex. Just a few years ago biologists were guessing that asexual daphnia lineages could be millions of years old, and most recent estimates put it between 1,000 years and 172,000 years. But new calculations for the molecular evolutionary rates of the two chromosomes implicated in asexuality date the establishment and spread of the asexual lineage to just 1,250 years ago. Some current asexual lineages, in fact, were only decades old, younger than Lynch himself.
“A pond of asexual daphnia may go extinct quite rapidly owing to these deleterious-gene-exposing processes, but the small chromosomal regions responsible for asexuality survive by jumping to new sexual populations where they again transform the local individuals to asexuality by repeated backcrossing,” Lynch said. “Soon after such a transformation, the processes of gene conversion and deletion restarts, thereby again exposing resident pre-existing mutations leading to another local extinction event. As far as the sexual populations are concerned, asexuality is infectious, spreading across vast geographic distances while undergoing no recombination.”
Perhaps further study will show that the spread of asexuality is a common cause of extinction of susceptible species.