Death by Asexuality: Geneticists Uncover New Path in Evolutionary Mutations

Scientists have discovered a little bit more about the process of evolution. They've found that asexual lineages of species are doomed from fast-paced gene conversion processes that unmask pre-existing deleterious recessive mutations. The findings reveal a little bit more about how new species emerge and thrive while others simply go extinct.

When it comes on betting who will win the evolutionary race, geneticists usually choose sexual reproduction. This is mainly due to Muller's ratchet, which is the mechanism by which a genome accrues deleterious and irreversible mutations after the host organism has lost its ability to carry out the important gene-shuffling job of recombination. Yet this new work reveals a little bit more about why mutations arise.

In order to learn more about the process of evolution and genetics, the researchers sequenced the entire genomes of 11 sexual and 11 asexual genotypes of Daphinia pulex, also known as the water flea. After examining these genomes, the researchers discovered that every asexual genotype shared common combinations of alleles for two different chromosomes transmitted by asexual males without recombination.

In fact, asexual males spread the genetic elements for suppressing meiosis, the type of cell division necessary for sexual reproduction, into sexual populations. While this may make females asexual, their sons will not necessarily also be asexual. Instead, they may have the ability to spread the asexual gene.

"One might think of this process as a transmissible asexual disease," said Michael Lynch, one of the researchers, in a news release.

The researchers also determined that the spread of asexual linages occurred just 1,250 years ago. In fact, some of the lineages were mere decades' old. That's a huge leap from the previous estimate of millions of years old.

"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," said Lynch in a news release. "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."

The findings reveal a little bit more about how sexual and asexual populations survive. In addition, it shows how a gradual accumulation of new mutation is less of a contributor for asexual decline. The research shows how evolution has moved forward in these populations.

The findings are published in the journal Proceedings of the National Academy of Sciences.