Death by asexuality: a path for mutations.

• Position: Genetics

Geneticists long have bet on the success of sexual reproduction over asexual reproduction based, in a large part, on the process known as Muller's ratchet, 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.

The work from the laboratory of Michael Lynch, professor of biology at Indiana University, Bloomington, instead indicates that most deleterious DNA sequences contributing to the extinction process actually are present in the sexual ancestors, albeit in recessive form, and simply become exposed via fast-paced gene conversion and deletion processes that eliminate the fit genes from one of the parental chromosomes.

After sequencing the entire genomes of 11 sexual and asexual genotypes of Daphnia pulex, a model organism for the study of reproduction that is more commonly known as the water flea, the team discovered that every asexual genotype shared common combinations of alleles for two different chromosomes transmitted by asexual males without recombination.

Asexual males then spread the genetic elements for suppressing meiosis, the type of cell division necessary for sexual reproduction, into sexual populations. The unique feature of this system is that, although females become asexual, their sons need not be, and instead have the ability to spread the asexuality gene to sexual populations--in effect, by a process of contagious asexuality.

"One might think of this process as a transmissible asexual disease," notes Lynch. Exposure of preexisting, deleterious alleles is, incidentally, a major cause of cancer, he adds.

In another remarkable finding from the genome-wide survey for asexual markers, the team also was 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 at more than 100,000 years, but new...