Iowa City, IA, USA. Living organisms have good reason for engaging in sexual, rather than asexual, reproduction according to biologist Maurine Neiman. Only sexual females produce offspring (both sons and daughters), but asexual females make only daughters. There is a cost of males, predicting that sex should be rare because asexual females will leave many more descendants.

One of the most important unanswered questions among evolutionary biologists remains: given that we know asexual reproduction results in more progeny, and the presence of males does bear an evolutionary cost, why then does (binary) sex persist and thrive?

We do know that sex predominates by a wide margin in the natural world and results in the production of genetically variable offspring, unlike asexual reproduction. A more precise understanding the benefits of sex is of direct relevance to understanding the value and preservation of genetic diversity within and among species, populations, and ecological communities.

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Genomics

In an article published in a recent issue of the journal Molecular Biology and Evolution, Neiman and her colleagues, including John M. Logsdon Jr., associate professor of biology, examined the theory that sexual reproduction, while requiring more time and energy than asexual reproduction, is also much more common among living organisms and, therefore, must be very beneficial.

The expectation that sex should be rare is based on the assumption that sexuals and asexuals are similar. In other words, the two-fold cost of sex will be diminished or even negated if asexual females experience disadvantages related to asexuality.

Accordingly, the research team used a variety of approaches to identify and understand the consequences of the many ways in which asexuals and sexuals might differ, with the goal of better understanding why sex is so common in natural populations.

Neiman and her colleagues tested the hypothesis that asexuality is rare because sex is required to prevent the accumulation of harmful mutations. They used the DNA sequence data of sexual and asexual, varieties of the New Zealand freshwater snail, Potamopyrgus antipodarum, to address these questions.

The scientists sequenced mitochondrial genomes and found that the sexually reproducing snails had accumulated harmful DNA mutations at about half the rate of the asexual snails.

"This is the first study to compare mutation accumulation in a species where sexual individuals and asexual individuals regularly coexist, and thus provides the most direct evidence to date that sex helps to counter the accumulation of harmful mutations," said Neiman.

Neiman plans to continue her evolutionary biology research such that a clearer understanding of the advantages of sex will offer a better understanding of the value of preserving genetic diversity within and among populations, species, and ecological communities.

CitationAccelerated Mutation Accumulation in Asexual Lineages of a Freshwater Snail. Maurine Neiman, Gery Hehman, Joseph T. Miller, John M. Logsdon, Jr. and Douglas R. Taylor. Molecular Biology and Evolution 2009; ePub ahead of print. doi:10.1093/molbev/msp300

Abstract

Sexual reproduction is both extremely costly and widespread relative to asexual reproduction, meaning that it must also confer profound advantages in order to persist. One theorized benefit of sex is that it facilitates the clearance of harmful mutations, which would accumulate more rapidly in the absence of recombination. The extent to which ineffective purifying selection and mutation accumulation are direct consequences of asexuality, and whether the accelerated buildup of harmful mutations in asexuals can occur rapidly enough to maintain sex within natural populations, however, remain as open questions. We addressed key components of these questions by estimating the rate of mutation accumulation in the mitochondrial genomes of multiple sexual and asexual representatives of Potamopyrgus antipodarum, a New Zealand snail characterized by mixed sexual/asexual populations. We found that increased mutation accumulation is associated with asexuality and occurs rapidly enough to be detected in recently-derived asexual lineages of P. antipodarum. Our results demonstrate that increased mutation accumulation in asexuals can differentially affect coexisting and ecologically similar sexual and asexual lineages. The accelerated rate of mutation accumulation observed in asexual P. antipodarum provides some of the most direct evidence to date for a link between asexuality and mutation accumulation, and implies that mutational buildup could be rapid enough to contribute to the short-term evolutionary mechanisms that favor sexual reproduction.

Keywords:sex, asexual, parthenogenetic, Muller's ratchet, mtDNA, Hill-Robertson, potamopyrgus antipodarum.

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