For tens of millions of years, microscopic, multicellular organisms called bdelloid rotifers have been reproducing asexually. Evolutionary theories suggest that it is difficult for an organism to adapt to its changing environment without the ability to create diversity through sex. Now, a group of scientists headed by Alan Tunnacliffe from the University of Cambridge has shown how asexual reproduction could by itself be a mechanism for generating diversity. In particular, by studying two proteins that allow bdelloid rotifers to survive through dry conditions, the scientists found that two copies (or alleles) of a single gene could diverge over time, eventually having different complementary functions. The study, published in the Oct. 12 edition of the journal Science, is the first time this allelic divergence has been demonstrated in asexual animals. The group also highlighted their findings in an online press release at the University of Cambridge web site.

Rotifers are small, usually freshwater, animals about 0.5 mm in length and composed of a few hundred cells. These organisms have specialized organ systems and their name rotifer (or "wheel-bearer" in Latin) derives from the crown of cilia (tail-like structures) that surround their mouth; when rapidly moved, the cilia cause these organisms to whirl around. Bdelloid rotifers, a common variety of rotifers found in freshwater, are known for their remarkable ability to survive for many years in a desiccated state when exposed to dry conditions, eventually resuming normal behavior when water becomes available.

As bdelloid rotifers are asexual, they do not have access to the means for genetic exchange that sexual organisms rely on to survive and adapt in natural environments. Such means lead to the possibility of combining advantageous genes from different organisms while purging disadvantageous genes, thereby occasionally generating organisms with favorable combinations of genes that make them better suited for survival. While other asexual organisms are known, they are thought to become extinct relatively quickly due to their inability to adapt. Thus how rotifers have continued to survive for tens of millions of years remained a mystery to scientists; in the Science paper, a group of scientists seeks to address this question in a species of bdelloid rotifers, Adineta ricciae.

Sexual organisms like humans have two copies (or alleles) for almost all genes in their cells, with each parent contributing one or the other copy. These two nearly (or completely) identical copies have the genetic information to code for two nearly identical proteins. In asexual organisms like rotifers that have two alleles for each gene, the alleles can accumulate mutations (or changes) in the absence of genetic recombination through sexual reproduction. Such mutations can cause these genes to become different or divergent over time. The researchers found that in Adineta ricciae, there are two genes that provide resistance to desiccation, identified through their similarity to the genes coding for the late embryogenesis abundant (LEA) proteins that protect plant seeds from drying. Based on their analysis, the scientists found that these two genes appeared to be former alleles that had diverged over time, now coding for proteins with different functions that protect the animal during dehydration. One of these proteins prevents essential sensitive proteins in the cells from clumping as the animal dries, while the other helps maintain the cell membrane that surrounds each cell.

Professor of Biology at Amherst College, Patrick Williamson, commented on the study, "The evolutionary persistence of these asexual (diploid) rotifers is an important fact to be explained by any theory for the existence of sexual reproduction. Nice to know that natural selection made use of this genetic peculiarity."

Principal investigator of the study, Alan Tunnacliffe, summarized his findings, saying, "We've known for a few years that gene copies that would have the same DNA sequence in sexual creatures can be quite different from each other in asexuals. But this is the first time we've been able to show that these gene copies in asexuals can have different functions."

"It's particularly exciting that we've found different, but complementary, functions in genes which help bdelloid rotifers survive desiccation, he added. Evolution of gene function in this way can't happen in sexual organisms, which means there could be some benefit to millions of years without sex after all."

Photo Credit: Diego Fontaneto; source: Gross L (2007) Who Needs Sex (or Males) Anyway? PLoS Biol 5(4): e99 doi:10.1371/journal.pbio.0050099