As discussed in the Biology and Society section, some species can reproduce via both sexual and asexual routes. But although some animal species can also reproduce asexually, very few animals reproduce only asexually. In fact, evolutionary biologists have traditionally considered asexual reproduction an evolutionary dead end (for reasons we’ll discuss in the Evolution Connection section at the end of the chapter). To investigate a case where asexual reproduction seemed to be the norm, researchers from Harvard University studied a group of animals called bdelloid rotifers Picture given on top . This class of nearly microscopic freshwater invertebrates includes more than 300 known species.Despite hundreds of years of observations , no one had ever found bdelloid rotifer males or evidence of sexual reproduction. But the possibility remained that bdelloids had sex very infrequently or that the males were impossible to recognize by appearance. Thus, the Harvard research team posed the question , Does this entire class of animals reproduce solely by asexual means?
The researchers formed the hypothesis that bdelloid rotifers have indeed thrived for millions of years despite a lack of sexual reproduction. But how to test it? In most species, the two versions of a gene in a pair of homologous chromosomes are very similar due to the constant trading of genes during sexual reproduction. If a species has survived without sex for millions of years, the researchers reasoned, then changes in the DNA sequences of homologous genes should accumulate independently, and the two versions of the genes should have significantly diverged from each other over time. This led to the prediction that bdelloid rotifers would display much more variation in their pairs of homologous genes than most organisms. In a simple but elegant experiment , the researchers compared the sequences of a particular gene in bdelloid and non-bdelloid rotifers. Their results were striking.
Among non-bdelloid rotifers that reproduce sexually,the two homologous versions of the gene were nearly identical, differing by only 0.5% on average. In contrast, the two versions of the same gene in bdelloid rotifers differed by 3.5–54%. These data provided strong evidence that bdelloid rotifers have evolved for millions of years without any sexual reproduction.
Source- Book by Campbell biology