Evolution without sex? No problem for rotifers
Washington: Researchers have discovered how a group of animals can abandon sex, and still produce more than 460 species over evolutionary time.
Rather than the standard way of using sexual reproduction to weed out harmful mutations to its DNA, tiny aquatic animals called bdelloid rotifer (Adineta vaga) appear to have adopted other strategies to maintain lineages over millennia that aren`t burdened by genetic damage or killed off altogether, David Mark Welch of the Marine Biological Laboratory (MBL) in Woods Hole, said.
Mark Welch and his MBL colleague, Irina Arkhipova, are the US. leads on the international project to sequence the rotifer genome and analyze what it reveals.
Neither males nor meiosis (cell division to produce sperm or eggs) have ever been observed in a bdelloid rotifer.
Instead, the unfertilized eggs just divide to produce offspring.
This reproductive strategy, which for most animals would be an evolutionary dead end, is borne out by the rotifer`s genome, the structure of which "is completely consistent with what you would expect to see with a long-term absence of meiosis," Welch said.
"It`s hard to prove a negative, and we can never say there is no chance the rotifer is ever having sex. But it would have to be some kind of crazy meiosis," he said.
In most animal species, alternative forms of the same gene (alleles) are found in the same spot on two different chromosomes-one from the mother, one from the father-which pair during meiosis, and segregate into new sperm and egg cells.
In the bdelloid genome, gene copies either don`t match up positionally along chromosome pairs or are located on the same chromosome.
This means the alleles would not be able to pair up normally during meiosis and segregate evenly into new sperm and egg cells.
If bdelloids are not having sex, how can they avoid the accumulation of deleterious mutations or generate new diversity?
The bdelloid genome shows evidence for other ways of maintaining healthy genes and viable lineages.
One is gene conversion, in which one allele replaces another through DNA repair mechanisms or other strategies.
The other is horizontal gene transfer (HGT), the transfer of DNA from one organism to another, which is common among microbes yet rarely seen in animals.
At least 8 percent of the rotifer`s genes, more than in any other animal, are likely to have been acquired by HGT.
The findings are published in the journal Nature.
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