Sexual reproduction is thought to be the most valuable evolutionary lifestyle as it allows for genetic variation. However, there are some species that do not adhere to this lifestyle and have changed our views on reproduction, as they are able to reproduce a-sexually (figure 1) through processes such as parthenogenesis also known as “Virgin birth”. One of the major concerns with this method of reproduction is that there is a limit to the amount of genetic diversity that can be introduced to the genome. One interesting group of organisms that reproduce this way are the bdelloid rotifers.
Figure 1, the difference between sexual and asexual reproduction*
The bdelliod rotifers are a group of zooplankton within a group including over 450 different species. This study addresses some very interesting points about our understanding of a-sexual reproduction. They answer some important questions about how a species can reproduce a-sexually and still preserve double the number of genes compared to humans, approximately 40 000 genes, in ten times smaller genomes. How they keep all these different genes when they only reproduce a-sexually we are still yet to find out.
There is evidence that the bdelliod rotifer Adineta vaga have undergone ameiotic evolution meaning there is no pairing of chromosomes during meiosis, the evidence is based on bdelloid rotifer genomic structure. They have a tetraploid genome (four sets of chromosomes) and when compared to a sexual species there are certain allelic regions that have been completely rearranged. These allelic regions may be found on the same chromosome and sometimes also together with other sexual reproduction associated genes that have been lost in the genome. Therefore, it is unlikely that the pairing of homologous chromosomes can take place and so they seem incompatible in conventional meiosis; this makes them incapable of meiotic pairing. Due to the lack of ability of the chromosomes to pair it seems that there is extensive evidence for gene conversion. Gene conversion seems to be the method that prevents the accumulation of any deleterious mutations, there have been a significant increase in certain gene families to allow for protection against the accumulation of harmful alleles. These may be gene families such as those that are involved in the resistance to oxidation.
There seems to be no real benefits or problems for A. vaga to be reproducing a-sexually. There appears to have been the evolution of mechanisms that inhibit the accumulation of deleterious mutations and thus allow the rotifers to live happy, problem free a-sexual lives.
*figure found at:
http://www.ansci.wisc.edu/jjp1/ansci_repro/misc/project_websites_08/tues/Komodo%20Dragons/what.htm
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