Off peak

Precisely this pattern is what Welch and Meselson found recently when they took a closer look at more DNA surrounding the hsp82 locus in bdelloids — the same gene that was initially assumed to harbour divergent 'Meselson effect' alleles. It turns out that the regions around the four copies of the hsp82 gene do come in two pairs. Between them, the pairs have four genes in common and the order and orientation of these genes is the same in all four regions. This is a clear indication that this region must be the remnant of an ancient regional or global duplication event. Most of the other genes in the four hsp82 regions are only found in one or the other pair and probably reflect site-specific insertions or deletions, but, within each pair, gene order and orientation is the same, so in a way they look just like chromosome pairs of sexual diploids that engage in meiosis. The nucleotide divergence within one pair varies from 0–20% and is on average about 7%, which is still higher than the average observed in other animals. This indicates that the bdelloids were perhaps originally tetraploid and that the divergence after the duplication may account for the Meselson effect. So does this mean that the collinear chromosome pairs observed are simple meiotic homologues? If not, how come they have not diverged much more as predicted by the Meselson effect? There must be other, non-meiotic homogenisers at work, clearly. Sex and the lifestyle The bdelloids' unusual lifestyle may provide an explanation for both the Meselson effect and the excess of foreign genes, as well as perhaps for why they are asexual. When you pick up some dry moss and put it in water for a while, chances are you will find bdelloid rotifers. Their near ubiquity in aquatic habitats, even ephemeral ones that dry out entirely, is owed to the fact that bdelloids are particularly hardy when it comes to desiccation. In that piece of moss, you're likely to find another type of microscopic animal, a little water bear or tardigrade, member of a related phylum, which is also highly resistant to drought, and some groups of which also reproduce asexually. This ability to survive waterlessness is a unique selling point in ecological terms because it allows bdelloids and tardigrades to colonise habitats that are not readily accessible for other animals, which means there will be …