Friday, 14 March 2014
Bdelloid rotifers are microscopic invertebrates living in fresh water and moist soil. As I recall, they are commonly observed in soil samples (with paramecia, rotifers are what I remember most from my high school classes involving a microscope). They travel around by swimming or by crawling in a leech-like way, using their head and foot in an alternative step. ("Bdelloid" means "Leech-like").
Their most remarkable characteristic is that they have been reproducing in an asexual way for millions of years. There are no male bdelloid rotifers; only females whose eggs are produced by mitosis instead of meiosis. Genetic analysis of the roughly 360 parthenogenetic species of bdelloid rotifers suggest that their most recent common ancestor lived around 80 million years ago.
Why is that interesting? After all, parthenogenesis is common in the living world. Bacteria reproduce asexually. Yeast can reproduce either sexually or asexually. Plants can be spread by apomixis as well as by sexual reproduction. Even among animals, where sexual reproduction is the norm, we know of cases of "virginal births". For example, turkeys have given birth to babies without fertilization (all males, because in birds the sexual chromosomes ZW give females and ZZ to males, and the eggs involved here were the result of haploid eggs doubling their genetic content -the WW combination being lethal). Komodo dragons have given birth to babies without mating. Sharks were born to partner-less mothers in aquarium located in Nebraska and Virgina. We know of strains of the brine shrimp Artemia that have been parthenogenetic for quite along while. And the famous lizard Cnemidophorus neomexicanus is only one of several parthenogenetic lines of lizards where only females are present. So what's the big deal about bdelloidae? Well,it,s really the 80 miilion years figure.
From what we can tell, a general rule regarding the development of parthenogenesis in animals is that it's bad news in the long term. We came to that conclusion by observing that parthenogenetic animal lines die out sooner or later, but sooner more often than later. That is understandable from different view points: naturally, a parthenogenetic line has little allelic variation (only that conferred by individual mutations carried down a specific sub-line), and so the chances of developing individuals better adapted to this or that change in selective pressures is smaller than in a sexual species. Furthermore, with no meiotic recombination, each chromosome (even if they are paired) is on its own when it accrues point mutations, deletions or insertions; it is a victim of Muller's ratchet. (It is actually quite interesting to observe how the two chromosomes in each pair are growing increasingly different from their partner)(1)!
So, the Bdelloidae are really oddballs when it comes to parthenogenesis, having not only survived 80 million years but showing no apparent distress. A possible explanation for their amazing success in compensating for the sex-derived genetic variation that we all take for granted is that bdelloidae are amazingly good at appropriating foreign DNA sequences, even that of their bacterial food. They apparently do so when repairing their own damaged cells, and this massive horizontal gene transfer could very well compensate for what is lost with the abandonned meiosis.
As for purity rings, they are cultural artifacts indicating that the wearer intends to remain chaste until their marriage. They were made popular in the 1990s among American conservative-minded groups who promoted abstinence for teens.
(1) Dawkins, R. The ancestor's tale, Mariner Books, New York, 2004, page 429.