BDELLOID ROTIFERS: SIXTY MILLION YEARS WITHOUT SEX
Most living things reproduce by sex. It may not be the coupled intimacy we relate to when we read romance novels, because we do not usually imagine sex in bacteria, fungi or similar microscopic organisms. But sex is the rule and two strains of a species of bacteria can couple and swap genes or two fungal cells can fuse and form a new cell. What is rare among living plants and animals are species that have no known sexual activity
How does one show that sexual activity is absent in a species? It’s easy if there is one sex that lays eggs and no males are known to exist. We describe such species as reproducing by parthenogenesis (virgin birth). Even in such species there may be special occasions when males are produced and sex takes place or where a species is self-fertilizing.
There is one group of animals that have never given birth to males and show no evidence of self-fertilizing. They are called Bdelloid rotifers. They are multicellular microscopic animals. What makes them unusual is that they lost their sexuality about 60 million years ago. They were at the time unusual in having four of each chromosome instead of two. We and most animals and plants have only two of each chromosome represented in our body cells.
Matthew Meselson, whose laboratory studies rotifers at Harvard, told me that the story is by no means complete and he and his students are still following many leads to work out the past history of these sexless creatures. One unusual feature is that they gave up a process called meiosis which reduces the chromosome number in half for the production of eggs or sperm (in our own eggs or sperm there are 23 chromosomes instead of the 46 found in our body cells). Another lead they are following is their use of chunks of DNA from the food they eat which become incorporated in some of their cells.
The absence of meiosis means the eggs produced in rotifers have the same chromosome number as their body cells. Their chromosomes do not pair while eggs are formed and while some of their genes show 4 copies, considerable numbers of other genes have 1, 2, or 3 copies. Whatever the number present, no genes have identical sequences unlike our own genes where, with few exceptions, there are two of each kind of gene having identical sequences. This demonstrates that since the dinosaurs died out the rotifers have slowly been battered by newly arising gene mutations and what were once four sets of chromosomes is becoming one single set with approximately four times the DNA of its original set of genes in its sexual past.
Rotifers compensate for sex by their immense numbers. They are found on every continent and they are carried on the feet of birds and tag along in the scales of fish. They remind me of patchwork quilts, reshaping their genes as a quilter deftly converts pieces of old dresses into a repetitive design, both pleasing and functional.