While collecting material for my PhD research I had the great good luck to meet Paul Hebert from the University of Windsor. He was already on the way to becoming a pre-eminent cladoceran geneticist (although when I spoke to him last week he said he hadn’t looked at one in years). His lab unraveled many of the mysteries of Daphnia population genetics and most of my stories are those I learned while a post-doc in his lab. All sorts of facets were examined to match his varied interests and those of his students and post-docs.
About two years into my stay in Windsor it was decided to do a region wide survey of all the Daphnia populations in the Windsor area. This was no small task as the area is riddled with ponds. We were aided by the fact that over 90% of the land in nearby counties had been converted to agriculture and the ponds drained and filled. That still left many ponds but was a sad reminder of what can happen when the land is “improved.”
This survey was spurred by earlier research in his lab on Daphnia population structure. The results of the earlier work were startling. The typical parthenogenetic Daphnia life cycle was not at all typical around Windsor. The old story was that ephippia hatched, female juveniles emerged, grew up, reproduced asexually, and then when stressed produced males, had sex and produced more ephippia, at which point the pond was done for the season. By having a bout of sex at the end of the season genes were mixed like they would in any sexual species. And this is the case in many Daphnia populations so there are literally hundreds or thousands of clones in most ponds.
But for whatever reason the Daphnia of Windsor were different. All the steps of population development were the same but the results were markedly different. Any one pond had only a small handful of clones, as few as one or two, sometimes with a few more, but rarely ever approaching the numbers one would expect. For that matter, we found the same set of clones in a many of the ponds. This is not all what we should have found.
Briefly, all the developmental steps were followed but at one point the system had changed. I detailed before that Daphnia produce their regular eggs by mitosis, which allows them to produce exact copies of themselves, clones. To have sex at the end of the season females switch and produce males, also by mitosis, and then their eggs by meiosis to be fertilized by males. This last step was broken in Windsor. Females found a way to produce ALL their eggs by mitosis. Males were still produced but superfluous. The males were normal and even mated, but the sperm weren’t used. We later found the males were functional (their sperm worked) but functionless.
With this scenario, a few best-adapted clones could dominate a pond and this is exactly what happened. There are a number of interesting evolutionary implications as well as providing an opportunity to explore the movement of clones between habitats. This is critical knowledge because it tells us about the importance of individual ponds in maintaining the genetic integrity of the species.
I’ll pick up with these ideas in my next post. I hope they have whetted and wetted your curiosity.
