It has been a few weeks since my last post and it is time to pick up my pen (or laptop) and continue my story. When I last wrote, I was describing the life cycle of Daphnia. I described the normal life cycle of cyclic parthenogenesis, asexuality followed by “normal” sex, and the rare condition called obligate parthenogenesis in which males have no role. Although there are many facets to the male part of the story, for example, where do males come from, how do they find mates, and how do they mate, I thought I’d stick with the story of the obligate parthenogens because there are interesting results that transcend simple population genetics.
One of my interests has long been the distribution of pond dwelling invertebrate species. I have discussed previously there is distribution of species in time and space. My initial, naïve, view of the pond world, that many species are broadly distributed, was shown to be wildly wrong once I understood the differences in parthenogenetic styles of Daphnia populations. A result that has been shown to me repeatedly over the years is that appearances are deceiving and scale is much more important than we realize.
In 1970 it was thought that Daphnia pulex was widespread across much of North America; not so much in the southern part of the U.S. but pretty much everywhere else. By 1980 Paul Hebert’s lab showed that some D. pulex populations reproduced by obligate parthenogenesis. Further worked revealed that not all the asexual clones were the equally widespread. A few clones were relatively common, at least around Windsor (but only on the Canadian side and not in Michigan which is a whole other story), but the majority weren’t found in many places.
The implication of this result is important for conservation biologists and those interested in preserving vernal habitats. Although D. pulex has the outward appearance of living in many ponds, the populations are actually genetically distinct. The only explanation is that Daphnia, as a convenient marker species, doesn’t get around much. It appears in many ponds only because there has been a long period of time for them to colonize habitats. We would have a hard time recognizing this without the convenience of obligate parthenogens.
Further evidence for the lack of mobility came from the study of a series of ponds separated by a few meters. All had Daphnia, but each pond had geneticslly distinct clonal assemblages. The explanation I offer is ponds were successfully colonized by one, or a few, Daphnia. If this is the case for Daphnia, it is likely the case for other pond invertebrates as well.
Let me repeat because this is so important: a broad distribution of a species does NOT mean they have high vagility (a fancy word for ease of getting around). Individual vernal ponds are much more important than they might appear. If I were interested in preserving a habitat I would want to be able to show how unique it is. And just because it has some of the same species as some other ponds does not mean that there is much immigration going on. Every pond is likely a unique island regardless of how similar the species composition appears. All this revealed because of how Daphnia reproduces.
I’ll provide some other examples of this lack of immigration before getting back to Daphnia sex tomorrow.
You should really turn this series, and the future postings, into a book!