As I mentioned in my last post, obligately parthenogenetic (asexual) Daphnia produce males that are functional but functionless to the females that produce them. Their functionality was suggested by one simple test and then proven by another much more complicated. Both of these tests are interesting stories.
Our first question was whether superfluous males still produced sperm. That was easily determined by squashing a few males and looking for sperm. This proved a good lesson to me. Not every animal has sperm that looks like what we learn about in biology class. They come in many shapes and sizes depending on the species. My favorite example is from another pond animal, the ostracod, known as seed shrimp (the term “shrimp” is used for all sorts of small, non-shrimp animals, like fairy shrimp and tadpole shrimp). Some ostracod species have sperm six times longer than their body. The sperm is coiled inside the male and when they were first observed it was assumed the sperm were parasites. As a graphic illustration, this is equivalent to a human male producing sperm 36 feet long. I’ll leave the rest of imagery to you.
The sperm of Daphnia are amoeboid. With no flagella to propel them, there are problems that have to be overcome. But I digress and I’ll get back to the mating habits of Daphnia and other pond dwelling animals later. For now, I’ll just say that we found sperm in the males. That leaves the question of whether they were functional.
My fellow post-doc Dave Innes and I did a series of experiments to determine if the males could still do their job. The test was seemingly simple. We took males produced by obligate asexuals and had them mate with cyclically asexual females that could use their sperm. Sounds easy enough, but there is a reason pioneering geneticists used fruit flies instead of Daphnia to do all their experiments 60 – 70 years ago. The individuals have to be induced to mate and then the resting eggs have to be induced to hatch. The first step is only a little tricky but the second step was a major stumbling block not reliably resolved until the mid 1980’s by the Hebert lab.
Using these techniques we discovered something about the males and something about obligate asexuals. Firstly, the males were functional. A male and a female homozygous (both alleles the same) for two different alleles of the same gene produced offspring that were heterozygous (two different alleles). The only way to have this result is if the males did their job. That’s a neat result.
The other result was much neater. We raised these heterozygous offspring, allowed them to produce resting eggs on their own, and then hatched the resting eggs. Here’s the cool part. All of these new offspring were heterozygous. That doesn’t happen when there is normal sex going on. The best explanation is if the female parents were obligately asexual. The male carried a gene that imparted obligate asexuality on their children.
The implication for natural populations is this: an obligately asexual female introduced into a pond with cyclically asexual individuals can “infect” the new population with this gene. Over some long period of time the population will be dominated by obligate asexuals and likely just a few, best adapted clones. That this has happened in only a small portion of the distribution of Daphnia is another story related to my others about the lack of dispersal in pond dwelling invertebrates
More soon.
