I don’t want to get mired in the genetics of Daphnia (although fascinating) but rather move on to other topics about vernal pools. But before I do, I want to finish a few last thoughts on invertebrate sex. That means a few words on males, where they come from, and how they do what they do. This is not as obvious as it may seem.
Firstly, what is the source of male Daphnia? I’ve already mentioned that females produce clones by generating eggs that are identical to the mother. That’s the whole point of parthenogenesis. Most animals use chromosomes to determine gender. There are many variations on the theme, but most people remember from high school biology that humans females have two X chromosomes (XX) and males an X and a Y (XY) with the genes on the Y chromosome making men what they are. This is called chromosomal sex determination. As female Daphnia make identical copies of themselves, there is no way to use this method. So, as I began the paragraph, where do male Daphnia come from?
Cladocerans, of which Daphnia is one, have evolved a different mechanism, called environmental sex determination. There are no sex chromosomes but rather conditions in the environment trigger changes in the expression of DNA so eggs develop into males instead of females. Male Daphnia are distinct from females in look as well as behavior, but they are genetically identical to their mothers.
What triggers the changes in how the genome is expressed? That leads to my favorite answer to all questions: it depends. The best answer is stress and you should now ask, “what sort of stress?” The answers get more and more murky as I tell you that stress can be not enough food, the wrong kind of food, not enough oxygen, too much Daphnia waste, or all the biochemical changes that come from being too hot or too cold.
This explanation is based on the observation that cladoceran males are rarely seen early in the season, when there is plenty of food and oxygen in the cool waters of vernal ponds. Males are absent until the pond is crowded and then they become relatively common, approaching 50% of the population just as in sexual species. No stress, no males; lots of stress, lots of males. How much stress is needed? Some. I don’t feely badly in not knowing the answer because even cladocerans don’t know. Depending on the degree of stress, Daphnia respond by producing a continuum of gender confused offspring that range from mostly female all the way to true males. These intersex individuals may produce eggs or sperm depending on how far along they are in the continuum.
The other neat observation is something I have mentioned before, the hatching of males from resting eggs in Daphnia ephemeralis. I believe this rare example is the result of the fact that this species only tolerates cold water (less than 50F) and has such a narrow time period in the ponds that there is isn’t sufficient time to produce a few generations of females before producing males. Producing males from the start ensures that mates are available if there is a heat wave that shortens the life of the pond, at least as D. ephemeralis experiences it.
Tomorrow: sex in the pond.
