Growing up in a vernal pond is about more than just avoiding being eaten. Once a cladoceran reaches maturity it starts to produce daughters. It does this through a process called parthenogenesis. The ability to produce offspring without a mate is one of the physiological features that unite cladocerans. It is not all that uncommon among invertebrates but somewhat rare (but not unknown) in vertebrates. There was an article in the paper last week about a shark having a virgin birth and this was considered news. Cladocerans do it daily by producing eggs through the cell division process of mitosis. This is the same process that you probably learned about in high school and describes how organisms achieve being multicellular.
The process of mitosis ensures that genetically all of our cells are identical. Differential gene expression allows our cells to become spleen, skin, muscles and nerves. As different as these are they all have the identical DNA. The fact that you are alive and able to read what I am writing is a testament to how the outstanding quality of your DNA. Congratulations.
The difficulty is what happens when we reproduce. So that our offspring have only as much DNA as we do, our eggs or sperm go through cell divisions that halve the amount of DNA they carry, the process of meiosis. When the sperm and egg unite, the new individual has a full complement of DNA. The new combination is your offspring that carries half of your DNA and half from your spouse. But your combination works, at least well enough for you to have grown up and be able to reproduce. The evolutionary question is, why ruin a combination that works? I don’t want to get to far a field from vernal ponds so I’ll just say that new combinations are good for all sorts of reasons but aren’t always necessary. Just most of the time.
Part of evolutionary race of which we are all part is about getting copies of your DNA into the next generation. The “survival of the fittest” is about who gets the most copies of their DNA into subsequent generations, not about who can do the most push ups. That is why you need at least two offspring, each carrying half of your DNA, so there is one complete copy (in two halves) that carries on your tradition. This loss of a good thing, your DNA that clearly works, is part of what is referred to as the cost of meiosis.
Imagine if each your offspring had a complete copy of your entire DNA instead of half. You would be the grand prizewinner of the DNA sweepstakes by getting multiple, complete copies of your genome into the next generation. That is exactly what cladocerans have figured out how to do. They produce eggs by mitosis, the same process used to produce all the other body cells. Each egg has the complete genome of the mother undiluted by the DNA of a mate. One of the consequences of this type of reproduction is that when you collect individuals from a population it is likely that you are collecting grandmothers, mothers and daughters that are identical to one another. All these individuals (and they are distinct individuals) that are genetically identical are called genets.
Learning to distinguish genets led to all sorts of findings important to our understanding of the connectivity, or lack thereof, among vernal ponds. More on this to follow.
