Saturday, December 26, 2015

Parthenogenesis a.k.a. Virgin Birth

Note: This show first aired on December 26, 2015.

Late December is the dark time here in the northern hemisphere, and many world religious and spiritual traditions mark the celestial event, the winter solstice with introspection and a celebration of the return of the light. From here the days only get longer again, until June at least.

A central narrative to one of these coopted solistice worshiping holidays though, has nothing to do with darkness and light, and everything to do with an event that theoretically occurred 9 months before what has become a major American commercial holiday. I’m talking of course about virgin conception and birth, a major article of faith in the Christian tradition. But virgin birth also has biological precedents, though not in humans or any other mammals for that matter.

First lets quickly review what we consider “normal” sexual reproduction. Genetic material from one parent and genetic material from a second parent combines to make an individual with a uniquely mixed set of genes. Each parent gives half of the genes, via the gametes, the sperm and the eggs. Sperm and eggs each carry only half of the genetic material required to make up a new individual. In genetic terminology this is called being haploid, sperm and eggs are both haploid cells, they only have one set of genetic material, all of the other cells in the body have two sets of genetic material, called diploid. Yes there are many species that have MORE than two sets of genetic material in their cells, this is called polyploidy and it is very common in plants, and is most well represented in the animal kingdom by fish. We’re going to focus on typical diploid animals for the purposes of this explanation, though.

In virgin birth, more accurately called parthenogenesis, a new individual is created without the benefit of having genetic material from two different parents. There are two basic ways this can happen. One is that the genetic material in the haploid egg replicates itself changing the egg from haploid to diploid, or with two sets of genetic material. That cell then develops like any fertilized embryo, except that it only has genetic material from the mother, and only one set of genetic material from the mother at that, so it is an exact and complete clone of the mother. The other way is that there is some genetic recombination that occurs either as a result of fusion occurring during meiosis (or gamete production) or via the egg being fertilized by what is called a polar body (which is a cell left over from when the egg was produced). These second methods all lead to an individual that is not a full clone of the mother, but what is called a half clone. Half of the genetic material is exactly the mother’s, while the other half has undergone some degree of recombination depending on the exact mechanics of this type of parthenogenesis, there are actually several routes to this end.

Parthenogenesis does occur in nature, in the animal kingdom alone it is common in invertebrates, and of the vertebrates, amphibians, reptiles, birds and especially fish exhibit this behavior. There are at least 80 species of non mammalian vertebrates that are fully unisexual, meaning there are no males, and reproduction is obligate parthenogenesis. Many more vertebrates demonstrate facultative parthenogenesis, meaning they can reproduce this way, but more typically (we think) reproduce sexually.

So the Christmas story has some biological basis, though parthenogenesis is not documented in mammals, and because mammals have an XX XY sex determination system, and sex is determined by genes carried on the sperm, if mammals could do parthenogenesis, all of the offspring would be female, which certainly adds a twist to the classic Christmas story.

Happy Holidays everyone.


About Komodo dragons (who have exhibited parthenogenesis)

Vrijenhoek, R.C., R.M. Dawley, C.J. Cole, and J.P. Bogart. 1989. A list of the known unisexual vertebrates, pp. 19-23 in: Evolution and Ecology of Unisexual Vertebrates. R.M. Dawley and J.P. Bogart (eds.) Bulletin 466, New York State Museum, Albany, New York