Parthenogenesis in Livebearing Snakes...Explained

by Donald L. Blanchard

This article was originally published in The Cold Blooded News, the newsletter of the Colorado Herpetological Society, Vol 25, #11, November,1998.

The following report is derived in part from a presentation given to the Tucson Herpetological Society by Gordon W. Schuett, Department of Life Sciences, Arizona State University West, Phoenix, Arizona, on September 16, 1997, as reported in The Sonoran Herpetologist, the Newsletter of the THS, Vol.11, No.9, September, 1998.

Parthenogenesis, reproduction without fertilization by a male, has been reported in only a few different groups of reptiles, most notably New World lizards of the genus Cnemidophorus (family Teiidae), Old World lizards of the genus Lacerta (family Lacertidae), and the Brahminy blind snake (Ramphotyphlops braminus; family Typhlopidae). In all of these cases, the populations are composed entirely, or almost entirely, of genetically identical female individuals, or clones. Thus, it came as quite a surprise when Dr. David Chiszar of the University of Colorado, Boulder, found that a timber rattlesnake (Crotalus horridus) in his lab that had never been with a male since its birth had produced a litter of offspring: one live, two stillborn, and three infertile eggs. More surprising was the fact that the live and stillborn offspring were all males. Clearly, this parthenogenetic reproduction was different than that practiced by the known unisexual reptiles. (This event was reported to the CHS by Dr. Chiszar in January of 1996.)

In October of 1995, Dr. Chiszar reported this odd occurrence by telephone to Gordon W. Schuett, at Arizona State University. Schuett had been studying sperm storage in reptiles, and was faced with a puzzle of his own: a wandering garter snake (Thamnophis elegans vagrans) that had been producing litters, some with viable offspring, in the absence of males for about 10 years. Schuett utilized the now familiar method of DNA fingerprinting to test for any paternal contribution to the offspring. DNA fingerprinting separates the genetic material of an individual, so that it appears as a long series of bands, each corresponding to a particular genetic component in the individual's DNA. Parents are determined by matching shared bands; the more bands that match, the greater the probability that a parent has been found. A perfect match of band-sharing between two individuals (in number and location of bands) indicates a very close genetic relationship, such as found in identical twins or clonal species like the unisexual Cnemidophorus.

Schuett found that the mother garter snake had up to twice as many bands as the offspring, not an anticipated result, but that those bands possessed by the offspring matched almost perfectly with those of the mother, indicating that all the DNA in the offspring came from the mother, but that not all the mother's DNA was present in the offspring. Thus, this was a true parthenogenesis, with no male contribution (which would have provided genetic material to the offspring that was different from that provided by the mother). But the missing DNA from the mother, coupled with the fact that all the offspring were males, lead Schuett to the conclusion that the reproduction observed was a form known as automictic parthenogenesis (AP). AP had been previously described in domestic turkeys and chickens, and the offspring produced are all diploid males. (Diploid is the normal condition for sexual reproduction, indicating that all chromosomes occur in pairs.)

To understand automictic parthenogenesis, one must first understand meiosis, the process by which sex cells (eggs and sperm) are produced. In normal cell division, or mitosis, every chromosome is first duplicated, then one copy of every chromosome is drawn to each end of the cell. Then, when the cell divides, each daughter contains exactly the same genetic compliment as the parent cell. Meiosis, on the other hand, is a two stage process, ultimately producing four cells. In the first division, the chromosomes are not duplicated; rather the paired chromosomes line up together (remember, all chromosomes are paired in diploid cells) and one of every pair is drawn to each end of the cell. The daughter cells now contain only half the genetic material of the parent cells (one chromosome from each pair), and are called haploid cells. The second stage of meiosis proceeds similarly to normal cell division, with each chromosome being duplicated before division occurs. Thus meiosis produces four haploid cells, two of them containing one half of the parent's original DNA, and the other two the remaining half. In the female, three of these four cells contain the genetic material and little else; the majority of the cytoplasm, or cell fluids, is retained by the fourth cell, which becomes the egg. The other three cells, called polar bodies, are generally reabsorbed into the female's body.

In AP, the second polar body -- the daughter cell produced along with the egg in the second stage of meiosis -- acts like a sperm and re-enters the egg. Essentially the egg fertilizes itself! As this polar body contains identical genetic material to the egg, having been produced by normal division of an already haploid cell, the resulting diploid cell has only half the genetic diversity of the female's original cells. Thus fewer bands appear in the DNA fingerprint of the offspring.

So why are all the viable offspring males? In most mammals, sex is determined by the X and Y chromosomes, two of which constitute a pair. A pair of X chromosomes and the individual is female; one X and one Y and the individual is male. (As the mother has only X chromosomes -- generally, a YY combination isn't possible, and wouldn't be viable if it were.) In those diapsids (including birds, lizards, and snakes) where AP has been described, females have dissimilar sex chromosomes (ZW), while males have two copies of the same chromosome (ZZ). In AP, if the egg and the second polar body each contain a Z chromosome, when they are combined, a male offspring will be produced. If they both contain a W chromosome (a 50% chance), the egg will be non-viable (WW). This is what causes the high proportion of infertile eggs in AP parthenogenesis.