Parthenogenesis in snakes

[Warren_Booth]

Absolutely, I am more than happy to.

I started working on parthenogenesis in snakes in 2010, with a case in Boa constrictor presented to me while I was a post-doctoral researcher at NCSU. The resulting study went viral, and was picked up by hundreds of news organizations across the globe. This opened a door to numerous new cases being presented, and as a result, my lab at The University of Tulsa actively researchers parthenogenesis; primarily though undergraduate research projects, but also with visiting researchers. We just embarked on three massively significant studies this year and should be presenting our findings very soon.

So, due to the number of cases we now have (around 20, however we have since added three additional species since this paper was published at Christmas), that span both the "ancient" and "advanced" snakes (boas/pythons, and pitvipers, watersnakes, etc, respectively), we can now start to look at the characteristics of parthenogenesis across the phylogenetic tree of snakes. What we find is very interesting.

A variety of characteristics are noted:

1) Parthenogenetic mode - In general we find that cases of parthenogenesis are reported in species that are normal sexually reproducing. This transition to producing asexually (parthenogenetically) at some point is known as facultative parthenogenesis. We know of one species of snakes that is obligately parthenogenetic - The Brahminy blind snake (Indotyphlops braminus). This species lacks males, and is thus unisexual.

2) Ploidy - this relates to the number of chromosome sets an organism has. Humans, for example are diploid - inheriting a set of chromosomes from their mother, and then a complimentary set from their father. All snakes are known to be diploid also, with the exception of the Brahminy blind snake that is triploid. Thus, it has inherited two sets from one parent, and one set from the other. It has not been confirmed yet, but we believe that this may result from it being a species of hybrid origin. I.e. two closely related species bred and produced offspring that where triploid and obligately parthenogenetic. We have seen this in many lizard species that also exhibit obligate parthenogenesis and lack males.

3) Mode of parity - this relates to being an egg layer, or live bearer. We have seen facultative parthenogenesis in live bearers and egg layers, however in the former we find the most species exhibiting this behavior. We have not yet found it in species that are egg layers withing a primarily live bearing lineage (e.g. Bushmasters are egg layers, but the majority of other pitvipers are live bearers), or visa versa.

4) Sex chromosome morphology. This relates to whether differentiation occurs in the size of the sex chromosomes. In many species (snakes included) size variation is noted between the sex chromosomes, be they X and Y, or Z and W. For example, the Y chromosome of mammals is very reduced in relation to the X. This is thought to result from the lose of non-essential genes and due to a lack of recombination between sex chromosomes (the tend not to exchange genes, whereas non-sex chromosomes do - known as crossing-over - generates genetic variation in offspring compared to the parents and siblings). In ancient snakes the sex chromosomes are actually the same size and can not be distinguished easily. They also recombine (cross-over), which is very interesting. In advanced snakes, they exhibit size variation (known of heteromorphic), and these do not recombine. We find that in species with homomorphic sex chromosomes (ancient snakes), the parthenogens are females, whereas in those with heteromorphic sex chromosomes, only males are produced.

5) The sex of parthenogens produced - As mentioned above, we find that ancient snakes produce females, but advanced snakes produce males. Regardless of this, the same mechanism is used - automictic parthenogenesis. This is were the egg nucleus fuses with a polar body (when sex cells are produced, 4 daughter cells are made. 3 of these are considered non-viable, however they do contain a set of chromosomes. In the 2nd polar body, these are identical, or near identical, to the egg nucleus. This results in an offspring the is essentially homozygous across its genome. Think of it as being as inbred as you can be. This has issues - being highly inbred means numerous mildly deleterious mutations are expressed. Explained below.

6) Viability of the parthenogens - Due to be homozygous across the genome,and due to the accumulation of deleterious mutations, many offspring are deformed, or still born. We frequently see cranio-facial deformaties, spinal kinking, loss of features (e.g. no eyes). This is very common in the advanced snakes, whereas in the ancient snakes most parthenogens are born relatively healthy. This changes over time and many succumb to various illness within two years and die. Not all die however. Some do thrive.

So, with all of this in mind, and specifically due to the production of only males in the advanced snakes, and females in the basal snakes, along with some new data we have, we actually believe that ancient snakes may have XY chromosomes, and advanced snakes ZW. We now have targeted genomic data that is specifically addressing it. Previous studies (e.g. Vicosa et al. PLos One. 2013) failed to provide evidence for this however. We will report on this soon.

Altogether, facultative parthenogenesis is widespread across the snake phylogeny. In many lineages it is common - e.g. boids, pythonids, natracines, and pitvipers. We have multiple occurrences within many species, so it is not rare, and we have found it in the wild.

Our next studies related specifically to the viability of these parthenogens, whether they can reproduce (and the evolutionary significance of this), and sex chromosome evolution.

Hope this helps,

Warren