Issue 1

Asexual reproduction, the ability to reproduce without a mate is something we would associate with many invertebrate and plant species. But, although rare, some vertebrate species such as birds, sharks, lizards and particularly snakes, are capable of ‘switching’ their normal mating behaviour in favour of reproduction with no male input. This process is called facultative parthenogenesis and involves the mother contributing all the genetic information required for offspring development. Until recently such a shift in reproductive behaviour had only been observed in captivity; however, in September 2012 exiting new reports were made on the first documented case of vertebrate parthenogenesis in the wild. This evidence is key for the conservation of many reptile species as it suggests that small amounts of asexual reproduction in natural vertebrate populations are viable without any highly adverse effects. This can be used to increase species number if the genetic variation in these populations is monitored correctly.

Sex determination in most animals, including humans, is controlled by the allocation of X and Y chromosomes. These tightly-wound segments of DNA code for sexual characteristics; with females carrying two X chromosomes, and males carrying one of each. When human gametes fuse, the mother always contributes an X chromosome and therefore it is the father’s sperm that determines offspring sex.  However, snake gender is instead controlled by Z and W chromosomes, and unlike humans, it is the females that is heterozygous.  The males are homozygous for the Z chromosome and therefore they do not contribute to the gender of their offspring. Fundamentally, it is this system that allows the female to produce litters of mixed gender sibling via facultative parthenogenesis.  It is still uncertain what encourages females to undergo this process, but when they do, the production of an embryo occurs via a system known as ‘Terminal fusion automixis’. During which, instead of an egg and sperm cell fusing, two abnormally small egg cells fuse together and develop in the same way as a normal embryo. This not only leads to the possibility of female clones (with a WZ genotype), but male-half clone (ZZ genotype) and ‘Super-Females’ with a WW genotype.

Many reports have been made of parthenogenesis occurring in captive snakes. For instance, a female Colombian rainbow boa (Epicrate maurus) isolated from males 3 days after her birth in 1987, bore a litter of 6 live, 2 deformed and 2 stillborn neonates (young snakes) in 2006 and 3 stillborn neonates the year after. The maximum period of time for sperm storage in snakes is recorded at only 7 years, suggesting facultative parthenogenesis to be the only viable possibility.

It has been suggested that it is isolation from males that’s causes captive females to reproduce asexually. However, a virgin birth by a Common Boa (Boa constrictor imperator) kept in captivity with four males has caused cracks to form in this theory. The unusual characteristics of her litter gave rise to suspicions that the offspring were asexually conceived, suspicions later confirmed by satellite DNA fingerprinting. All offspring were female and exhibited an unusual caramel colour (the same rare recessive phenotype as their mother). The four males she was housed with were not believed to be heterozygous for this trait and the chances of these exact phenotypes randomly occurring in each individual made long-term sperm storage seem highly unlikely. This left parthenogenesis as the only viable hypothesis.

The widespread occurrence of facultative parthenogenesis in captive reptile species has led scientists to believe that it could possibly be a natural phenomenon, and in late 2012, evidence was finally found to support this theory. A team of researchers investigating facultative parthenogenesis in wild snake species collected 22 pregnant Copperhead Vipers (Agkistrodon contortrix) and 37 Cottonmouth Vipers (Agkistrodon piscivorus). One litter born from each species of Viper were molecularly analysed after showing signs they were produced asexually and found to be offspring of a single parent.

Facultative parthenogenesis is important to the study of both the past and future. Not only does it suggest new theories on evolution, it provides insight on the conservation of endangered species, such as the Komodo dragon (Varanus komodoensis), that are capable of such reproduction. It also warns of the risks associated with accumulative deletion mutations that can be cause by a lack of genetic recombination.  Many questions still remain about what triggers facultative parthenogenesis, but we can be sure it will play a key part in the conservation of many endangered reptiles. If it is a lack of mates that causes females to reproduce asexually, we may start to see a decline in recombinants before a species is even recognised as endangered. This shrinking of the gene pool could influence the effectiveness of environmental adaption in snakes, and with our now rapidly changing biosphere, this really is an alarming prospect for our scaly friends.