Snakes that Give Virgin Birth

Virgin birth (a form of asexual reproduction) has fascinated humans for centuries. Recently, biologists have uncovered many of the mysteries associated with the ability of some animals to produce offspring without ever mating. This phenomenon is common in bacteria, most fungi, many plants, and some invertebrate animals, where it takes many forms. It is relatively uncommon in vertebrates, although a few species of fishes, amphibians, and reptiles reproduce using a form of asexual reproduction called parthenogenesis, which is when an embryo develops from an unfertilized egg cell. Parthenogenesis can be facultative or obligate. Species with facultative parthenogenesis can also reproduce sexually (and usually do), whereas species with obligate parthenogenesis cannot and are usually all-female. Both types of parthenogenesis are found in snakes, and several new examples have been documented in the past few years.

Only one species of snake is known to have obligate parthenogenesis. It is a member of the Scolecophidia, or blindsnakes, called the Brahminy Blindsnake or Flowerpot Snake (Ramphotyphlops braminus). This tiny egg-laying species is made up only of females and is extremely widespread, partially thanks to the ability of just a single individual to colonize new areas. Unlike mammals, most reptiles (and birds) have a ZW chromosomal sex determination system, so instead of the males being XY and the females XX (click here for a review), male snakes are ZZ and female snakes are ZW. However, in common with other obligate parthenogenetic species, Brahminy Blindsnakes are triploid, meaning that they have three sets of chromosomes rather than two. Examination of the karyotype (a picture of one complete set of chromosomes) of Brahminy Blindsnakes has revealed evidence of hybridization, which has also played a role in the origin of other polyploid obligate parthenogenetic vertebrates, including certain lizards, salamanders, and fishes.

Boa constrictor (Boa constrictor)

Facultative parthenogenesis has been documented in a number of species of snakes that normally reproduce sexually. Most of the time this takes place when someone has kept a female snake in captivity for a long period of time. Although it can be difficult to distinguish parthenogenesis from long-term sperm storage, which is possible in snakes over periods of up to at least 5 years, new molecular methods have allowed biologists to differentiate offspring that were produced parthenogenetically from those that were produced from sexual reproduction following prolonged sperm storage. Looking back at supposed cases of lengthy sperm storage may reveal facultative parthenogenesis in unexpected places.

In contrast, facultative parthenogenesis in caenophidians is fraught with difficulties. Most of the offspring produced this way are not viable because they have suffered a loss of some genetic information. Many are stillborn or have deformities or other abnormalities. All are males, and the litters are unusually small. Nevertheless, parthenogenesis has been documented in both captive and wild Cottonmouths (Agkistrodon picivorus) and Copeprheads (Agkistrodon contortrix), and in captive Eastern Diamondback (Crotalus adamanteus), Timber (C. horridus), and Aruba Island (C. unicolor) Rattlesnakes, four species of gartersnakes (Thamnophis couchii, T. elegans, T. marcianus, and T. atratus), and Arafura filesnakes (Acrochordus arafurae). Most of these species are commonly kept in captivity, and they span the gamut from the most basal caenophidians to the most derived, but the infrequent occurrence and low viability of facultative parthenogenesis in these species suggests that although all caenophidians may be capable of parthenogenesis, it is probably not very ecologically or evolutionarily significant. The reproductive potential of the few captive-born parthenogenetically-produced Copperheads that have survived is currently being assessed.

The next steps in this area of herpetology are to discover more about the different cellular and developmental mechanisms that control and influence parthenogenesis, document parthenogenesis in species and taxonomic groups where it is not so far known, and understand more about the hybrid origins of obligate parthenogenetic species. We still don't know what is required to induce parthenogenetic reproduction in either facultative or obligate species - some lizards require copulation with other females, and many salamanders require egg activation by the sperm of a male salamander of a different species. Who knows what bizarre adaptations parthenogenetic snakes await discovery?

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