Boa Constrictor Produces Viable WW Offspring Via Parthenogensis
Faculative parthenogensis has been previously reported in numerous families of squamates. In some lizards parthenogenic species results from hybridization. Female snakes, isolated in captivity from males,have produced offspring asexually. It has generally been thought that females fertilize their own eggs using the secondary polar body produced during the second division of meiosis. The polar body is haploid (N) and therefore could act as a sperm by contributing a complete set of chromosomes to the offspring. Snakes normally have a ZW sex chromosome system, males are ZZ, females are ZW, and offspring that would receive the chromosome combination WW, would be females but were believed not to survive development (this would be similar to having a YY male in mammals). However, WW females have been experimentally produced in the lab in fish and amphibians. Today, Warren Booth of the University of North Carolina and colleagues have published an early on-line article that describes a female boa constrictor producing two litters of all female offspring that were WW, and the female had been housed with males. In an interview with the BBC Booth told the reporter, "The female [boa] has had not one virgin birth, but actually two, in spite of being housed with and observed to be courted by multiple males. All offspring are female. The offspring share only half the mother's genetic make-up..." The researchers used microsatellite DNA fingerprinting, to document the first evidence of facultative parthenogenesis in the Boa constrictor and identified multiple, viable, non-experimentally induced females for the first time in any vertebrate lineage. They hypothesized in the paper that the elevated homozygosity of the offspring in relation to the mother may be the product of terminal fusion automixis (fusion of the secondary polar body to the egg nucleus). The fact that no males were produced suggests maternal sex chromosome hemizygosity (WO) - that is the female had only one sex chromosome, and when the chromosome replicated, the sister chromosomes did not separate (non-disjunction). This report is the first evidence of parthenogenesis in the family Boidae and suggest that WW females may be more common within basal reptilian lineages than previously assumed.