Two studies on reproduction in reptiles have made me go “wow, thats cool” this week.
Firstly, the report of a boa constrictor giving birth to two litters of offspring without the need for a father. This sort of “virgin birth” is called parthenogenesis, and is not that uncommon in itself, having been previously observed in a few other reptile and fish species, and numerous invertebrates. What separates this latest report from the others is the unusual complement of sex chromosomes observed in the offspring.
When vertebrates reproduce by parthenogenesis, the offspring are usually “half clones” of the mother. Chromosomes come in pairs, and normally you get one half of the pair from your father and the other half from your mother. But in parthenogenesis, both halves come from the mother – that is, two copies of one half of the mother’s chromosomes are inherited. This means that when it comes to the sex chromosomes, the offspring end up with two of the copies of the same chromosome. In all other recorded instances of pathenogenesis in vertebrates, species like snakes with the ZW sex chromosome system (where males have ZZ and females have ZW chromosomes) produce only male offspring, and species with XY chromosomes (males XY and females XX) produce only female offspring. In other words, only the sex where the two sex chromosomes are the same is produced and the opposite scenario (WW females or YY males) was thought to result in non-viable offspring. Until this boa constrictor came along, that is – yep, her litters are made up entirely of WW female snakes, a finding which “up-ends decades of scientific theory on reptile reproduction”.
The second study, published online in Nature this week, is an great example of just how malleable sex determining systems can be. For many reptiles, sex is determined not by sex chromosomes, but by what temperature the egg is incubated at. In tuatara for example, incubating eggs at high temperatures produces males, while low temperatures produce females. You might think that chromosomal and temperature-dependent sex determination are two fundamentally different, mutually exclusive ways of determining sex. However, in some species the line between these two systems is blurred, suggesting that switching between systems is easier than you would think. This study found that the Tasmanian snow skink (Niveoscincus ocellatus) has both types of sex determination, and all it has taken to switch between the two is a shift in climate.
The snow skink lives in both the warm lowlands and cool highlands of Tasmania. The study found that in the highlands, the skink uses chromosomal sex determination, producing an equal ratio of male and female offspring regardless of the temperature. However, in the lowlands its sex determination is temperature-dependent – cool cloudy days produce males, and warm, sunny days produce females. The researchers speculate that the divergence in sex determination mechanisms was caused by temperature differences, enabling the lizards to maximise their reproductive output in the differing climates.