The evolution of sex chromosomes

An interesting study in this week's edition of Nature by Organ and colleagues looks at the evolution of sex chromosomes. While humans use the XY system for determining sex (XX for females, XY for males), this is by no means the only system for determining sex. Most reptiles, for example, determine sex by the temperature at which the young develops. For example crocodiles develop as males if the eggs are between 31.7°C and 34.5°C, and females if the eggs are above or below this temperature.

A chromosome-based method for determining sex has arisen not just once, but several times. Mammals use the XY system, but birds use the ZW system (where ZZ is male and ZW is female). These systems create problems, such as the dosage compensation question (how to stop excess / insufficient production of genes on the X or Z chromosomes in the gender with two copies / one copy), however they have a major advantage. This advantage is most evident in mammals - mammals are endothermic, meaning that we keep a constant body temperature. We also bear live young. Obviously, this combination of characteristics would be fatal to a species with temperature-dependent sex determination - all offspring would be of one sex.

In this paper the Pagel laboratory has used an evolutionary analysis to consider the relationship between bearing live offspring and having a chromosome-dependent sex determination system. There are multiple examples of animals with chromosome-dependent sex systems that lay eggs (all birds) and even examples of animals with temperature-dependent sex systems that bear live offspring (some lizards). However in one group of animals the relationship was very strong - amniotes that have fully returned to the sea (sea snakes, sirenians and cetaceans) are all live-bearing and have chromosome-dependent sex systems. An evolutionary analysis predicts that other extinct lineages of sea reptiles, mosasaurs, sauropterygians and ichthyosaurs, also developed chromosome-dependent sex systems before evolving life birth and spreading out over the ocean.

Like mammals with endothermic body temperatures, the constant temperatures of the ocean would have spelt doom to any species that evolved life oceanic birth before evolving a chromosome-based sex system. This is probably the reason why otherwise entirely aquatic species that use temperature-based sex determination systems (such as crocodiles and sea turtles) remain bound to laboriously climb out of the water to lay their eggs.