Press Releases

Phylogenetic analysis predicts the mechanism of sex determination in extinct marine reptiles – University of Reading

Release Date : 17 September 2009

Long extinct sea reptiles not only had live births, but the sex of their offspring was genetically pre-determined, according to research published this week in the journal Nature.

Most reptiles lay eggs but these cannot survive underwater, so live birth gave these newly-evolved reptiles the ability to invade the seas. The research from the University of Reading and Harvard University shows that the evolution of live births required the prior acquisition of a genetic mechanism for determining the sex of offspring. It was this pair of traits which enabled reptiles to adapt to a variety of marine conditions producing many species similar to present-day whales.

Sea snakes are the only present-day seagoing reptiles that have sex chromosomes to determine the sex of their liveborn offspring. Such genetic pre-determination of sex is a trait that has independently been gained and lost throughout the evolution of reptiles. Other reptile species, both present day and now extinct, have had their offspring's sex determined by environmental factors, such as temperature.

Professor Mark Pagel from the University of Reading and colleagues at Harvard University used phylogenetic analysis to predict the mechanism of sex determination in three extinct Mesozoic marine reptiles. Each reptile was from an independent evolutionary lineage, although from fossil evidence all were known to have evolved the capacity to give birth to live young, rather than lay eggs. The scientists found that each of the three Mesozoic reptiles evolved genotypic sex determination before acquiring live birth.

Professor Mark Pagel explained: "Genetic sex determination is advantageous because it balances the male-to-female ratio in the species and buffers its survival from environmental fluctuations. If you give birth to live young, as opposed to laying eggs, they come out with their sex already determined. This requires a genetic mechanism for determining sex, such as in humans. Many reptiles rely on the temperature of the environment to determine the sex of offspring. The oceans have a relatively stable temperature, so a genetic determination for sex was necessary to enable colonisation of this environment.

This research is published in the Journal Nature on Thursday 17 September. The abstract can be found at


Further information from Alex Brannen, Media Relations Manager at the University of Reading

Notes to editors:

Professor Mark Pagel is currently in the USA and is not available for broadcast interviews.

The Centre for Advanced Computing and Emerging Technologies (ACET) at the University of Reading made the ThamesBlue supercomputer available for this research.


Search Form

Main navigation