The green lizard (Lacerta viridis, Laurenti 1768) have ZW genetic sex determination systems, whereas many other reptiles exhibit XY sex determination. Credit: Andras Liker

Who wins the battle of the sexes?

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Whether or not a species has more males than females can be predicted by which genetic sex determination system - the parts of the DNA that determine gender, such as XX and XY for human females and males respectively - is in play, say international researchers. The authors show that, when the female has two different sex chromosomes, as is the case with female birds who have ZW (males have ZZ), this tends to result in a male dominated population, whereas when the male has two different sex chromosomes, as is the case in mammals, including humans, this leads to a female dominated population.

Journal/conference: Nature

Organisation/s: University of Bath, UK

Media Release

From: Springer Nature

Predicting tetrapod sex ratios

The adult sex ratio (ASR; the proportion of males to females) in tetrapod populations differs considerably depending on the genetic sex determination system, finds a study published inNature this week. The ASR of tetrapods — birds, mammals, reptiles and amphibians — is known to vary in nature. Such variations affect population dynamics, social behaviour and ecology, with unbalanced ASRs being linked to mate-choice decisions and parental care. However, until now, the causes of ASR variation in wild populations have been unclear.

András Liker, Tamás Székely and colleagues performed phylogenetic analyses on data from 344 species in 117 families of tetrapods. They show that when the female has two different sex chromosomes (heterogamety), as is the case in birds, this tends to result in a more male-biased sex ratio, and when the male has two different sex chromosomes, as is the case in mammals, there tends to be a more female-biased sex ratio. In other words, the heterogametic sex tends to become underrepresented in a given population.

Although the authors explore various genetic mechanisms that may contribute to this pattern, the underlying causes remain unclear. Further theoretical, experimental and comparative studies are therefore needed to understand the links between sex determination, sex ratios and social behaviour.

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