Revenge of the sex chromosomes

Male (left) and female eastern bluebirds, Sialia sialis. In birds, males are homogametic (with two Z chromosomes) and females are heterogametic (with one Z and one W) — and this means that, all things being equal, he has a slightly longer life expectancy than she does. (Flickr: Vicki DeLoach)

Sex chromosomes are one of evolution’s stranger inventions, when you think about it. Many sexually dimorphic species, including most reptiles and amphibians and lots of plants have sex determination via gene complexes on otherwise conventional chromosomes, or even as a response to environmental variation; and those that do have sex chromosomes don’t all have the same kind.

In mammals, most of us learned in high school biology (or possibly via dusty sitcom jokes), individuals carrying two X chromosomes typically develop as female, while those with one X and one Y chromosome typically develop as male. But, as you find out if you stick around for university-level biology or do the right kind of extracurricular reading, a number of non-mammalian taxa, like birds, do it the other way around, with males carrying two Z chromosomes and females carrying a Z and a W chromosome. It’s a classic evolutionary kluge: there’s more than one way to genetically encode separate features for males and females, and which solution is in use within a given group of species depends largely on the happenstance of their shared ancestry. And, like all kludges, it comes with a downside — recessive deleterious mutations on the X or Z create a genetic burden borne only by XY or ZW individuals — and a paper recently published in Biology Letters finds that, across a wide range of animal taxa, this is indeed the case.

The cost of sex chromosomes lies in the weird pattern of inheritance that they create, which irritates introductory genetics students and can seriously inconvenience the heterogametic sex — whichever one carries differing chromosomes, whether it’s an XY male or a ZW female. This is because, for the heterogametic sex, there’s no such thing as a recessive allele on a sex chromosome. In the homogametic sex, a deleterious mutation on one X or Z chromosome can potentially be masked by a non-mutant version on the other X or Z; but in the heterogametic sex, there’s no matching X or Z to mask the effects of such a mutation. In humans, red-green color blindness is a non-lethal example: people carrying a single color blindness allele on an X will be color blind if their second sex chromosome is a Y, but people with two Xs will only be color blind if they carry color blindness alleles on both. More serious examples include haemophilia and muscular dystrophy. But even weakly deleterious recessive mutations add up, and it’s been hypothesized, and even shown in a few taxa, that the unmasking of deleterious recessive mutations in the heterogametic sex should mean heterogametic individuals tend to have shorter lifespans.

Log ratio of lifespan estimates for homogametic vs heterogametic individuals across 229 animal species, with their phylogenetic relationships. Red points are for ZW species, blue for XY species. Xirocostas et al. (2020), Figure 1.

The new study tests that hypothesis by comparing lifespan data for animal species with XY- or ZW-type sex determination. The authors, Zoe Xirocostas and colleagues at the University of New South Wales in Australia compiled data for altogether, 229 species of mammals, birds, reptiles, amphibians, insects, and other arthropods. They then estimated the ratio of typical lifespan of homogametic individuals to the typical lifespan of heterogametic individuals, and tested the very simple hypothesis that this ratio was greater than one.

The data show a fair bit of variation, but overall the signal is there: on average, homogametic individuals live 17.6% longer than heterogametic individuals. Phylogenetic effects did not account for a significant fraction of variation, but the type of sex determination system did. Specifically, in ZW species, homogametic individuals individuals had a significantly smaller lifespan advantage than in XY species. That suggests there is something about males that may depress lifespan, separate from the effect of carrying an "unguarded X". Xirocostas et al. suggestion a number of reasons for this, including the possibility that the Y chromosome in XY males is more apt to evolve to a reduced state — or even be completely lost — than the W chromosome of ZW females. They also note that standard-model sexual selection, in which males compete, sometimes violently, for mating opportunities, carry costly ornamental traits, or perform mate-attraction behaviors that can also attract predators, may also contribute.

Whatever the reason, this is a tidy project in establishing broad support for a basic prediction of the population genetics of sex determination. It’s more a starting point than a conclusion, but I expect it will be cited quite a bit in future studies of mutation accumulation on the sex chromosomes.

References

Charlesworth D. 2002. Plant sex determination and sex chromosomes. Heredity 88, 94–101. doi: 10.1038/sj.hdy.6800016

Eggert C. 2004. Sex determination: the amphibian models. Reproduction Nutrition Development 44:539-549. doi: 10.1051/rnd:2004062

Xirocostas ZA, SE Everingham, AT Moles. 2020. The sex with the reduced sex chromosome dies earlier: a comparison across the tree of life. Biology Letters 16: 20190867. doi: 10.1098/rsbl.2019.0867

About Jeremy Yoder

Jeremy Yoder is an Assistant Professor of Biology at California State University, Northridge. He also blogs at Denim and Tweed, and tweets under the handle @jbyoder.
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