26 October 2018
Jennifer Marshall Graves
Department of Ecology, Environment and Evolution
La Trobe University, Melbourne, Australia
Sex chromosomes are a great example of what I call "Dumb Design", biological systems that make no sense functionally, but are the result of quixotic evolution.
Sex in vertebrates may be determined by genetics (GSD) or by the environment, usually temperature (TSD). Sex determining genes may define either a male-specific Y chromosome (XX female: XY male system of male heterogamety), or a female-specific W (ZZ male: ZW female system of female heterogamety). The sex-specific chromosome (Y or W) progressively loses homology with its erstwhile partner (X or Z), and degrades rapidly, losing or repurposing active genes, and accumulating repetitive elements and amplified copies or pseudogenes.
Any system with differentiated sex chromosomes must cope with practical problems of segregating non-homologous chromosomes at meiosis (often XY or ZW don’t pair very well), and the evils of monosomy (think haemophilia or X linked mental retardation in men). Different dosages of the X or Z in males and females may be mitigated by complex epigenetic dosage compensation systems.
I use comparisons between distantly related vertebrates to deduce how sex chromosome systems begin, how they differentiate and how they are replaced. Nearly all mammals share the XY system of humans, but marsupial mammals reveal a more ancient mammal sex pair and monotremes have multiple sex chromosomes related to the ZW of birds. This gives us a start time of 166 MY for the evolution of our XY, and a rate for the degradation of the mammal Y that suggests it will disappear in a few million years; indeed it has already been replaced in two groups of rodents. Intriguingly, the three mammal groups are separated by major sex chromosome rearrangement and turnover, suggesting that speciation can be driven by sex chromosome changes.
Sex determination is more flexible in reptiles and fish, which show many different sex genes and sex chromosome systems, even between close relatives. Some taxa include GSD and TSD species, and our dragon lizard model does both: sex is determined by a ZW system, but male development is overridden at high temperatures. The fertility of ZZ sex reversed females allowed us to flip the entire sex system from GSD to TSD in a single generation.
How can so many different triggers activate the same genetic pathway? The pattern of sex genes from all over the vertebrate phylogeny suggests independent recruitment of certain genes (or copies of them), or genes downstream or upstream that modify their actions. In TSD reptiles, epigenetic pathways activate the same genes. So the answer to the oft-asked question "why so many ways to determine sex?" lies in the self-destruction of sex chromosomes and the availability of many alternative ways – genetic or environmental – to rescue a species from Y- or W-driven extinction.