Sex Chromosome Evolution

Sex chromosomes…sounds…sexy

You bet it is! Sex chromosomes are the portions of the genome that hold the key to the differentiation of females and males. Sex chromosomes are common in animals, and appear elsewhere in the tree of life too. Some animals use temperature to distinguish sex and others have just a single sex. One pair of chromosomes (in humans there are 23 pairs) are the sex chromosomes and, unlike all the other chromosomes, they are not equally shared between the sexes. This unequal sharing has very interesting evolutionary consequences.

Sex determination typically controlled through either a dominant gene (left panel) or a dosage difference (right panel). There may be a male specific Y chromosome or a female specific W chromosome. Figure design modified from Graves 2012

In mammals this pair of chromosomes are called the X and the Y. The X chromosome occurs in two copies in females, and only one copy in males. The Y chromosome holds the gene that leads to male development and thus only occurs in males (Fig. 1). Another system also exists, where the sex chromosomes are called the Z and the W. This system is the opposite of an XY system. In ZW systems, the Z chromosome is in two copies in males, and one copy in females. The W is present only in females and, in some taxa, it will contain a gene that leads to female development. Alternatively, sex may be determined by a gene on the Z chromosome, and males develop because they have two copies and females develop because they have one copy of the gene; this is the case in birds (well…mostly).

This unequal sharing of chromosomes and sex specificity of chromosomes is what gives the sex chromosomes their unique properties. They experience different levels of selection, compared to the rest of the genome (owing to their suppressed effective population size), and they disproportionately contribute to speciation, infertility and other similar phenomena.

More curiosities

Proper development of female and male is critical to being able to breed, promoting the continuation of genetic lineages. The cascade that determines sex is quite tightly regulated and largely similar across the a wide variety of taxa. For vertebrates most of the cascade is the same (or quite similar), and the same genes can even been found in the genetic cascade controlling sexual development in vertebrates and invertebrates.

Surprising it is then to learn that the master regulators of sex (i.e. the genes that set off this cascade of genetic interactions) are highly variable. Even very closely related species may have very different master regulators. For placental mammals, SRY controls sex, but this gene has been lost in numerous taxa (leaving the job up to yet to be discovered genes). In birds, the gene is (mostly) dmrt1, which is a very curious gene. For most other taxa, the genes in charge are an absolute mystery. For taxa fish, lizards, and frogs, there is immense variability in sex chromosome systems (ZW or XY) and perhaps equal variability in the master regulators…or perhaps not…

My Project

I study Xenopus, the African clawed frogs, which are very curious aquatic frogs. For some species we know that they have ZW sex chromosomes and in some taxa there exists a gene called DM-W, which sits on the W chromosomes and its expression leads to female development.

My research focuses on X. borealis, a species that appears to lack DM-W. What I have confirmed is that, as suspected, X. borealis has a novel set of sex chromosomes. Furthermore, this sex determining region contains similar genes to that of mammals, another distantly related frog, and a lizard species. This represents at least 4 times that this genomic region has become established as sex chromosomes.

With this project, we have found support for a model where certain genetic elements independently evolved as the master regulators of sex determination different species. Basically, when there is a change in the sex determining system, it might not be a completely novel genetic element that takes over. We are continuing this work and investigating recombination in X. borealis and exploring genome-wide patterns of sex linkage.

We performed phylogenetic analyses using over a thousand genes to establish phylogenetic relationships among representatives from the major lineages of Xenopus. From this, we can infer that the ancestor of extant Xenopus had DM-W and the system in X. borealis is a new system, that arose after DM-W.