Origins and functional evolution of Y chromosomes across mammals

By Olivia Niblock and Aikaterini Liarelli

When determining sex, you must take into account the sex chromosomes.  In most mammalian cases, the females are XX and the males XY, i.e. it is the masculine genes on the ‘Y’ chromosome which determine the gender of the offspring. However, sex determination does not have to follow the exact pattern of defining the males with a different chromosome, or indeed by a genetic element at all. For example, there is a variant of the XX/XY system where females have two copies of the X chromosome, as in humans, but males have only one (X0), and sex is expressed depending on the dosage of hormones on the chromosomes. Additionally, there is the ZW sex-determination system which determines femininity, not masculinity. In these cases, females have two types kinds of chromosomes, ZW, whereas the males are homozygous (that is, have two copies of the same chromosome) for Z. The ZW system is found in birds, reptiles and some insects.

Figure 1: Y or W protein coding repertoires and their origins: Strata are divided from S1 to S5. Genes are ordered within strata, differentiated sex chromosome genes are in the ‘Added’ section, and independently recruited genes are indicated in red.

In this 2014 study, a Mexican scientist Juan Cortez and his team, traced the evolution of the Y chromosome across 10 mammals and 5 birds (15 species in all). Since the Y chromosome underlies sex determination, its evolutionary path is worth exploring and many evolutionary studies have dealt with it in the past.  By using high-throughput genome and transcriptome sequencing, Cortez et al. produced a phylogenetic tree, with an attached table showing individual genes which are classed as ‘Y’ specific – or only in males (Figure 1). Interestingly enough, their goal was to explore the evolution of the Y chromosome across mammals, but they decided to use 5 birds for comparison (the birds in question being chicken, turkey, finch and ostrich).

The results showed three sex chromosome originations: the Y chromosome found in Eutherians and Marsupials such as giraffes and kangaroos respectively, the Y 1-5 in the Monotremes, which include animals like the dark-billed platypus, and finally the W chromosome in birds.

In Eutherians and Marsupials, SRY gene played a role in sex determination. This is a sex determining gene and it is considered the decision-maker in sex determination. Individuals with the gene become males, whereas individuals without the gene become females instead. The SRY gene was traced back to its original ancestor, approximately 180 million years ago whilst the Y chromosomes found in monotremes arose independently. Later on, approximately 140 million years ago, the W chromosome arose in birds, as shown in Figure 1.

The most interesting part of this study was that even though Y/W chromosomes arose millions of years ago, Y/W genes conserve a lot of their initial expression patterns, despite undergoing selection and expression decreases. Cortez and his team concluded that dosage constrains were the case; one gene is not overexpressed in relation to the others, and thus the individual can turn up with “testis-specificities through differential regulatory decay”, which affects the level of protein of an individual.

It is important to study the Y chromosome and trace back its evolutionary history, as it is thought that sex was determined non-chromosomally initially (i.e. sex was determined by temperature, perhaps, as it is in turtle species) and only took on a chromosomal form fairly recently in evolutionary history . Not so many years ago, people thought that Y chromosome’s function was to only determine sex, but it has other properties which have only recently been discovered. It is thought to be an X chromosome that has decayed across the ages of evolutionary time, and it is well known that the X chromosome does indeed carry genes not specific to sex determination (like colour-vision capabilities, for example). The Y chromosome is not only important in sex determination, but is involved in male disease susceptibility, and most importantly, phenotypic differences between the two sexes in health and disease. It would be interesting to see the scope of this research broadened to include many other species, as opposed to simply 15 subjects as in this study, as the results would be very useful to future studies.

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