The Human Y Chromosome in the Light of Evolution
In many dioecious taxa, the karyotype defines the sex. Sex-chromosome pairs then
have arisen among those lineages from separate pairs of autosomes. The
pseudoautosomal regions (PARs) at the ends of the human Y chromosome can vary in
size among mammals. In male meiosis, it occurs the recombination of the PARs of the
X and Y chromosomes at high rates. Although the human PARs mostly resemble
autosomes in base composition and in gene density and diversity, a few
pseudoautosomal genes have been identified, which may elude X inactivation, as
should be expected from genes with sex-uniform dosage. The genetic drift might
explain the sweep of the inversions into fixation on sex chromosomes in the presence
of the sex-determining gene (SRY), which seems to have been the first active Y gene to
stop recombination with the X chromosome.
The null hypothesis of the article should be that evolution has no significant effect on
differences of sex chromosomes (X and Y). Therefore, the alternative hypothesis is
based on the evolutionary role in shaping distinctive properties of sex chromosomes.
Although the sex chromosomes vary largely in any aspects, they usually tend to equalie
gene activity between the sexes, which hints to the common factors that led their
evolution. The human Y chromosome shows only a few distinct expression profiles and
it presents extremely low gene-functional diversity. The distribution of the genes in the
human Y chromosome reflects the evolutionary elements associated to sex-specific
chromosomes.
This paper reviews a broad of previous studies, which results in no raw data. Thus, the
Y chromosome was analyzed in many living systems, such as Poecilia reticulata,
mouses and the fruitfly Drosophila miranda. Meanwhile, both X and Y chromosomes
were studied in humans and a few others placental mammals. The results allow a gross
classification of the genes found in the NRY (non-recombining region of the Y
chromosome) into three classes (1, 2 and 3) based on expression profile and homology
to the X chromosome.
Table 1: Classification of human Y-chromosome genes. Image extracted from Lahn et al., 2001.
The table above shows the categorization of many studied genes found in the Y
chromosome. Accordingly, most of the pseudoautosomal genes that were studied
present a tendency to inactivate the X homologue in females, except for the SYBL1
and HSPRY3 genes, which undergo both X and Y inactivation in males and females,
pointing out the complexity of the evolutionary history inherent to this region and
its recent X-to-Y translocation. The three gene classes of the NRY show significantly
limited functions if compared to other human chromosomes. This highlights the
three evolutionary fates of NRY genes: specialization in male-specific function,
preservation in ancestral or functional decay. Also, this functional specialization
points towards two evolutionary processes associated with the Ys: the
accumulation of genes that benefit only male fitness and genetic decay.