But size doesn’t matter, because the Y contains the genes that set the male embryo on the glorious path to developing facial hair, antisocial behaviour, and all the rest.
More worryingly (to some geneticists, at least), the Y degenerates over time, losing the genes it once shared with its partner, the X. The Y and X are derived from the same ancestral chromosome, but over millions of years of evolution they have gradually ceased to exchange genetic material (to recombine, in technical terms), as other homologous pairs of chromosomes are wont to do.
This genetic divorce is not much of a problem for the X, because Xs recombine whenever they are paired in a female’s (XX) genome. But the poor Y – quite literally the odd-man-out among mammalian chromosomes – has no-one at all to recombine with, and this results in a so-called ratchet effect that’s thought to lead invariably to degeneration.
Why should the Ys lonely state lead to degeneration? Consider what happens when a mutation – and mutations are usually bad – occurs on an ordinary chromosome. Recombination provides an efficient mechanism for separating that mutation from the good parts of the chromosome, because some lucky offspring will, just by chance, get a healthy chromosome and pass lots of copies of it to the next generation.
The offspring that inherit the bad mutation will probably shrivel and die without issue, and the mutation will be discarded on natural selection’s refuse heap.
But for the Y there is no such luck. Every bad thing that happens, as long as it’s not bad enough to actually kill or sterilise its bearer, will tend to stick around, and get added on to previous bad things (hence, the ratchet analogy). And, eventually, the Y acquires so many bad bits it hardly matters for fitness when it actually loses a piece, and then another, and another … Over many generations, in other words, the Y degenerates and shrinks.
Now, the big question keeping some evolutionary geneticists awake at night is this: if the Y keeps shrinking, will it eventually disappear?
Indeed, some animals, such as the Transcaucasian mole vole, have already lost their Y chromosome. Could the same happen to us?
Don’t worry. Even if the human Y were to vanish some day, purged from our genomes like a bad memory, the male-determining factor would presumably just relocate to some other chromosome (and set in motion the evolution of a new Y, which would eventually degenerate like the old …)
But, still, the nagging question: is our Y slowly fading into oblivion?
Jennifer Hughes, of the Whitehead Institute for Biomedical Research in the USA, and colleagues set out to answer this question by comparing the human Y chromosome to the Y of the rhesus monkey – a bad-tempered, hirsute cousin from which out lineage split about 25 million years ago. And they discovered something surprising.
It’s been known for a while that the genetic divorce between the X and Y happened in five stages over the course of our evolution. At each stage, a section of the Y ceased to recombine with the X, and thus began to evolve independently and acquire differences in the DNA-sequence.
By comparing the degree of differentiation with the X in different sections of the Y, it’s possible to identify the regions (“strata”) affected at each stage of the genetic divorce, and estimate how long ago recombination ceased.
Such genomic analysis has revealed that the first stage occurred over 240 million years ago, while the most recent stage occurred just before our ape lineage split off from old-world monkeys such as the rhesus.
Hughes and colleagues report in Nature today that, since our split with the rhesus monkey, our Y chromosome has indeed lost many genes from the youngest stratum (layer), testifying to ongoing degeneration. Intriguingly, though, not a single gene has been lost from the four older strata. In both humans and rhesus monkeys, these strata contain the very same 18 ancestral genes.
This is good evidence that parts of the Y are just too important to lose. Within each stratum, degeneration eventually slows, and perhaps stops, because powerful natural selection continually cleanses these chromosomal regions of deleterious mutations.
After all, genes on the Y control some crucial male functions, such as the formation of testes. And, evolutionarily speaking, a male without testes may as well be pushing up daisies.
So rest assured. The human Y chromosome looks to be a survivor after all.
Russell Bonduriansky is a Senior Research Fellow in the Faculty of Science at UNSW.
This piece was published in The Conversation.