Wild platypus, in a Tasmanian stream.
Disappearance of the Y chromosome has already happened in an unknown number of mammals, particularly those with a short generation time such as many rodents, leading to their extinction. With no DNA, there is no way to determine if this was the cause of extinction, and in any case, extinction, at least locally, could occur very quickly if caused by a lost Y chromosome or at least the one remaining essential gene responsible for sex determination.
However, some rodents, especially some Eastern European and Asian mole voles and Japanese spiny rats evolved an alternative method of sex determination, so, unless humans evolve a similar replacement, they too will go extinct.
The replacement in the rodents was a sequence of DNA adjacent to a gene known as the SOX9 gene located on one of the autosomes which controls the development of maleness in the developing foetus. This gene is activated by the SRY gene on the Y chromosome, so, with no Y chromosome and no replacement, no males and no reproduction.
However, there is a problem which will arise during the replacement of Y with some other mechanism, and that mechanism could vary by region. During that period there will be at least two mutually exclusive methods for determining the sex of the offspring, during which any hybrids will be unable to reproduce even if they are viable. This means there will be an evolutionary 'war' between these two methods, during which any mechanism which acts as a barrier to hybridization will give the carriers an advantage because they won't be producing sterile offspring.
These evolving barriers will quickly cause the emerging species to diversify, so, even if humans survive the loss of their Y chromosome, there will inevitably be two or more species of humans.
This has already happened in those families of rodents such as the Eastern European mole voles and the Japanese spiny rats where some species have an alternative sex determination mechanism involving a SOX9 enhancer in males on the same chromosome as the SOX9 gene and the loss of the Y chromosome, so the chromosome with the SOX9 enhancer has picked up the sex-determining baton, while related species have retained the XY method.
The enhancer consists of a simple duplication of a short sequence of DNA that is present in mice but in which it has no known function. Somehow, this activates the SOX9 gene at the right time, taking over the function of the Y Chromosome’s SRY gene.
In the following article reprinted from The Conversation under a Creative Commons licence, professor Jenny Graves Jenny Graves, Distinguished Professor of Genetics, La Trobe University, Australia, explains that it could have been the cause of divergence between the marsupials and the monotremes and between the marsupials and the placental mammals, and so this played a key role in major steps in our own evolution.
The article has been reformatted for stylistic consistency. The original can be read here:
Did sex drive mammal evolution? How one species can become two
There’s a difference in the sex chromosomes between various mammals, such as the platypus compared to humans.
Jenny Graves, La Trobe University
How new species are created is at the very core of the theory of evolution. The reigning theory is that physically separated populations of one species drift apart gradually.
But changes in chromosomes, particularly sex chromosomes, can interpose drastic barriers to reproduction. Mammals may be a good example. Comparisons of the sex chromosomes of the three major mammal groups show that there were two upheavals of sex chromosomes during mammal evolution.
The first corresponded to the divergence of monotreme mammals (platypus and echidna) from the rest, and the second to the divergence of marsupials from placental mammals (including humans).
In a paper published in BioEssays, I propose that drastic sex chromosome changes could have played a direct role in separating our lineage (placental mammals), first from the egg-laying monotremes, then from marsupials.
In humans and other placental mammals, such as mice, dogs and elephants, sex is determined by a pair of chromosomes. Females have two copies of the X while males have a single copy of the X and a small Y that contains the male-determining gene SRY.
Other vertebrate animals also have sex chromosomes, but they are different. Birds have an unrelated sex chromosome pair called ZW, and a different sex determining gene called DMRT1.
Snakes also have a ZW system, but again it is a different chromosome with different genes. Lizards and turtles, frogs and fish have all sorts of sex chromosomes that are different from the mammal system and from each other.
The rise and fall of sex chromosomes
Sex chromosomes are really weird because of the way they evolved. They start off as ordinary chromosomes, known as autosomes. A new sex gene arises on one member of the pair, defining either a male-determining Y as in humans or a female-determining W as in birds.
The acquisition of a sex factor on one member of the pair is the kiss of death for that chromosome, and it degrades quickly. This explains why only a few active genes remain on the human Y and the bird W.
When old sex chromosomes self-destruct, a new sex gene and sex chromosomes may take over. This is fraught with peril because the interaction of old and new systems of sex determination is likely to cause severe infertility in hybrids.
Rival sex genes may be at war with each other, causing intersexual development, or at least infertility. For instance, what will be the sex of a hybrid that has both a male-determining Y and a female-determining W?
Added to this are problems with gene dosage because the degenerate Y and the W have few genes. If an XY male mates with a ZW female, most of the progeny will be short of genes. There may also be problems with gene dosage because genes on the X and the Z are used to working harder to compensate for their single dosage.
Rearrangement of sex chromosomes with autosomes also causes severe infertility because half the reproductive cells of a hybrid will have too many, or too few, copies of the fused chromosome.
Such hybrid infertility poses a reproductive barrier between populations with the new and the old sex system. So could such barriers drive apart populations to form distinct species?
Reproductive barriers and new species
The idea that chromosome change could drive the formation of new species was popular 50 years ago.
But it was thoroughly dismissed by evolutionary geneticists in favour of the idea that speciation, the formation of new and distinct species, must occur in populations already separated by a physical barrier such as a river or mountains, or behaviour such as mating time, and occupied different environments.
Small mutations would accumulate slowly and the two populations would be selected for different traits. Eventually they would become so different that they could no longer mate with each other and would form two species. This allopatric speciation relied on external factors.
The alternate view, that sympatric speciation can happen within a population because of intrinsic genome changes, fell out of favour. Partly this was because it is hard to demonstrate speciation of populations sharing the same environment, the argument always being that the environment could be subtly different.
The other problem was imagining how a major chromosome change that occurred in one animal could spread to a whole population. Sex chromosome change is especially drastic because it directly affects reproduction. But our comparisons show that sex chromosomes have undergone dramatic changes throughout vertebrate evolution.
It is important to examine closely examples of evolutionary divergence that were accompanied by drastic sex chromosome change. Strangely, mammals may offer us a window into this evolutionary past. Their sex chromosomes are extremely stable, yet they have undergone rare dramatic changes, each of which lines up near when one lineage became two.
Sex chromosome change and mammal divergence
Placental mammals all share essentially the same XY. Marsupials, too, have XY chromosomes, but they are smaller; genes on the top bit of human X are on autosomes in marsupials.
Comparisons outside mammals shows that this bit was fused to ancient marsupial-like X and Y chromosomes before the different lines of placental mammals separated 105-million years ago.
Monotreme mammals (platypus and echidna) have bizarre multiple X and Y chromosomes. Surprisingly, comparing the genes they bear showed that they are completely unrelated to the XY of humans and marsupials. In fact, platypus sex chromosomes are related to bird sex chromosomes.
The human XY pair is represented by an ordinary chromosome in platypus. So our XY and SRY are quite young because they must have evolved after monotremes diverged from our lineage 190-million years ago.
Sex chromosome change has occurred very rarely in mammals, so it seems significant that each change corresponds to a major divergence. That’s why I propose that sex chromosome turnover separated monotremes from the rest of the mammals, and sex chromosome fusion occurred later to separate our lineage from marsupials.
Strengthening the argument that sex chromosome turnover begets speciation is evidence of a new round of sex chromosome change and speciation.
In Japan and Eastern Europe, species in two rodent lineages have completely eliminated the Y chromosome and replaced SRY with a different gene on a different chromosome. In each lineage the Y-less rodents have recently diverged into three species.
What does this mean for our own lineage? The primate Y seems to be more stable than the rodent Y. But if it continues to degrade at the same rate, it will disappear in about 4.6 million years.
Will it be replaced by some different gene and chromosome? And if so, will this unleash a new round of hominid speciation? We may have to wait another 4.6 million years to find out.
Jenny Graves, Distinguished Professor of Genetics, La Trobe University
Professor Graves open access paper in BioEssays gives more detail:
AbstractA sex determining system that means frequent extinctions or speciation to avoid extinction, only to come up with an alternative that is no better in the long-term than its predecessor! Creationists regard this sort of thing as evidence of intelligent design - which probably says more about Creationists than it does about biology.
Comparative mapping and sequencing show that turnover of sex determining genes and chromosomes, and sex chromosome rearrangements, accompany speciation in many vertebrates. Here I review the evidence and propose that the evolution of therian mammals was precipitated by evolution of the male-determining SRY gene, defining a novel XY sex chromosome pair, and interposing a reproductive barrier with the ancestral population of synapsid reptiles 190 million years ago (MYA). Divergence was reinforced by multiple translocations in monotreme sex chromosomes, the first of which supplied a novel sex determining gene. A sex chromosome-autosome fusion may have separated eutherians (placental mammals) from marsupials 160 MYA. Another burst of sex chromosome change and speciation is occurring in rodents, precipitated by the degradation of the Y. And although primates have a more stable Y chromosome, it may be just a matter of time before the same fate overtakes our own lineage.
Video abstract
Graves, J.A.M. (2016)
Did sex chromosome turnover promote divergence of the major mammal groups?
BioEssays, 38: 734-743. DOI: 10.1002/bies.201600019
Copyright: © 2022 The authors.
Published by WILEY Periodicals, Inc. Open access.
Reprinted under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International license (CC BY-NC-ND 4.0)
Utilitarian evolution, with its inability to look forward or plan, is quite capable to coming up with something which works in the short term of the next generation but is indifferent to the long-term survival of the species. The fact that humans will either go extinct, or speciate into two or more new species, and that the latter possibility is purely a matter of chance, is of no concern to a mindless process like Evolution.
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