A A A

Male Promiscuity Boosts Role of Chance in Sex Chromosome Evolution

By Claire Asher, on 19 March 2015

Humans, like all mammals and birds, determine sex with chromosomes. Whether a fertilised egg develops into a male or female depends on what chromosomes it carries Scientists have long recognised that genes evolve a little differently on the sex chromosomes, and recent research in GEE suggests this may be due to differing patterns of inheritance that favour the influence of chance on gene sequence change. Furthermore, promiscuity in males has a large influence on the magnitude of this effect, with chance playing an even greater role in sex-chromosome evolution in highly promiscuous species. Using genetic sequence data in combination with physical and behavioural measurements of promiscuity in birds, Dr Alison Wright and Professor Judith Mank report strong evidence for the role of neutral forces in sex chromosome evolution.

In birds and mammals, along with some invertebrates and reptiles, sex is determined by the chromosomes you carry – the sex chromosomes, as they are aptly named. If you are a male mammal, you carry one X and one Y chromosome; a female mammal carries two X chromosomes. Similarly, if you are a male bird, you carry two Z chromosomes; a female carries one Z and one W. Whether it’s the XY system, the ZW system or even the UV system used by some species of algae, the result is more or less the same. Sex is determined by the presence or absence of particular chromosomes. This isn’t always the case – some species determine sex using temperature during development, other species determine sex based on social conditions, while others do away with fixed sexes altogether and are either hermaphrodite or possess the ability to switch sex. However, one of the most common, and certainly the best studied, systems among living organisms is to determine sex with chromosomes.

Unlike autosomal chromosomes (all our chromosomes that are not sex chromosomes), sex chromosomes are not inherited and expressed equally across the sexes. The Y and W chromosomes only ever appear in one sex, for example. This has some interesting consequences for evolution. For example, scientists have found that the ‘major sex chromosomes’ (X and Z chromosomes) tend to evolve faster than the autosomes. Known as the Faster-X (or Faster-Z) effect, this phenomenon is now well documented in a range of different species, and scientists have suggested a number of possible explanations for why this might be the case. Faster evolution on the major sex chromosomes might be caused by more effective natural selection favouring beneficial mutations (adaptive hypothesis) or due to less effective natural selection failing to remove harmful mutations (neutral hypothesis).

Why would natural selection act differently on sex chromosomes than autosomal ones? In a paper published in Molecular Ecology this month, Dr Alison Wright explains that the differences between chromosomes arise because of differences in the pattern of inheritance, which ultimately influences the number of chromosomes that are passed on to the next generation, called the effective population size. An individual who never reproduces is an evolutionary dead end, and as their genes are not passed on, and they are not counted in the effective population size. Individuals that do mate contribute sex chromosomes unevenly, and this can have a significant impact on the course of sex-chromosome evolution.

When two individuals mate, they each pass one of each pair of chromosomes to the offspring. Each chromosome has an equal likelihood of being carried by the offspring, and the effective population size (ie chance of being passed on) of all autosomal chromosomes is the same. But for the sex chromosomes, things are a bit more complicated. Each time a pair of individuals mate, between them they bring three major sex chromosomes and one minor chromosome to the table. This translates to major sex chromosomes having an effective population size three times larger than the minor sex chromosomes. And both sex chromosomes have a smaller effective population size than the autosomes.

But that’s only if everybody is monogamous. As soon as promiscuity is involved, things get even more complicated. If males are promiscuous (and they often are, in the animal kingdom), then this means some males in the population are likely to be very successful, while others fail to reproduce at all. In other words, the variance in male mating success is much higher. Promiscuity reduces the effective population size of the minor chromosomes even further.

promiscuitysexchromosomes

Why does effective population size matter? Well, the effective population size determines the relative influence of chance on gene sequence evolution. Although we generally think of evolution progressing as natural selection favours beneficial mutations and purges deleterious ones, chance also has a big role to play. Chance, known in this context as genetic drift, has a bigger impact on small populations, and rare mutations. This is because when a particular mutation is rare, it only takes a little bit of bad luck for it to be lost forever. Just think of the times you’ve walked home in the rain only to hear the characteristic crunch of the end of a snail’s life – here your foot is the agent of genetic drift. The death of that snail had little or nothing to do with the genes it carried, but your foot has altered the course of evolution, slightly. The effective population size of autosomal genes reflects the population size of the organisms they are found in, but for the sex chromosomes, their effective population size is even smaller, making them more prone to genetic drift.

Dr Alison Wright, Professor Judith Mank and colleagues from GEE sequenced expressed genes in six species of birds, spanning 90 million years of evolution, to investigate the rate of evolutionary change in genes on different chromosomes. They compared sequence data from monogamous species like the Swan Goose (Anser cygnoides) and the Guinea Fowl (Numida meleagris) with promiscuous species like the Mallard duck (Anas platyrhynchos), wild Turkey (Meleagris gallopavo), and Peafowl (Pavo cristatus) to investigate how gene sequences and gene expression patterns vary both within and between species. They then matched data on the rate of evolution with characteristics of species that are associated with promiscuity, such as testes weight and sperm number. Their results indicate that natural selection is less effective on the Z chromosome in general, and this becomes even more pronounced in promiscuous species. The authors therefore conclude that Faster-Z evolution in birds is not adaptive, but is driven by neutral processes.

mec13113-fig-0002

Differences in gene sequences within and between species can tell us a lot about the rate of evolution for different lineages. This is because the genetic code has some redundancy in it – DNA is split up into three-letter words or codons, and there are many cases where different codons translate into the same amino acid. So, it is possible to have genetic sequence change that is essentially invisible to natural selection – it doesn’t alter the resulting protein sequence and so has no influence on the organism. Changes in gene sequence that swap between these ‘synonymous’ codons can therefore give us a rough baseline of neutral change. Non-synonymous differences (the ones that do have an effect on the organism), between individuals or between species, represent the rate of evolution. More non-synonymous changes suggests either positive selection, where evolution favours those changes because they are beneficial, or genetic drift, where selection is weaker and cannot remove slightly harmful mutations from the population. The authors found that genes on the Z chromosome show a faster rate of non-synonymous change than autosomal genes. Further, the ratio was significantly correlated with measures of promiscuity, with more promiscuous species having more non-synonymous changes.

Although this could be a mark of positive natural selection, the authors found no difference in the number of genes undergoing positive selection between sex- and autosomal-chromosomes, suggesting the Faster-Z effect is driven by genetic drift rather than positive selection. In fact, differences within species indicate that natural selection is less effective at removing mildly deleterious mutations from the Z chromosome than the autosomes. Combined with other analyses on gene expression, these results show strong support for the neutral explanation for Faster-Z evolution in birds.

Interesting, promiscuity increases the effective population size of X chromosomes, and that may explain why previous studies have found evidence that Faster-X chromosome may well be due to positive selection. These differences suggest that Z chromosomes may be less important in adaptation than X chromosomes.

Original Article:

nerc-logo-115ERC

This research was made possible by funding from the Natural Environment Research Council (NERC) and the European Research Council (ERC).

The Battle of the Sexes:
Sexually Antagonistic Genes in Small Populations

By Claire Asher, on 18 July 2013

By definition, males and females are physically different. In some cases, these differences are restricted to the sex organs, but in many species males and females also differ in their physical appearance and behaviour. What makes you a good male does not necessarily make you a good female, and the benefit of some genes depends upon which sex they find themselves in. These antagonistic genes have sex-dependent fitness effects, and can be a source of genetic variation in natural populations. Research in GEE shows that in small populations, chance events can alter the frequency of antagonistic mutations, and alter the balance of fitness between the sexes. Small populations tend to produce either good males and poor females, or good females and poor males, but not both. This phenomenon could have interesting implications for dispersal and fitness, especially in fragmented populations.

Differences in physical appearance between the sexes is known as sexual dimorphism, and occurs because characteristics that make you good at being male are often different from the characteristics that make you good at being female. A brightly coloured male may get more females because he is more visible or attractive, but a brightly coloured female doesn’t lay more eggs, she just gets eaten.

Sexual dimorphism allows males and females to independently adapt to different optima. The problem is dimorphism is actually quite hard to achieve. Males and females of the same species share, more or less, the same genome. Some species, such as birds and mammals, have sex-specific genes and chromosomes (e.g. the Y chromosome), but these represent a very small proportion of their genes, and many species achieve sexual dimorphism without any sex chromosomes at all. For the most part, traits that differ between males and females are generated from the same set of genes. The evolution of dimorphic traits such as body size, plumage colour and antlers proceeds not through changes to gene function, but changes in gene expression.

Gene Expression and Sexual Dimorphism

Male and female Red Deer
(Cervus elaphus)

A gene that codes for a trait that is beneficial to males but detrimental to females can become sexually dimorphic if it is expressed only when it appears in males (or vice versa). This same gene, when it finds itself in a female body, simply isn’t expressed and does not exert its effect. So a female red deer carries the gene for antlers, but never grows any because the gene is silent.

Sex-specific gene expression is an elegant solution to the problem that males and females face different challenges in life. However, the road to sexual dimorphism is not that straight forward. For a new sexually dimorphic trait to evolve, it must first appear as a novel mutation in a pre-existing gene. At first, it will be expressed just like most other genes – equally in both males and females. This mutation might do great when it finds itself in a male, allowing him to produce more offspring, but when it appears in a female, it would be harmful, reducing the number of offspring produced. For this new mutation to be successful and spread through the population, it needs to avoid the negative fitness effects that come from being expressed in a female. At this stage, a second mutation altering gene regulation – so that the gene is expressed only in males – would be hugely beneficial, but this mutation may take time to arise by chance.

There are many examples in nature of sexually dimorphic traits – genetic variants expressed in one sex but not another. But scientists are increasingly finding mutations that haven’t made it to this happy equilibrium yet – their fitness effect is sex-dependent but they are still expressed in both males and females. These genes are known as ‘sexually antagonistic’ and are thought to be a major source of genetic variation within populations.

Generating Variability
Normally, a gene that has a detrimental effect (i.e. reduces how many offspring you produce) is quickly removed from the population by natural selection. Sexually antagonistic genes are different because their detrimental effect is only seen by natural selection when the mutation appears in one sex. Half the time, the mutation is beneficial, and half the time it is harmful. Overall, the effect of the gene can even out to be essentially neutral. Neutral mutations can stick around in a population for ages, although they may be lost, or spread, by chance.

Chance, also known as genetic drift, can change the frequency of mutations in a population irrespective of their fitness effects, and the effect of chance is much greater in smaller populations. Sexually antagonistic genes may be particularly susceptible to genetic drift because their effect is, on average, neutral. Most theoretical and empirical studies of sexual antagonism use very large populations with high genetic diversity, where the influence of genetic drift is very small. Researchers in GEE have been investigating the evolution of sexual antagonism in small populations to see how chance might influence their frequency.

The fruit fly,
Drosophila melanogaster

The Role of Chance
In a recent paper in Evolution, Jack Hesketh, Kevin Fowler and Max Reuter from GEE looked at how small populations of the fruit fly Drosophila melanogaster evolved, and how sex-specific fitness changed under the influence of genetic drift, through the random spread of sexually antagonistic genes. Four populations of 100 flies were maintained for 80 generations, and the fitness of both larvae and adults was measured in terms of survival (larvae), fertilisation success (males) and egg-laying (females).

Over 80 generations of evolution in the lab, the four fruit fly populations diverged in terms of fitness. Average fitness across the sexes didn’t differ significantly between populations, but there was a strong interaction between population and sex – some populations produced very fit females and unfit males whilst others produced fit males and unfit females. This effect was not present for larval fitness however, which makes sense since larvae are primarily concerned with growth and not reproduction. In the original source populations, some individuals carried mutations beneficial to females, whilst others carried mutations beneficial to males. In this experiment, through the process of genetic drift, different populations ended up with a different selection of these mutations – some populations kept hold of the genetic variants favourable to females and consequently produce very fit females but relatively crap males (and vice versa).

This study shows that in real populations, genetic drift may lead to population-divergence in sexually antagonistic genes. This could have interesting repercussions for fragmented populations where different fragments may diverge differently in terms of sexual antagonism. Dispersal in this situation could be extremely favourable – a fit male in a population full of fit males would be faced with stiff competition, but if he wandered over into a neighbouring population full of unfit males, he would presumably have his pick of the ladies. Max Reuter and colleagues in GEE hope to continue to investigate this phenomenon and its consequences in natural populations.

Original Article:

This research was made possible by funding from the Natural Environment Research Council (NERC) and the Biotechnology and Biological Sciences Research Council (BBSRC).