Male ejaculates are able to modify female behaviour in advantageous ways, often increasing male fitness. This is particularly beneficial to species where the sperm from the last male takes precedence. Male fitness can be enhanced by:
- Promoting the female to lay eggs shortly after copulation
- Induction of refractoriness in the partner to remating i.e. Creating a period of time in which the female is unwilling to mate
Promoting egg laying shortly after copulation
The promotion of egg laying in female drosophila (fruit flies) by the male soon after copulation increases his fitness. This is achieved by secreting the accessory gland protein ‘Acp26Aa’ (also known as ovulin) along with sperm, during copulation.
Ovulin is typically found only in the male as it is expressed in the accessory glands (glands which cooperate with the male reproductive organs to produce various peptides). However, in an experiment by Heifetz et al. (2005), female fruit flies were modified to produce ovulin by inclusion of the Acp26Aa gene into their genome. The peptide was inducible following a heat shock.
The results of the experiment showed that when the Acp26Aa gene was inactive, the number of eggs produced by the female was fairly low. However when the gene was activated i.e. following a heat shock, the number of eggs being produced was relatively large. This is due to the production of ovulin, following the activation of the Acp gene.
The experiment showed that in the presence of ovulin, the female produces many more eggs. In the wild, it is the males which produce ovulin and by secreting ovulin into the females, he can ensure that the female lays eggs soon after copulation. This is beneficial to the male as it means the eggs are highly likely to be fertilised by his sperm.
Ovulin works by stimulating female reproductive tissues (specifically the muscles) associated with oviposition, thus increasing the rate at which eggs are passed through the reproductive tract by the muscles.
Inducing refractoriness to remating
Remating refractoriness can be induced by another Acp gene, Acp70A. This gene encodes for the protein, Sex peptide.
Sex peptide is transferred along with male semen to produce refractoriness in the female, thus reducing her willingness to remate. This is beneficial to the male as it means his sperm is less likely to be subject to sperm competition from other males. Sex peptide requires the presence of sperm in order to produce remating refractoriness i.e. the male cannot simply secrete sex peptide alone to induce refractoriness. This is due to the fact that sex peptide binds to the tails of the sperm.
The secretion of sex peptide into the female (bound to sperm tails) reduces the willingness of the female to remate, even after 48 hours.
Slow release of Sex peptide
Because sex peptide is bound to the tail of sperm cells, it is able to be released slowly into the female. Release of sex peptide from sperm cells is slow because the peptide must be cleaved from the tail of the sperm by enzymatic action. Because of this, it can be present in the female reproductive tract for up to five days. This has the obvious benefit of causing longer periods of refractoriness in the female.
Release of accessory fluid peptides according to perceived risk
The release of accessory fluid peptides varies according to the perceived risk of sperm competition. An experiment by Wigby et al. (2009) attempted to show this. When sperm competition may be high, (produced in this experiment by keeping males in pairs thus the male presumes competition) the release of accessory peptides is greater. When the males were kept alone i.e. No perceived threat, the release of peptides was much lower.
Accessory fluid constituents and the cost of female mating
Accessory fluids can reduce the survivability of females, the presence of such peptides in the female reproductive tract can decrease the fitness of the female. Sex peptide is no exception to this and it is a large contributor to the cost of mating.
An experiment by Wigby & Chapman (2005) showed that females mating to males which had the sex peptide gene removed (and thus did not produce the protein) had a greater total egg production, thus showing greater survivability – reaching the conclusion that sex peptide causes harm to females.
Other constituents can also cause harm for example, the accessory gland peptide Acp62f. In an experiment, after around 14 days of initial copulation with a male producing Acp62f peptides, 0% of drosophila females survived. However in the control group (not mated to a male), there were still around 80% of females which had survived over the same time period.
Acp62f was the only gene from 8 which were tested, that caused harm to the female. The conclusion was that around 10% of Acp genes can cause females harm.
The peptide product of the Acp62f gene was characterised as a sine protease inhibitor, an enzyme which prevents the breakdown of proteins. This is possibly a useful accessory gland protein as it will likely maintain the sperm, keeping them in a good condition.
Problems arise from constituents when they leave the female reproductive tract. Whilst 90% of Acp62f remains in the female tract, 10% therefore moves into other female tissues. In other Acp peptides, this may be either purposeful or accidental. However, it is the presence of the Acp peptides in the female tissues which is responsible for causing harm.
Why do males cause harm to females?
Damage may be accidental, for example, the accidental leakage of accessory peptides in to the female tissues. Damage may also occur during copulation, due to the friction etc.
Damage could also be deliberate. Deliberate damage may be beneficial for a number of reasons:
- Allows for longer copulation (e.g. Barbed penis, prevents removal from female), this in turn allows the transfer of more sperm and thus a higher fertilisation rate for the male.
- Damage to the female may alter her behaviour, which increases male fitness, for example; a damaged female is more likely to reproduce as she is aware of her decreased fitness – she is intent to produce offspring before her death.
- Damage can also dissuade females from remating with other males
Females however, can grow resistant to the damage done to them, further experiments on drosophila species showed that:
- Females in a male biased environment are more likely to mate and females in a female biased environment are less likely to mate
- Females in the males biased environment were subject to more copulations and were exposed to more sex related damage
- Due to the increased number of copulations, future copulations posed less of a risk to the survivability of the female
- It was concluded that females can thus build up a resistance to the damage caused to them during copulation
Accessory fluid evolution
When mating with males that cause damage to the females, the females will develop a resistance to the damage done. The female engages in an evolutionary ‘arms race’ with the male to protect herself from the constantly evolving accessory peptides. (A co-evolutionary Red Queen cycle)
Evolution of the accessory fluid peptides can occur in two ways; either the sequence of the existing proteins is altered (thus enhancing their activity) or new proteins evolve with new biological activity.
To evolve, the DNA sequence of the proteins must alter. To measure the rate of adaptive evolution i.e. how fast a species is evolving in a co-evolutionary manner, you must compare the nucleotide sequences between closely related species.
The two types of DNA changes which can occur are:
1. Silent – Change in nucleotides but no change in protein sequence
2. Active – Change in nucleotides and a change in protein sequence
The number of active changes is known as dN and indicates the rate of adaptive evolution.
The number of silent changes is known as dS and indicates the rate of background level evolution of genes.
A higher dN/dS ratio = a higher rate of adaptive evolution
Acp genes have a much higher rate of adaptive evolution (dN/dS). This is due to the greater number of active changes per silent change.
The opposite is true for ‘normal genes’ however, indicating that there is more background evolution occurring.
All the above is not just true for insects however, looking at the divergence of human genes, you can see that sperm and egg genes are evolving rapidly. This leads to the conclusion that male expressed genes are responsible for the fast rate of accessory gland protein evolution.