Sex is not much fun for female chickens. Even though they are likely to have many partners, female chickens have little choice over with whom they mate. On top of this, male chickens are anything but picky and will copulate with whoever comes their way, including their sisters. But female chickens can still have the last squawk—instead of choosing a partner, they select the sperm that fertilises their eggs.
It’s easy to understand why being promiscuous is advantageous for males: the more females they mate with, the more offspring they will produce. But female promiscuity (voluntary or forced) has long confused scientists. Mating is usually a dangerous affair for females; males are often so aggressive during sex that they seriously injure their partner. Besides, females (and ultimately their offspring) should in theory gain more from mating only with a champion male that carries the best genes—why bother with the others? In evolutionary terms, female promiscuity just doesn’t make sense. So why is it so widespread in nature?
It appears that promiscuous females can pick who fathers their children after copulation. This so-called ‘cryptic female choice’ has been described in insects, reptiles, snails, spiders and birds. Which takes us back to chickens. After forced mating with several males, female red jungle fowl—the ancestor of the domestic chicken—can squeeze out unwanted sperm and keep only the sperm from their favourite mate in their reproductive track. Fowls use cryptic female choice to avoid inbreeding, for example, by selecting against sperm from their brothers. But it’s also possible that sperm is selected based on genetic compatibility of particular sets of genes.
Researchers from the Universities of East Anglia and Oxford (UK) recently tested this hypothesis in fowls by looking at major histocompatibility complex (MHC) genes, which encode for key proteins involved in immunity. MHC genes come in a lot of ‘flavours’ that are linked to an effective immune response—individuals with a diverse mix of MHC genes are less likely to get sick and die from disease.
Hanne LØvlie and colleagues asked whether fowls use cryptic female choice to make sure their offspring inherits a mixed MHC gene pool. They singly mated females with related or unrelated males after sequencing the MHC genes in all animals. They then calculated the fertilisation rate of each mating by scoring the number of holes made by sperm cells in egg yolk membranes.
The researchers found that more sperm reached the eggs when males were unrelated to the females, and this effect was even stronger when these males had a very different MHC gene mix from their partner. However, when the females were inseminated artificially, the fertilisation bias disappeared—eggs were fertilised at a similar rate by all sperm. These results suggest that female fowls somehow pick the male with the best set of MHC genes during mating, and then get rid of the sperm from other males by cryptic female choice. Evolutionary speaking, girl power wins.
References:Lovlie H., Gillingham M.A.F., Worley K., Pizzari T. & Richardson D.S. (2013). Cryptic female choice favours sperm from major histocompatibility complex-dissimilar males, Proceedings of the Royal Society B: Biological Sciences, 280 (1769) 20131296-20131296. DOI: 10.1098/rspb.2013.1296
Images from Wikipedia Commons.
This article was originally published in Lab Times on the 19-11-2013 (print).