Paternal sex allocation: how variable is the sperm sex ratio?
Think back to the classes you took on cell division. You might recall that in male mammals, meiosis is an important part of spermatogenesis. Chromosomes in diploid cells separate as the cell splits in two: one half gets the X chromosome the other the Y. So this means that the ratio of X to Y chromosomes in sperm is 50:50, right? Well… maybe not! In this paper Edwards and colleagues used a very cool microscopy technique called fluorescence in situ hybridisation (FISH) to stain the X chromosomes (red) and Y chromosomes (green) in sperm from mice. They counted the ratio of X to Y, and found an overall bias towards sperm containing Y chromosomes, and specific individuals that produced a much higher or lower proportion of sperm containing Y chromosomes than expected.
Why do we care? Understanding X:Y chromosome ratios in sperm might help us to better understand how sex allocation works in wildlife populations that have skewed sex ratios. Traditionally this field has focused on the role of mothers in “selecting” whether to produce male or female offspring, under the assumption that the contribution of X and Y sperm from fathers was equal. Now we need to consider the possibility that males may also “select” their offspring’s sex (NB I am using inverted commas here because this is not necessarily a conscious “choice”, but it is much easier to think about this way!). This might also have implications for management of threatened species. For example, in species with very small population size, even a slightly skewed sex ratio can cause problems for conservation breeding programs. Understanding the mechanisms that lead to skewed sex ratios may help future conservation efforts.