Sex ratio conflicts

In social insects, sex allocation is complicated further by the relatedness between haplodiploids. Table 6.4 shows us that a reproductive female (queen) shares the same level of relatedness (r = 0.5) with both her sons and her daughters, and therefore her ideal sex ratio is 1:1. Female workers, on the other hand, who do not produce offspring, share a higher degree of relatedness with their sisters (r = 0.75) than their brothers (r = 0.25), and therefore their ideal sex ratio in the colony is 3 : 1 in favour of females. This leads to a conflict over sex allocation between workers and queens, particularly in monogynous colonies in which the queen has a single mate, because then the offspring will all be full-siblings of the workers. Because workers outnumber the queens and are also the ones that rear the larvae, they should have the upper hand in this conflict and we therefore may expect that sex ratios should approach the workers' ideal. This has prove to be the case in a number of monogynous species, for example in the ant species Colobopsis nipponicus and Leptothorax tuberum the proportion of females in numerous colonies was found to be around 0.75 (Hasegawa, 1994; Pearson, Raybould and Clarke, 1995).

Workers may control sex ratios in a colony either by killing male larvae or by controlling the proportion of females that develop into reproductive adults (potential queens) versus sterile workers. Since adult males and females can be easily identified in social insect colonies, their sex ratios can be obtained without the aid of molecular markers, but the mechanisms of sex ratio manipulation cannot be understood fully without using molecular data to compare primary and secondary sex ratios. In a study of the ant Leptothorax acervorum, researchers used microsatellite markers to genotype eggs, and from these data they learned that the sex ratio did not change between eggs and adults. They therefore concluded that workers were obtaining their optimal sex ratio by manipulating the proportion of females that developed into sterile workers and not by killing male larvae (Hammond, Bruford and Bourke, 2002). The situation is different in fire ants (Solenopsis invicta), which often have sex ratios that are intermediate to the ratios that should be favoured by workers and by queens. Once again, microsatellite data were used to genotype eggs and obtain a primary sex ratio, and these data revealed that queens were biasing the sex ratio of their eggs in favour of males, thereby forcing workers to raise a higher proportion of males than that dictated by their optimal sex ratio (Passera et al., 2001; see also Box 6.4).

6.4 Biased sex ratios in adult populations

Many surveys of wild populations have revealed an excess of either male or female adults. The reasons for this are not always well understood, although molecular sex probes have allowed researchers to test a number of possible explanations. Populations of western sandpipers (Calidris mauri) in the northern part of their range show a male to female ratio of around 3:1. Predation by peregrine falcons is high in these populations, and researchers wanted to know whether or not the shortage of females could be attributed to disproportionately high predation rates. They removed feathers from the remains of individuals that had been recently preyed upon and from these they amplified the CHD genes. This told them that around 24 per cent of the carcasses tested were female. Because the proportion of females was comparable in living and dead birds, sex-biased predation was not a plausible explanation for the male-biased sex ratios in these populations (Nebel, Cloutier and Thompson, 2004).

In contrast to western sandpipers, females often predominate in populations of the knobbed whelk (Busycon carica), sometimes outnumbering males by as much as 10:1. A polymorphic microsatellite locus has been identified as a sex-specific marker in this species, because it is usually heterozygous (two alleles) in females and hemizygous (only one allele) in males. Researchers used this marker to genotype the embryos from two knobbed whelk broods, and found that 383/768 embryos (49.9 per cent) were female. As a result, the female-biased sex ratios in knobbed whelk populations cannot be attributed to a skewed primary sex ratio, but instead must be explained by some other factor such as a relatively high mortality of juvenile or adult males, or the use of different habitats by males, and females (Avise, Power and Walker, 2004).

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