Adaptive sex ratios

The use of molecular data to obtain sex ratios has been particularly widespread in studies of birds. It is almost impossible to sex the adults of many bird species or the newly hatched chicks of virtually all bird species on the basis of external phenotypic characters, but they can be sexed from their genotypes. Recall from Chapter 2 that female birds are the heterogametic sex (ZW) whereas males are homogametic (ZZ). A chromo-helicase-DNA-binding (CHD) gene is located on each of the W and Z sex chromosomes of most bird species (CHD-W and CHD-Z, respectively). A pair of primers has been characterized that will anneal to a conserved region and amplify both of the CHD genes in numerous species (Griffiths et al., 1998). A variable non-coding region that is a different length in each gene means that the size of the product will depend on whether it was the CHD-W gene or the CHD-Z gene that was amplified. As a result, a single band (CHD-Z only) will result from the PCR of male genomic DNA, whereas two bands (CHD-Z and CHD-W) will result from amplified female genomic DNA (Figure 6.9).

Figure 6.9 A portion of CHD genes was amplified from male and female blue tits and chickens using primers P2 and P8 (Griffiths et al., 1998). Note that in both species two bands were generated from the female samples but only one band from the male samples. Photograph provided by Kate Orr and reproduced with permission

Figure 6.9 A portion of CHD genes was amplified from male and female blue tits and chickens using primers P2 and P8 (Griffiths et al., 1998). Note that in both species two bands were generated from the female samples but only one band from the male samples. Photograph provided by Kate Orr and reproduced with permission

These avian sex markers can be used on tissue that has been taken from eggs, although more accurate results are obtained from newly hatched nestlings. In a number of studies, the sex ratios determined from molecular data have added support to the theory of adaptive parental manipulation. Female blue tits (Parus caeruleus) produce more sons when mated to males that have a higher survival rate, a characteristic that females can gauge on the basis of the male's ultraviolet plumage ornamentation (Sheldon et al., 1999). In kakapo (Strigops habroptilus) and house wren ( Troglodytes aedon) populations, females were produced in excess when conditions were not conducive to the growth of particularly large and healthy offspring (Albrecht, 2000; Clout, Elliott and Robertson, 2002), presumably because weaker males are less likely to obtain mates than weaker females, particularly in polygynous species.

Many more examples of biased sex ratios have been found in birds (see Komdeur and Pen, 2002; Pike and Petrie, 2003). As yet there is no single theory that can explain this adaptive behaviour, in part because the reasons seem to vary both within and between species. Timing of egg production, parental quality, environmental conditions, and helpers at the nest may all influence sex ratios. Furthermore, the mechanisms for sex ratio manipulation remain unclear. Non-random segregation of sex chromosomes, selective resorbtion of yolk, selective ovulation, sex-specific fertilization, and sex-specific inhibition of zygote formation are just some of the mechanisms that have been proposed (Pike and Petrie, 2003, and references therein). So far, molecular data have helped to demonstrate the existence of sex ratio allocation in birds, but there is considerable work to be done before we understand the adaptive reasons and the mechanisms for producing an excess of males or females.

Birds are not the only taxonomic group in which molecular markers have been used to identify the sex of morphologically similar juveniles. Neither embryos nor tadpoles can be sexed in amphibians on the basis of external phenotypes, but a sex-linked gene, ADP/ATP translocase, has been used to differentiate between the homogametic and heterogametic forms of the Japanese frog Rana rugosa. In this species, interpretation of molecular data depends on which form is being studied, because in different forms the heterogametic sex is either the male (XX/XY) or the female (ZZ/ZW), and in some forms both sexes are homogametic (Miura et al., 1998). In one study, embryos from two populations of the ZZ/ZW form were genotyped by PCR-RFLP analysis of ADP/ATP translocase (Sakisaka et al., 2000). These data showed a significant bias towards male offspring at the start of the breeding season and a female-biased sex ratio towards the end of the breeding season. The authors of this study suggested that this switch could be explained by the relatively fast development of males which typically metamorphose into adults by the autumn, whereas the more slowly developing female tadpoles often hibernate throughout the winter.

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