Bias in studies of heterozygosity and mate choice

A key question for molecular and behavioural ecologists who study mating systems is to understand why, in many species, females choose to mate with extra-pair males. Recently a possible explanation, 'genetic compatibility', has gained increasing empirical support (for a comprehensive review, see Kempenaers 2007 ). Genetic compatibility hypotheses assume that females seek extra-pair mates with alleles that complement their own. Typically, this will be achieved by mating with a male of a different genotype than her own, in order to maximise the heterozygosity of her offspring. Because numerous studies have indicated positive associations between heterozygosity and fitness (see Coltman & Slate 2003 ), it follows that mating with 'compatible' males will result in heterozygous, and therefore fit, offspring. Most empirical support for genetic compatibility has been obtained with microsatellite markers that have first been used to assess parentage and then to estimate relatedness and/or individual heterozygosity. A problem with this approach is a possible bias that favours the detection of extra-pair paternity when the extra-pair male has a genotype different from that of the female and her social mate. This in turn could lead to the erroneous conclusion that extra-pair males are less related to the female than within-pair males. In this issue of Molecular Ecology , Wetzel & Westneat 2009 (hereafter W&W), use simulation studies to assess the extent of this bias, using parameter estimates obtained from recent empirical data. They identify two forms of bias that may affect tests of the genetic compatibility hypothesis, and provide guidelines on how these biases may be avoided.     

No evidence for extrapair fertilizations in the merlin revealed by DNA fingerprinting

Broods of young merlins were compared with the adults in attendance at their nest by DNA fingerprinting. No offspring were found that mismatched genetically suggesting that intraspecific brood parasitism and extrapair fertilization are very rare in this population. The results are discussed in the light of the Paternity Assurance Hypothesis.     

Fitness consequences of hybridization between house sparrows (Passer domesticus) and tree sparrows (P. montanus)

Gene transfer may occur following hybridization between closely related species if hybrids are viable and able to breed with individuals of one or both of the parental species. House (Passer domesticus) and tree sparrows (P. montanus) occasionally hybridize and produce viable offspring. Previously, we concluded that male tree × house sparrow hybrids are most probably fertile based on the observation of a male F1 hybrid feeding the nestlings with a female house sparrow in two consecutive clutches. However, recent DNA analyses based on blood samples revealed that all nestlings (4) in the first clutch were sired by a neighbouring house sparrow male, whereas nestlings in the second clutch (2) were not blood sampled and most probably died before fledging. This indicates that extensive extra-pair fertilization confounded our previous conclusion, and indicates that social partnership and attending behaviour can be imprecise measures of paternity.     

COMPARATIVE EVIDENCE FOR THE EVOLUTION OF SPERM SWIMMING SPEED BY SPERM COMPETITION AND FEMALE SPERM STORAGE DURATION IN PASSERINE BIRDS

Sperm swimming speed is an important determinant of male fertility and sperm competitiveness. Despite its fundamental biological importance, the underlying evolutionary processes affecting this male reproductive trait are poorly understood. Using a comparative approach in a phylogenetic framework, we tested the predictions that sperm swim faster with (1) increased risk of sperm competition, (2) shorter duration of female sperm storage, and (3) increased sperm length. We recorded sperm swimming speed in 42 North American and European free-living passerine bird species, representing 35 genera and 16 families. We found that sperm swimming speed was positively related to the frequency of extrapair paternity (a proxy for the risk of sperm competition) and negatively associated with clutch size (a proxy for the duration of female sperm storage). Sperm swimming speed was unrelated to sperm length, although sperm length also increased with the frequency of extrapair paternity. These results suggest that sperm swimming speed and sperm length are not closely associated traits and evolve independently in response to sperm competition in passerine birds. Our findings emphasize the significance of both sperm competition and female sperm storage duration as evolutionary forces driving sperm swimming speed.