Sex-specific additive genetic variances and correlations for fitness in a song sparrow (Melospiza melodia) population subject to natural immigration and inbreeding

ABSTRACT Quantifying sex-specific additive genetic variance (V A ) in fitness, and the cross-sex genetic correlation (r A ), is pre-requisite to predicting evolutionary dynamics and the magnitude of sexual conflict. Quantifying V A and r A in underlying fitness components, and multiple genetic consequences of immigration and resulting gene flow, is required to identify mechanisms that maintain V A in fitness. However, these key parameters have rarely been estimated in wild populations experiencing natural environmental variation and immigration. We used comprehensive pedigree and life-history data from song sparrows ( Melospiza melodia ) to estimate V A and r A in sex-specific fitness and underlying fitness components, and to estimate additive genetic effects of immigrants as well as inbreeding depression. We found substantial V A in female and male fitness, with a moderate positive cross-sex rA. There was also substantial V A in adult reproductive success in males but not females, and moderate V A in juvenile survival but not adult survival. Immigrants introduced alleles for which additive genetic effects on local fitness were negative, potentially reducing population mean fitness through migration load, yet alleviating expression of inbreeding depression. Substantial V A for fitness can consequently be maintained in the wild, and be concordant between the sexes despite marked sex-specific V A in reproductive success.