C-RUI:Behavior and the Major Histocompatibility Complex in Savannah Sparrows

Skidmore College, Saratoga Springs NY

Investigators

Abstract

0116139-Freeman-Gallant 1. A lay abstract, with the title of the proposal and your full name at the top. Corey R. Freeman-Gallant and Nathaniel T. Wheelwright. "Inbreeding, Female Mating Fidelity, and the Major Histocompatibility Complex in Savannah Sparrows" Female birds often copulate outside of the pair-bond to produce broods of mixed paternity, and in some species, as many as 60% of young derive from extra-pair fertilizations. The ubiquity of this infidelity has important implications for our understanding of avian biology. Recent work has linked female mating fidelity to the ecology and evolution of male parental care, social mating systems, breeding dispersion and synchrony, and the form and function of elaborate sexual displays. Despite its importance, the adaptive significance of multiple mating remains controversial. While several recent studies support the idea that extra-pair copulations (EPCs) allow females to obtain good genes for their offspring (by trading-up) on the genetic quality of sires), many others have found no relationship between female mating fidelity and traits likely to reflect male quality. Indeed, much variation in female mating fidelity remains unexplained, and few feasible alternatives to the good genes hypothesis have been pursued in the field. A corollary to the good genes hypothesis proposes that females do use EPCs to increase the quality of young, but it is the interaction between maternal and paternal genomes"and not male quality per se"that is the target of female choice. This individual optimization or genetic compatibility hypothesis has now been explored in snakes, lizards, and pseudoscorpions, but few studies in birds have considered the importance of genotype-dependent mating preferences and optima. Preliminary work with Savannah sparrows (Passerculus sandwichensis), however, strongly suggests that females pair nonrandomly (and disassortatively) with respect to the major histocompatibility complex (Mhc), a group of genes responsible for eliciting the adaptive immune response of vertebrates. Moreover, a female's genetic similarity to her social mate predicts her fidelity: females paired to similar males are more likely to produce extra-pair young than females paired to dissimilar males. These results parallel similar studies in mice and humans, and suggest either that the benefits of genetic diversity (at the Mhc) drive female mating patterns or that the avoidance of inbreeding is an ultimate cause of social and genetic mate choice in Savannah sparrows. With NSF funding, new molecular and analytical techniques will applied to samples obtained during previous field work. Preliminary results with Savannah sparrows are based on 38 pairs nesting on Kent Island, New Brunswick, Canada in 1995. To determine whether females choose within- and extra-pair sires according to their Mhc genotype, and to incorporate other potential determinants of female mating fidelity (such as adult age, timing of breeding and phenotypic quality), the analysis will be extended to pairs nesting in 1994. In addition, the importance of overall levels of genetic relatedness to female mating fidelity, the occurrence of egg (embryo) mortality, and the survivorship of nestlings and fledglings will be assessed by surveying sparrows during the 2002 and 2003 breeding seasons. Both the costs of inbreeding and the benefits of Mhc heterozygosity predict that females should avoid fertilizations from genetically similar males. This research benefits from the prior development of a species-specific Mhc probe and nearly 15 years of field and molecular work on the Kent Island population of Savannah sparrows. This study will provide one of the first tests of the importance of the Mhc to social and genetic mating patterns in a free-living population of songbirds.