Sexual selection and the MHC in the fowl

In many species females prefer to mate with specific males. In addition, when they mate with multiple males, females often favour fertilisation by the sperm of specific partners. Why do females favour certain males over others? It has been suggested that females may select males that carry genes which help the individual cope with their environment and by doing so the females thereby obtain the same high quality paternal genes for her young. This idea remains contentious and little tested. The major histocompatibility complex (MHC) is a group of genes that help determine resistance to parasites and diseases in higher animals (i.e. vertebrates, including humans). The type and variety of MHC genes an individual carries determine how well they can cope with parasites and diseases. Females may therefore increase the health of their offspring by selecting mates on the basis of MHC characteristics. This may happen in three different ways. First, females may favour fertilisations by males carrying an MHC that is different from their own. This will ensure that the offspring will inherit many different types of MHC genes (alleles) from their parents, and thus will be able to defend themselves from a wider range of diseases. Second, females may increase the MHC diversity of the offspring by favouring males that have a diverse MHC. Third, females may prefer males carrying specific MHC genes that provide more efficient resistance against diseases than other MHC types currently available. Understanding how MHC diversity influences reproduction is important for our understanding of the evolution of female mate choice, male sexual ornaments (e.g. stags antlers) and reproductive strategies. It may also allow us to better manage fertilisation and fertility problems in domestic and pest species and in species at risk of extinction. Despite much interest in the MHC and reproductive behaviour, the mechanisms through which the MHC may influence reproductive success remain unresolved. Our proposed research investigates the role of the MHC in the reproductive strategies of birds, and aims to determine if and how the different possible MHC-based mechanisms outlined above operate at specific reproductive stages: from mate choice to the selection of sperm within the female reproductive tract after insemination. We will study two semi-natural populations of fowl, Gallus gallus. The fowl is an ideal animal to study for two important reasons. First, it is a model system of reproductive physiology and behaviour and much is known about the mechanisms that determine mating and fertilisation success. Second, the MHC of the fowl is relatively small and easy to study. This has enabled extensive studies of the fowl MHC and its associations with disease resistance: more is known about the link between the MHC and diseases in the fowl than in any other animal. We will first: (a) determine how the MHC of males and/or their similarity with the females MHC females accounts for differences between the reproductive success of different males in small natural groups, (b) experimentally investigate how much the MHC determines male traits (e.g. social status, comb size and sperm quality) which influence a males ability to mate with females, and the fertilising efficiency of their ejaculates, (c) experimentally test whether female preference for certain mates is explained by MHC mechanisms. We will then focus on events occurring during/after insemination, by testing whether: (e) males invest more sperm in certain females based on female MHC characteristics, (f) females preferentially utilise the sperm of certain partners based on male MHC characteristics, and (g) the outcome of the competition between ejaculates that occurs when females copulate with multiple males (as it normally happens in the fowl), is determined by the MHC characteristics of males involved.