Female wild house mice nest (and often nurse) communally (Sayler & Salmon, 1971). In addition to themselves, their immediate siblings and their mother, developing pups are likely to experience their mother’s nestmate(s) (who may or may not be a related) and any of the
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nestmate’s pups. Other mice may enter the nest environment, including the dominant male in whose territory the nest lies (which may or may not be their father), and other adult, sub-adult and juvenile males and females. Given the range of potentially unrelated individuals that may enter the nest, mother and self are likely to be the most reliable relatives from which to learn a phenotypic template.
Studies of laboratory and house mice for match-to-self and match-to-maternal phenotype matching mechanisms is mixed. Cross-fostered adult house mice spend more time investigating the odours of conspecifics that are genetically more similar to themselves than those that are more distant (Heth et al., 2003). In a study of inbreeding avoidance in free- breeding house mice, female house mice avoided mating with males that shared both MUP haplotypes with themselves, and found no evidence for a match-to-maternal MUP type (Sherborne et al., 2007). The results found in Chapter 2 also suggest a match-to-self mechanism using MUP sharing, as subject females spent longer in the cages of related stimulus females if they shared more MUP peaks with the related stimulus female compared to with the unrelated stimulus female. However in that experiment maternal MUP patterns were not investigated.
In a study using cross-fostered female semi-wild mice (wild crossed with laboratory strains to produce mice with controlled MHC type) females preferred males with different homozygous MHC types from the foster-parents that reared them (Penn & Potts, 1998a). Similar studies using cross-fostered male laboratory mice have demonstrated an avoidance of females that have the same MHC type as the foster-parents that reared them (Beauchamp et al., 1988; Yamazaki et al., 1988). Yamazaki et al (2000) demonstrated laboratory mouse pup preference for odours of the MHC-type on which they were reared, however this preference was not wholly reversed by cross-fostering, suggesting that perhaps both self and maternal cues are important. Manning et al. (1992) also suggest that both self and maternal cues may be important as females mice resulting from crosses between laboratory strains and wild- caught house mice prefer nest partners that share the same MHC type as themselves, and the same MHC type as their parents, suggesting both match-to-self and match-to-maternal mechanisms.
3.2.4 Chapter Aim
The aim of this chapter is to investigate the origin of phenotypic recognition templates in female house mice. This was addressed by asking the following questions:
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i) Can female house mice discriminate urine from a half-sister from urine from an unrelated female, and do they show any attraction towards either?
ii) Do female house mice preferentially associate with half-sisters over unrelated females?
iii) Do female house mice show kin discriminative behaviour when allowed to interact with half-sisters compared to with unrelated females?
iv) Does kin discriminative behaviour for half-sisters compared to unrelated females depend on maternal and/or paternal lineage?
v) Does urine from half-sisters appear more similar to females than the urine from unrelated females?
Female house mice were tested for their ability to discriminate half-sisters from unrelated females, and whether this discrimination depended on relatedness through the maternal and/or paternal lineage. A match-to-self mechanism matches own phenotype against that of encountered novel individuals. An individual’s own phenotype is a product of the two alleles, one inherited from the mother and one from the father. The ability to recognise maternal and paternal relatives should therefore be equal. A match-to-maternal mechanism matches a maternally learnt template against that of encountered novel individuals. All maternally- descended relatives will share one allele at every locus with the mother, however all paternally-descended relatives may not share any alleles at any locus. The ability to recognise maternal and paternal relatives should therefore be different if a match-to-maternal mechanism is used, with maternal relatives being discriminated but not paternal relatives. Female discrimination was assessed using a scent discrimination and attraction assay, a nest partner choice assay and a female-female interaction assay. Additionally, the perceived similarity between urine from half-sisters was assessed using an odour-genes covariance assay.
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3.3 Methods
Consecutive litters of unfamiliar maternal and paternal half-sisters were bred and females tested for their ability to recognise half-sisters based on maternal or paternal relatedness. A match-to-maternal phenotype mechanism may enable maternal but not paternal half-sisters to be discriminate, whilst equal discrimination to both maternal and paternal half-sisters may be shown using a match-to-self mechanism.
3.3.1 Animal Housing and Handling
Subjects and stimuli were captive-bred adult females from an outbred colony of house mice. The colony was established using individuals captured from populations in the North West of England and outbreeding was maintained with regular introductions of wild-caught mice. Animal housing and handing methods were the same as detailed previously (see Section 2.3.1).