Teorías que se centran en el Texto: Gramáticas de los cuentos (Rumelhart) y Teorías de

There are many different kinds of benefit that can be accrued by choosing certain mates rather than others. Whereas males generally increase their fitness by mating with many females, females maximise their fitness through a choice for male 'quality' (Andersson, 1994). Fitness benefits that choosy females may accrue include higher fecundity (highly fertile males); increased offspring survival (the result of a choice for male parental abilities); immediate gains (if males contribute or hold access to limiting resources); and increased fitness benefits from choosing mates whose genotypes are complementary. Breeding male naked mole-rats do not appear to care for pups any more than do non-breeders nor do they hold access to or contribute limiting resources. I shall therefore restrict the discussion of mate-choice to choice for genetic benefits and increased fecundity.

1.7.1 Choice for genetic benefits

The ability to learn genetic distinctions, allows animals to make decisions based on them in different contexts. One of the proposed functions of kin recognition is the avoidance of extreme inbreeding and/or outbreeding (Bateson, 1983; Hepper, 1991). Inbreeding is the mating of individuals related by ancestry. Since all animals are related if their ancestry is traced back far enough, in general practice, inbreeding is used to describe matings between close relatives (first cousins or closer).

Inbreeding tends to increase homozygosity in an originally outbred population and maintains homozygosity in a continuously inbred population, although mutation, gene flow and selection can modify the outcome of a particular pattern of inbreeding

(Falconer, 1981; Charlesworth & Charlesworth, 1987). In many laboratory studies an artificially imposed increase in the extent of inbreeding often results in a decline in many aspects of offspring fitness (Falconer, 1981); and in natural populations; inbred matings have been shown to have a similar effect (Packer, 1979; Greenwood, Harvey & Perrins, 1978). This decline in fitness is known as inbreeding depression. There are two classical rival theories proposed to account for inbreeding depression (Charlesworth & Charlesworth, 1987). One theory proposes that a decline in fitness associated with inbreeding depression is a result of inbred lines becoming fixed for recessive or partially recessive deleterious alleles (Charlesworth & Charlesworth, 1987). The other theory argues that inbreeding depression is due to the superiority of heterozygotes (heterosis or overdominance) over homozygotes (Patridge, 1983; Charlesworth & Charlesworth, 1987). A review of theoretical and empirical evidence suggests inbreeding depression results mainly from the unmasking of deleterious recessive alleles and, to some extent, the loss of heterosis (Charlesworth & Charlesworth, 1987; Pusey & Wolf, 1996). Another consequence of inbreeding is that heterozygosity is reduced and with it the opportunity for recombination. It has been suggested that recombination is an adaptation for life in saturated and/or heterogenous environments known as Bell's Tangled bank hypothesis (Bell, 1982) or temporally heterogenous environments, known as the Red Queen hypothesis (Jaenike, 1978). Outbreeding counteracts the effects of inbreeding depression by increasing genetic variability in a population. Even in predominantly inbreeding species, the loss of evolutionary potential may be alleviated to some extent even if they outcross occasionally (Maynard Smith, 1978). However there may be costs associated with outbreeding, other than outbreeding depression, which include the cost of finding a mate, dispersal from the natal area and establishment of a new one (Bengtsson, 1978).

1.7.2 Dissasortative mating in mammals

In rodents, two lines of evidence indicate that mate choice is influenced by kin recognition and that individuals typically avoid inbreeding. Firstly, non-sibling mating pairs are usually found to reproduce at higher rates that full-siblings (e.g..

Microtus spp.: McGuire & Getz, 1981; Boyd & Blaustein, 1985; Peromyscus spp.: Dewsbury, 1982). Secondly, adult animals prefer opposite-sex animals (or their urine odours) according to kinship in several choice situations (e.g.. Microtus spp.: Bolhuis

et al., 1988; Peromyscus leucopus; Grau, 1982). In laboratory strains of house mice, preference tests yield conflicting results. Preference tests using wild house mice appear to yield more consistent results, with individuals avoiding close inbreeding (Winn & Vestal, 1986; Krackow & Matuschak, 1991).

Perhaps the strongest evidence for dissassortative matting with respect to genotype comes from studies of the MHC in rodents and the f-complex in mice. The results of several lines of direct evidence suggest that selective neutrality is not responsible for the high levels of polymorphism exhibited by some MHC loci (Hughs & Nei, 1988; 1989). It is argued that the avoidance of inbreeding is one function of MHC-based disassortative mating (Brown, 1983; Partridge, 1988; Uyenoyama, 1988; Alberts & Ober, 1993; Brown & Eklund, 1994), if not the main function (Potts & Wakeland, 1990; 1993; Potts et al., 1994; reviewed by Jordan & Bruford, 1998). Inbreeding depression is a strong and persuasive phenomenon in living systems (Charlesworth & Charlesworth, 1987), and many specific mechanisms have evolved throughout the plant and animal kingdoms to avoid inbreeding (Uyenoyama, 1988; Blouin & Blouin, 1988). Inbred mice and rats in some cases express mating preferences only on genetic difference at the MHC (Yamazaki etal., 1976; Singh et al., 1987; Beauchamp et al., 1988). Extending the previous work by Boyse (1987) by employing a different genetic background and using MHC haplotypes recently isolated from wild populations, Egid and Brown (1989) demonstrated that female mice of two congenic mice strains prefer to mate with males of a different MHC type than their own. In contrast males of the same congenic strains showed no such odour nor mate choice preference for MHC dissimilar haplotypes (Eklund et a l., 1991). Their work supports the findings of Beauchamp et al. (1988) that the expression of a preference within a congenic strain may vary between the sexes. Laboratory studies should however be treated with caution. The results obtained may be an artifact of the artificial social conditions and the use of inbred strains of mice, thus making it impossible to draws strong conclusions about wild mice (Manning etal., 1992).

The most convincing evidence for MHC-disassortative mating preferences in mice comes from studies of wild-derived mice in semi-natural enclosures. In such a study Potts et al. (1991) found that 27% fewer offspring were homozygous for the MHC than expected from random mating and that this value was not the result of neonatal mortality suggesting that mating preferences alone could result in the observed deficiency in H-2 homozygosity. Hedrick (1992) developed a theoretical model to determine whether this observed deficiency in H-2 homozygosity in semi­ natural populations of mice could be the result of female mating preferences and concluded that female preferences for MHC-dissimilar mates could alone, rather than disease, account for the observed deficiency of H-2 homozygotes. This supports Potts

et al. (1991) argument that mating preferences alone are strong enough to account for most of the MHC genetic diversity found in natural populations of mice. Finally, Penn & Potts (1998) have demonstrated that female wild-derived mice in semi-natural enclosures avoid mating with males carrying familial genes and that the MHC-related

mating preferences of mice are the result of familial chemosensory imprinting. Mice learn the MHC identity of their parents and avoid mating with individuals carrying familial MHC genes (Beauchamp et al., 1985; Penn & Potts, 1998). Although most research on the role of the MHC in mate choice has focused on rodents, it has also been found that humans prefer the odour of MHC-dissimilar individuals (Wedekind et al., 1995; Wedekind & Furi, 1997). There is also evidence that humans have MHC- disassortative mating preferences (Ober etal., 1997).

Mate choice studies in mice carrying the f-complex also provide strong evidence for dissassortative mating. The f-complex is a well known, highly polymorphic genetic system in mice that has been implicated in mate-choice. The allelic variants are referred to as either wild type (+) or 'f-' haplotype (0; there are at least 15 f-haplotypes. Around 10-20% of wild mice carry the f-complex inversion on chromosome 17 (Baker & Pomiankowski, 1995). Animals that are homozygous for the same f-haplotype die before birth. Animals that have two different f-haplotypes have more or less decreased survival and males are sterile. Lenington et al. (1994) showed that females mate disassortatively by haplotype and their degree of discrimination is related to the fitness advantage of disassortative mating.