MHC-هتروزیگوسیتی و جذابیت چهره انسان

Females gain direct or indirect fitness benefits by choosing between males with traits indicating “good genes,” but we usually know very little about the nature of these genes. However, it has been suggested that genetic quality may often be defined as heterozygosity at certain loci. Here, we show that heterozygosity at three key loci in the major histocompatibility complex (MHC) is associated with facial attractiveness: Faces of men who are heterozygous at all three loci are judged more attractive by women than faces of men who are homozygous at one or more of these loci. MHC genes code for proteins involved in immune response. Consistent with this function, faces of MHC heterozygotes are also perceived to be healthier. In a separate test, in the absence of any other cues, patches of skin from the cheeks of heterozygotes are judged healthier than skin of homozygotes, and these ratings correlate with attractiveness judgements for the whole face. Because levels of MHC similarity can influence mate preferences in animals and humans, we conducted a second experiment with genotyped women raters, finding that preferences for heterozygosity are independent of the degree of MHC similarity between the men and the female raters. Our results are the first to directly link facial attractiveness and a measure of genetic quality and suggest a mechanism to help explain common consensus concerning individual attractiveness. In a relatively monogamous species like humans, evolutionary benefits from choosing heterozygous mates could include prolonged parental care and reduced risk of contracting disease for females and their offspring.

Sexual selection theory asserts that males maximise reproductive success through seeking multiple matings, while females achieve this goal through discrimination of mate quality, or choosiness (Bateman, 1948 and Trivers, 1972). Through mate choice, females gain direct and indirect benefits, where direct benefits include resources provided by males to females or offspring, and indirect benefits refer to genetic properties that enhance fitness of resulting progeny (Andersson, 1994 and Kirkpatrick & Ryan, 1991). Females may base their discrimination on phenotypic or behavioural variation among candidate males in traits that signal genetic quality (Hasselquist et al., 1996 and Petrie, 1994) or genetic complementarity (usually dissimilar genotype: Potts et al., 1991 and Yamaguchi et al., 1981), or both (Colegrave et al., 2002 and Roberts & Gosling, 2003). In most cases, we know little about the exact nature of the good genes involved, although Brown, 1997 and Brown, 1999 has recently suggested that genetic quality, in this sense, may often be defined as heterozygosity at certain loci. He proposed that heterozygosity should be correlated with the expression of condition-dependent male traits and that females should value heterozygosity in their offspring and, in some cases, their mates. Many studies investigating influences of heterozygosity on mating patterns have focussed on genes in the major histocompatibility complex (MHC, known in humans as human leukocyte antigen loci, HLA). MHC genes encode proteins involved in immunological self/nonself recognition and are among the most polymorphic in the genome (Mungall et al., 2003). It is now well-established that MHC disassortative mating preferences exist in several species, with beneficial effects in terms of offspring heterozygosity (Bernatchez & Landry, 2003, Jordan & Bruford, 1998 and Penn & Potts, 1998), but the extent to which heterozygosity in mates plays a role in female mate preferences is less clear. Preferences for heterozygosity in mates could result in direct benefits to females, for example, through reduced risk of disease transmission or provision of high-quality paternal care (Kirkpatrick & Ryan, 1991) because heterozygous males are often less susceptible to infectious diseases (Carrington et al., 1999, McClelland et al., 2003, McClelland et al., 2003, Penn et al., 2002 and Thursz et al., 1997). Although heterozygote mating advantages have been demonstrated less often than predicted by theory (Brown, 1997 and Brown, 1999), higher reproductive success has been found in male rhesus macaques, Macaca mulatta, which were heterozygous at a Class II locus ( Sauermann et al., 2001), and in male spotless starlings, Sturnus unicolor, of intermediate heterozygosity ( Aparicio, Cordero, & Veiga, 2001). Mate choice for males with high MHC-allelic diversity occurs in sticklebacks ( Reusch, Haberli, Aeschlimann, & Milinski, 2001), while in humans, Thornhill et al. (2003) found that women prefer odors of men who are relatively heterozygous at MHC loci. Such odor effects may not be independent of other cues ( Cornwell et al., 2004), and this raises the intriguing possibility that MHC genes might also be involved in facial preferences because facial attractiveness plays a key role in human mate choice. Facial characteristics are thought to advertise underlying “good genes” to potential mates (Penton-Voak et al., 2001, Perrett et al., 1994 and Thornhill & Gangestad, 1999). These characteristics include indicators of healthiness and developmental stability (Fink et al., 2001, Fink & Penton-Voak, 2002, Jones et al., 2001 and Thornhill & Gangestad, 1999) and, potentially, immunocompetence (Penton-Voak et al., 2001, Perrett et al., 1998 and Rhodes et al., 2003). Only recently, however, has the potential link between MHC genotype and facial preferences been investigated. In addition to odor preferences, Thornhill et al. (2003) tested facial preferences for MHC heterozygosity but found no significant effects. While the reason for the discrepancy between facial and odor preferences is unclear, it emphasises the need for further investigation of the potential links between MHC and facial attractiveness. Here, we test the hypothesis that MHC heterozygosity influences women's preferences for male faces. We presented women with photographs of men genotyped at three MHC loci and asked them to score each image for attractiveness. We compare scores of men who were heterozygous at all three loci with those who were homozygous at one or more of these loci. In addition, we compare two potential condition-dependent facial characteristics of men in each group, each of which have been previously linked to facial attractiveness: fluctuating asymmetry (Penton-Voak et al., 2001 and Perrett et al., 1999) and visible skin condition (Fink et al., 2001, Jones et al., 2004 and Jones et al., 2004). Finally, because MHC similarity is known to influence the attractiveness of human odor (Jacob et al., 2002, Thornhill et al., 2003, Wedekind & Furi, 1997 and Wedekind et al., 1995), we tested whether these attractiveness judgements were independent of levels of MHC similarity.