Genetica 104: 301–309, 1999. c  1999 Kluwer Academic Publishers. Printed in the Netherlands. 301 MHC variation in birds and reptiles H˚ akan Wittzell 1 , Thomas Madsen 2 , Helena Westerdahl 3 , Richard Shine 2 & Torbj¨ orn von Schantz 3 1 Department of Theoretical Ecology, Ecology Building, Lund University, S-223 62 Lund, Sweden; 2 School of Biological Sciences A08, University of Sydney, NSW 2006, Australia; 3 Department of Animal Ecology, Ecology Building, Lund University, S-223 62 Lund, Sweden Key words: birds, genetic variation, major histocompatibility complex, polymorphism, reptiles Abstract The major histocompatibility complex (MHC) has been studied in a multitude of mammals by now, but much less is known about its organisation and variation in other vertebrate species. The mammalian MHC is organised as a single gene cluster, but recent studies on birds suggest that this paradigm of MHC organisation has to be supplemen- ted. The domestic chicken thus possesses two separate gene clusters which both contain MHC class I and class II B genes, and we have shown that the ring-necked pheasant Phasianus colchicus also has two unlinked clusters of class II B genes. We are studying the effect of the MHC on mate choice, survival and reproductive success in natural pop- ulations of birds and reptiles. For this reason, we are developing DNA techniques to determine the animals’ MHC genotype. The amplification of the hypervariable exon 3 of the class I gene from songbirds and reptiles has provided us with species specific probes that can be used in Southern blot analysis. The first results indicate very extensive variation in all studied species, that is starlings Sturnus vulgaris, great reed warblers Acrocephalus arundinaceus and water pythons Liasis fuscus. The restriction fragment length polymorphism (RFLP) analysis also suggests that the number of MHC genes is significantly larger in these species than in pheasants and domestic chickens. Introduction Much interest has recently been paid to the possible role of parasites and infectious diseases in mate choice and sexual selection. The reason is an already classical paper by Hamilton and Zuk (1982), in which they out- lined how the evolutionary arms race between hosts and parasites continuously could give rise to heritable fitness differences in parasite resistance and in this way drive mate choice. Females who choose mates on the basis of male characters that indicate health and vigour would then increase their reproductive success by securing resistance genes for their offspring. The major histocompatibility complex (MHC) is only one of the genetic factors that may affect resistance to dis- ease and parasites, but its central role in the immune system makes it particularly interesting when testing Hamilton and Zuk’s hypothesis. In for example the domestic chicken, the MHC genotype is associated with resistance or susceptibility to several infectious diseases and autoimmune disorders (Kroemer et al., 1990a). Furthermore, the role of the MHC in the mate choice of mice is well-documented (Yamazaki et al., 1976, 1988; Potts, Manning & Wakeland, 1991). This is the reason why we are studying the effect of the MHC on mate choice, survival and reproductive suc- cess in natural populations, especially of birds and reptiles. The MHC has been studied in a multitude of mam- mals by now, but much less is known about its organ- isation and variation in other vertebrates. The mam- malian MHC has therefore come to form a paradigm of MHC organisation. This paradigm is usually illus- trated by the MHC of humans (HLA) and mice (H-2), which have the immunologically important class I and class II genes organised as two distinct, but genetically linked, gene clusters separated by a region of unre- lated genes (Trowsdale, 1995). Recent results from fish and birds suggest that this basic organisation of the mammalian MHC is not ubiquitous. There is still