The impact of natural and sexual selection on genetic diversity has been intensively studied in both natural and captive-bred populations , but the majority of our current knowledge in this area is derived from species with conventional sex roles, with choosy females and competitive males [2, 3]. Sex-role reversed species, in which females compete for mating opportunities and males are choosy [4, 5], offer exceptional opportunities to investigate central tenets of sexual selection theory and the importance of sexual selection in the maintenance of genetic diversity.
The hypervariable major histocompatibility complex (MHC/MH) has proven to be a powerful model in which to investigate the importance of natural and sexual selection in shaping genetic diversity [6–8]. The MHC is an essential part of the vertebrate adaptive immune system, and includes a suite of more than 200 genes involved in the destruction of infected cells and the antibody response . There are two major antigen-presenting groups of MHC molecules, class I and class II genes, which differ in their function, structure and pattern of expression . The peptide binding region (PBR) of MHC loci encodes a groove that permits the binding of specific antigens, and this region typically exhibits the highest sequence polymorphism within the gene .
The investigation of MHC genes in a diversity of vertebrates indicates that these loci are more diverse than any other gene family . Natural selection on MHC is thought to be driven primarily by pathogens, leading to balancing selection that acts on the PBR of MHC genes . Balancing selection operates through either negative frequency-dependent selection, in which the relative fitness of individual alleles is influenced by their frequency (reviewed in ), or via heterozygote advantage. The advantage of MHC heterozygosity lies in the potential increase of the number of different parasite-derived antigens that can be detected by a MHC-heterozygous individual's immune system . MHC diversity can be further enhanced by selection on linked genes, due to genetic hitchhiking [13, 14]. In addition to the importance of MHC genes as an integral part of the adaptive immune system, MHC-mediated odor cues have been shown to be important in mate choice, kin recognition and inbreeding avoidance [15–19]. Disassortative mating is widely believed to promote MHC diversity and to increase the proportion of heterozygote individuals in natural populations [15, 20, 21]. Sexual selection can thus directly contribute to MHC allelic diversity via disassortative mate choice .
Despite consistently high levels of variation, there are major differences in the genomic organization of MHC genes in different vertebrate groups. While these loci are physically linked in mammals, class I and II genes are unlinked in bony fishes (class Actinopterygii) [22, 23]. Due to the lack of linkage of MHC genes in actinopterygians, Stet et al.  have suggested that major histocompatibility genes in these species are most accurately termed MH loci. The unlinked nature of MH genes may provide increased evolutionary flexibility and contribute to enhanced MH diversity in this group. MH gene diversity is highly variable in teleost fishes, and while some species have a single classical MH class II beta-chain gene (MHIIβ) (e.g. salmonids [24, 25]), most species have multiple copies of this locus (e.g. sticklebacks: 4-6 copies , perch: >8 copies , cichlids: >10 copies ). This variation may be due, at least in part, to ancestral chromosome or genome duplications .
While previous studies on teleosts have shown that both natural and sexual selection structure MH allelic diversity in species with conventional female-based mate choice [16, 30, 31], no study to date has investigated MH variation in sex-role reversed species in which mating decisions are made by the male. Males and females often differ in their ability to detect odor cues [32, 33], and sex differences in the production, processing and use of MH-mediated signals are expected to influence the relative efficiency of sexual selection in sex-role reversed and conventionally-mating species, potentially reducing the level of MH variation in species with reversed sex-roles.
The teleost family Syngnathidae (seahorses and pipefish) is a well-suited model system to study questions concerning the relationship between sex roles and MH diversity. Both conventional and sex-role reversed species exist in the family and sex-role reversal has evolved several times independently in this group . Studies of wild populations of the potbellied seahorse, Hippocampus abdominalis, have found evidence of female-female competition and male mate choice, suggesting that natural populations of this species are sex-role reversed .
Here, we characterize MH-variation in wild-caught and captive-bred individuals of sex-role reversed populations of the potbellied seahorse, a species with a highly developed form of male parental care. Genome sequencing and transcriptome screening confirm the existence of a single, highly variable copy of the MHIIβ locus in this species, with a pattern of variation identical to that detected in species with conventional sex roles. This pattern of genetic variation has been influenced by a combination of intralocus recombination and positive selection on sites believed to be important for peptide binding. MHIIβ is expressed in brood pouch tissues of male seahorses, suggesting that these molecules may be functionally active during male pregnancy. Our results indicate that sex-role reversed taxa such as the seahorse are capable of maintaining the high MHC diversity typical of vertebrate species with conventional sex roles.