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. 2005 Mar 22;102(12):4414-8.
doi: 10.1073/pnas.0408264102. Epub 2005 Mar 8.

Mate Choice Decisions of Stickleback Females Predictably Modified by MHC Peptide Ligands

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Free PMC article

Mate Choice Decisions of Stickleback Females Predictably Modified by MHC Peptide Ligands

Manfred Milinski et al. Proc Natl Acad Sci U S A. .
Free PMC article

Abstract

Sexual selection has been proposed as one mechanism to explain the maintenance of high allelic diversity in MHC genes that control the extent of resistance against pathogens and parasites in natural populations. MHC-based sexual selection is known to involve olfactory mechanisms in fish, mice, and humans. During mate choice, females of the three-spined stickleback (Gasterosteus aculeatus) use an odor-based selection strategy to achieve an optimal level of MHC diversity in their offspring, equipping them with optimal resistance toward pathogens and parasites. The molecular mechanism of odor-based mate-selection strategies is unknown. Because peptide ligands for MHC class I molecules function as individuality signals in mice, we hypothesized that female sticklebacks might assess the degree of MHC diversity of potential partners by means of the structural diversity of the corresponding peptide ligands in perceived odor signals. We show that structurally diverse MHC ligands interact with natural odors of male sticklebacks to predictably modify MHC-related mate choice. For a mating pair with suboptimal numbers of MHC alleles, peptides increase the attractiveness of male water, whereas for a mating pair with superoptimal numbers, attractiveness is decreased. Our results suggest that female sticklebacks use evolutionarily conserved structural features of MHC peptide ligands to evaluate MHC diversity of their prospective mating partners.

Figures

Fig. 1.
Fig. 1.
Schematic representation of structure of MHC/peptide complexes. (A) The peptide-binding groove of MHC class I molecules accommodates peptides (red) generally of 9 aa in length, with the N and C termini tightly fixed in the edges. The most significant contacts between peptides and the MHC molecules are mediated through the side chains of so-called anchor residues of the peptide (shown as stick symbols) fitting into pockets of the MHC molecule. The specificity and the precise location of these pockets vary among different MHC molecules; in this way, the structural polymorphism of MHC molecules influences peptide-binding specificity. (B) The peptide-binding groove of MHC class II molecules is not closed at the ends, allowing peptides (red) to protrude out of the pocket. As in MHC class I molecules, the central core of the peptide is locked into the groove by means of anchor/pocket interactions. The N- and C-terminal overhangs are thought to be prone to exonucleolytic degradation (symbolized by dashed lines and scissors), whereas the core sequence is protected (16).
Fig. 2.
Fig. 2.
Modulation of odor-based preference of females for a potential mate through four different stickleback 9-mer peptides. (A) In a two-choice flow channel, single gravid females chose between the water from the tank of a single male supplemented with either solvent only or four stickleback peptides in solvent. The time females spent in the side of the flow channel to which water from the tank of a single male plus peptides in solvent was added (of a total of 600 s) is shown as a function of the combined number of different MHC class IIB alleles of male and female. (B) Reversal of female response to added peptides on the day after spawning; results are shown otherwise as in A.
Fig. 3.
Fig. 3.
Structure/function relationship of peptides in modulation of mate choice. (A) Modulation of odor-based preference of gravid females for a potential mate through five xenogeneic 9-mer peptides, shown otherwise as in Fig. 2. (B) Structural requirements for peptides as odor cues in MHC-related mate choice. Females chose between four different xenogeneic 9-mer peptides and the same number and concentration of the corresponding modified peptides (see text for details), shown otherwise as in A.

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