Several studies have demonstrated that women show pre-copulatory mating preferences for human leucocyte antigen (HLA)-dissimilar men. A fascinating, yet unexplored, possibility is that the ultimate mating bias towards HLA-dissimilar partners could occur after copulation, at the gamete level. Here, we explored this possibility by investigating whether the selection towards HLA-dissimilar partners occurs in the cervical mucus. After combining sperm and cervical mucus from multiple males and females (full factorial design), we found that sperm performance (swimming velocity, hyperactivation, and viability) was strongly influenced by the male–female combination. This indicates that sperm fertilization capability may be dependent on the compatibility between cervical mucus (female) and sperm (male). We also found that sperm viability was associated with partners' HLA dissimilarity, indicating that cervical mucus may selectively facilitate later gamete fusion between immunogenetically compatible partners. Together, these results provide novel insights into the female-mediated sperm selection (cryptic female choice) in humans and indicate that processes occurring after copulation may contribute to the mating bias towards HLA-dissimilar partners. Finally, by showing that sperm performance in cervical mucus is influenced by partners' genetic compatibility, the present findings may promote a deeper understanding of infertility.

Human leukocyte antigen (HLA) immune genes play an important role in partner selection, but it has remained unclear if nonrandom pairing with respect to parental HLA genes could occur at the level of the gametes. We tested this possibility by investigating whether the sperm fertilization competence in humans is dependent on HLA genotype combination of the partners. We conducted a full-factorial experiment, in which the sperm physiological preparation for fertilization among multiple males was studied in the presence of follicular fluid (oocyte surrounding bioactive liquid) of several females. All the studied sperm pre-fertilization physiological parameters (motility, hyperactivation, acrosome reaction, and viability) were strongly dependent on male-female combination. In other words, follicular fluids (women) that induce strong sperm physiological response in some males often induce much weaker response in the other(s). Sperm physiological responses were stronger in HLA-dissimilar male-female pairs than in HLA-similar combinations, but none of the measured sperm traits were associated with genome-wide similarity. Together, these findings shed new light on the evolutionary and immunological mechanisms of fertilization. Furthermore, our results raise an intriguing possibility that against currently prevailing WHO's definition, infertility may not represent exclusively a pathological condition, but may also result from immunogenetic incompatibility of the gametes.

Abstract-Fatty acid composition in the muscle of eight endemic haplochromine cichlids and one tilapiine cichlid from Lake Malawi was studied by gas-liquid chromatography. The species studied represent a wide variety of niches: Algae combing, shell crushing, feeding on littoral and semipelagic zooplankton, detecting invertebrates hidden in the sediment using expanded lateral line canals, and fish predation in littoral and bathypelagic zones. The sampling was done during the early rainy season. The analysis of fatty acid composition suggests that, despite the reported plasticity in feeding, the niches of the species studied are established and stable. The proportion of eicosapentaenoic acid (20:5n-3) in relation to arachidonic acid (20:4n-6) illustrated the position of these species in the foodweb: equal proportions in algae feeders and clear 20: 4n-6 dominance in species feeding on benthic prey. Although the number of replicates was small, discriminant analysis with 31 fatty acids separated the specimens into dense centers according to species. These results encourage the use of chemometric methods for studying ecological relationships among freshwater fauna and ongoing evolutionary processes.

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