Structural dissimilarity of partners' immune genes increases sperm viability in women's reproductive tract

Female-mediated selective sperm activation may remodel major histocompatibility complex-based mate choice decisions in humans

Major histocompatibility complex (MHC) genes are known to mediate mate choice both at the individual and gamete level. However, it has remained unclear how different episodes of MHC-associated mate choice interact and contribute to the total selection on MHC genes. Here, we clarified this interaction in humans by performing a full-factorial experiment where 10 females first ranked the attractiveness and intensity of the body odours of 11 males. Then we studied whether female odour preferences in these same 110 male-female combinations predicted sperm performance in the presence of follicular fluid (sperm-stimulating female reproductive fluid). When analyzing the total MHC similarity (including classical and non-classical MHC genes) of the male-female combinations, we found that females preferred the body odours of MHC-similar males, but that sperm motility was positively affected by the MHC dissimilarity of the male-female combinations. No associations were found for classical MHC genes only. Furthermore, odour preferences were negatively associated with sperm motility at the end of the follicular fluid treatment. Together, our results indicate that individual and gamete-level mate choice processes may act in opposing directions and that the most attractive males are not necessarily the most optimal partners at the post-copulatory level. Finally, our findings suggest that gamete-mediated mate choice may have a definitive role in disfavouring genetically incompatible partners from fertilizing oocytes.

Applying an evolutionary perspective to assisted reproductive technologies

Assisted reproductive technologies (ART) are commonly used to address human infertility and to boost livestock production. During ART, procedures such as in vitro fertilization, artificial insemination, and intracytoplasmic sperm injection introduce gametes and embryos to unnatural and potentially stressful conditions that can influence offspring health, often via epigenetic effects. In this perspective we summarize these key risks of ART for embryonic and longer-term offspring fitness, emphasizing the need for experimental research on animal models to determine causal links between ART and offspring fitness across multiple generations. We also highlight how ART can bypass a range of naturally and sexually selected mechanisms that occur in the female reproductive tract and/or via female secretions that ultimately determine which sperm fertilize their eggs. We further argue that this curtailment of female-modulated mechanisms of sperm selection may have important consequences for ART-conceived offspring. We encourage the development of ART methods that better mimic natural processes of sperm selection and embrace the fundamental principles of natural and sexual selection. Ultimately, the aim of this perspective is to encourage dialogue between the fields of evolutionary biology and applied areas of animal and human reproduction.

Cryptic female choice within individual males - A neglected component of the postmating sexual selection?

Cryptic female choice (CFC) is commonly assumed to act only in polyandrous mating systems, which allows females to bias fertilization towards the sperm of particular males. However, accumulated evidence has demonstrated that sperm show significant phenotypic and genotypic variation also within single ejaculates, which have important consequences for offspring phenotype and fitness. Here, I argue that these neglected sources of intra-male sperm variation often allow CFC to act also within individual males and facilitate fertilization bias towards genetically compatible (or otherwise preferred) sperm haplotypes. In this article, I explain prerequisites for within-male CFC, the criteria for demonstrating it and summarize accumulated evidence for this emerging selection process. Then, I evaluate prevalence of within-male CFC and review its potential evolutionary consequences. The aim of this article is to broaden the current definition of CFC by demonstrating that CFC has potential to act in all mating systems, in both internally and externally fertilizing species. Incorporation of the within-male CFC concept into the current models of sexual selection may provide novel insights into the deeper understanding of selective factors driving the evolution of mating systems and reproductive proteins. Finally, within-male CFC towards particular sperm haplotypes may increase our understanding of non-Mendelian inheritance.

Sperm Physiological Response to Female Serum-Potential New Insights into the Reproductive Incompatibility Diagnostics

Infertility is assumed to arise exclusively from male- and female-dependent pathological factors. However, recent studies have indicated that reproductive failure may also result from the reproductive incompatibility of the partners. Selection against such incompatibilities likely occurs via female-derived reproductive secretions, including follicular fluid (FF), that mediate gamete-level mate choice towards the sperm of specific males. To facilitate potential development of diagnostic tests for human reproductive incompatibility, we examined whether sperm physiological response to female serum indicate male–female compatibility in the presence of FF. We performed a full-factorial experiment, in which the sperm of 10 males were treated with the FF and serum of 6 healthy females. We found that sperm motility and viability in both biofluids were highly similar and that in 70% of the males, sperm serum treatment predicted male–female compatibility. We also identified male human leucocyte antigen (HLA) alleles and female (FF and serum) anti-HLA antibodies and tested whether the number of allele–antibody matches predict sperm physiological response to female fluids. However, no association was found between measured sperm traits and the number of allele–antibody matches. Overall, the present results may open novel possibilities for the future development of reproductive incompatibility tests and may pave the way towards more accurate infertility diagnostics and treatments.

Genetic incompatibility of the reproductive partners: an evolutionary perspective on infertility

In natural fertilisation, the female reproductive tract allows only a strictly selected sperm subpopulation to proceed in the vicinity of an unfertilised oocyte. Female-mediated sperm selection (also known as cryptic female choice (CFC)) is far from a random process, which frequently biases paternity towards particular males over others. Earlier studies have shown that CFC is a ubiquitous phenomenon in the animal kingdom and often promotes assortative fertilisation between genetically compatible mates. Here, I demonstrate that CFC for genetic compatibility likely also occurs in humans and is mediated by a complex network of interacting male and female genes. I also show that the relative contribution of genetic compatibility (i.e. the male-female interaction effect) to reproductive success is generally high and frequently outweighs the effects of individual males and females. Together, these facts indicate that, along with male- and female-dependent pathological factors, reproductive failure can also result from gamete-level incompatibility of the reproductive partners. Therefore, I argue that a deeper understanding of these evolutionary mechanisms of sperm selection can pave the way towards a more inclusive view of infertility and open novel possibilities for the development of more personalised infertility diagnostics and treatments.