MHCI variability and fitness in blue tits (Cyanistes caeruleus)

Abstract

The Major Histocompatibility Complex (MHC) is a group of closely linked loci that play an important role in the immune system within all jawed vertebrates, and it has been shown to yield a higher fitness with an intermediate diversity. Several studies present how females choose their mates based on the MHC variability, hereby providing their offspring with the optimal variability. MHC class I (MHCI) mediates immunity against intracellular pathogens such as Haemoproteus, Plasmodium and Leucocytozoon. Here I investigate the MHCI composition in a population of blue tits (Cyanistes caeruleus) sampled in 2001 and 2021, and test whether social pairing departs from what is expected from random pairings. I also investigate the connection between the MHCI morphology, age and reproductive success. Additionally, I investigate the correlation between the MHCI and prevalence of the blood parasites Haemoproteus, Plasmodium and Leucocytozoon, and the correlation between these blood parasites, morphology, age and reproductive success. The younger females tended to be in social pairs with fewer positively selected site (PSS) alleles than expected from random pairings, whereas the older females tended to be in social pairs with more PSS alleles than expected. A higher number of MHCI alleles was linked to longer wings. Interestingly, there was an association between the number of MHCI alleles and Haemoproteus infection within the 2001 blue tits, with individuals with a higher number of alleles being more likely to be infected. Finally, a close to significant positive association was also found between the MHCI PSS allele IWYWYVGR and Haemoproteus infection. My results are most consistent with selection for a higher or lower number of alleles rather than an intermediate optimum, but additional studies are needed to further investigate these results.