Limited associations between MHC diversity and reproductive success in a bird species with biparental care

The selective pressure from pathogens on individuals can have direct
consequences on reproduction. Genes from the major histocompatibility
complex (MHC) are central to the vertebrate adaptive immune system and
pathogen resistance. In species with biparental care, each sex has
distinct reproductive roles and levels of investment, and due to a
trade-off with immunity, one can expect different selective regimes acting
upon the MHC of each parent. Here, we addressed whether couples combine
each other’s variation at MHC loci to increase their breeding success.
Specifically, we used a 23-year dataset from a barn owl population (Tyto
alba) to understand how MHC class Iα and IIβ functional divergence and
supertypes of each parent were associated with clutch size and fledging
success. We did not detect associations between MHC diversity or
supertypes with the clutch size nor with the fledging success. In
addition, to understand the relative contribution from the MHC of the
genetic parents and the social parents, we analysed the fledging success
using only a cross-fostered dataset. We found several associations of
weak-to-moderate effect sizes between the father’s MHC and fledging
success: i) lower MHC-Iα divergence in the genetic father increases
fledging success, which might improve paternal care during incubation, and
ii) one and two MHC-IIβ DAB2 supertypes in the social father decrease and
increase, respectively, fledging success, which may affect the paternal
care after hatching. Furthermore, fledging success increased when both
parents did not carry MHC-IIβ DAB1 supertype 2, which could suggest
conditional effects of this supertype. Although our study relied on a
substantial dataset, we showed that the associations between MHC diversity
and reproductive success remain scarce and of complex interpretation in
the barn owl. Moreover, our results highlighted the need to incorporate
more than one proxy of reproductive success and several MHC classes to
capture more complex associations.