Colony fitness increases in the honey bee at queen mating frequencies higher than genetic diversity asymptote

Abstract

Across the eusocial Hymenoptera, a queen’s mating frequency is positively associated with her workers’ genetic diversity and colony’s fitness. Over 90% of a colony’s diversity potential is achieved by its mother’s tenth effective mating (me); however, many females mate at levels of me > 10, a zone we here call hyperpolyandry. We compared honey bee colony fitness at mating levels near and above this genetic diversity asymptote. We were interested in how hyperpolyandry affects colony phenotypes arising from both common tasks (brood care) and rare specialized tasks (parasite resistance). We used an unselected wild line of bees and a Varroa Sensitive Hygiene (VSH) line selected to resist the parasite Varroa destructor. Virgin queens were instrumentally inseminated to replicate the following queen/colony conditions: (1) VSH semen/low polyandry (observed mating number = mo = 9), (2) VSH semen/high polyandry (mo = 54), (3) wild type semen/low polyandry, or (4) wild semen/high polyandry. There was a positive effect of polyandry on brood survival, an outcome of common tasks, with highest values at mo = 54. There was an interaction between polyandry and genetics such that differences between genetic lines expressed only at mo = 54, with fewer mites in VSH colonies. These results are consistent with two hypotheses for the evolution of mating levels in excess of the genetic diversity asymptote: hyperpolyandry improves colony fitness by (1) optimizing genotype compositions for common tasks and (2) by capturing rare specialist allele combinations, resisting cliff-edge ecological catastrophes.Significance statementPolyandry is a female’s practice of mating with several males, storing their sperm, and using it to produce one or more clutches of genetically diverse offspring. In the social Hymenoptera, polyandry increases the genetic diversity and task efficiency of workers, leading to improved colony fitness. Over 90% of the increase in a colony’s diversity potential is achieved by its mother’s tenth mating; however, many females practice hyperpolyandry, a term we reserve here for mating levels above this genetic diversity asymptote. We show that a token of colony fitness arising from common tasks, brood survival, improves universally as one moves from sub- to hyperpolyandrous mating levels. However, a colony phenotype arising from a rare parasite resistance task is only expressed in the presence of the controlling alleles and under conditions of hyperpolyandry. These results suggest adaptive mechanisms by which hyperpolyandry could evolve.