Similar Performance of Diploid and Haploid Males in an Ant Species without Inbreeding Avoidance

AbstractUnder haplodiploidy, a characteristic trait of all Hymenoptera, femalesdevelop from fertilised eggs, and males from unfertilised ones. Males aretherefore typically haploid. Yet, inbreeding can lead to the production ofdiploid males that often fail in development, are sterile or are of lowerfertility. In most Hymenoptera, inbreeding is avoided by dispersal ﬂightsof one or both sexes, leading to low diploid male loads. We investigatedcauses for the production of diploid males and their performance in ahighly inbred social Hymenopteran species. In the ant Hypoponera opacior,inbreeding occurs between wingless sexuals, which mate within themother nest, whereas winged sexuals outbreed during mating ﬂightsearlier in the season. Wingless males mate with queen pupae and guardtheir mating partners. We found that they mated randomly with respectto relatedness, indicating that males do not avoid mating with close kin.These frequent sib-matings lead to the production of diploid males, whichare able to sire sterile triploid offspring. We compared mating activity andlifespan of haploid and diploid wingless males. As sexual selection acts onthe time of emergence and body size in this species, we also investigatedthese traits. Diploid males resembled haploid ones in all investigated traits.Hence, albeit diploid males cannot produce fertile offspring, they keep upwith haploid males in their lifetime mating success. Moreover, by father-ing viable triploid workers, they contribute to the colonies’ work force. Inconclusion, the lack of inbreeding avoidance led to frequent sib-matingsof wingless sexuals, which in turn resulted in the regular production ofdiploid males. However, in contrast to many other Hymenopteran species,diploid males exhibit normal sexual behaviour and sire viable, albeit ster-ile daughters.Social insects generally reproduce in large outbreed-ing mating swarms, followed by independent nestfoundations by queens and, as in the case of termites,also kings. However, in some species, alternativesexual phenotypes evolved, which can be regarded asdispersal polymorphisms (Molet et al. 2009; Peeters2012). These wingless sexuals circumvent the risks ofdispersal and independent colony foundation (Pusey& Wolf 1996), but their restricted dispersal capabilitiesmay lead to inbreeding with all its potential negativeconsequences. Outbreeding can be maintained if atleast one sex, most often the male, maintains the abil-ity to ﬂy. In several ant species, wingless females canattract unrelated winged males by releasing sexualpheromones, a behaviour that has been termed‘female calling’ (Peeters 1991; Passera & Keller 1994;Boomsma et al. 2005). Inseminated queens thenreturn to their nest to reproduce or they take part ofthe work force to start a new nest in the vicinity.Wingless males however usually mate with nestmatequeens that are in most cases close relatives. Thisnon-dispersive male morph is much less commonthan winged males, but has been found in ninegenera (Heinze & Tsuji 1995). As wingless males do not disperse but mate withintheir natal nest, they compete with each other for the