Genetics and Sociality

Ambrosia beetle genetics

Fig: Sexual dimorphism in Xyleborini: diploid female (left) and haploid male (right) of Diuncus papatrae. Photo J.H.

The whole weevil subfamily scolytinae, which includes most groups of ambrosia beetles, abounds with genetic and reproductive oddities. For an overview, refer to Lawrence Kirkendall’s classic paper (Kirkendall, 1983).

Of all ambrosia beetles, the most peculiar in terms of genetic setup are one of our lab’s favorites, Xyleborini. These beetles combine haplodiploidy and inbreeding. Xyleborini males are haploid, flightless clones of their mothers with just 50% of her genetic material. They hatch from usually a single unfertilized egg, while most other eggs that each female lays are diploid, and develop into females. The small haploid males spend most of their life inside the natal gallery fertilizing their own sisters. Such sons allow single females to establish successful populations in non-native locations (Kirkendall, 1983).

Haplodiploidy is not an invention of Xyleborini and has nothing to do with fungus symbiosis. It is a property of a large subclade within Dryocoetini, a bark beetle group from which Xyleborini arose and radiated (Jordal, 2000).

Ambrosia beetle sociality

The variation in mating and breeding systems among ambrosia beetles (or bark beetles at large) is huge, and better covered in papers by other people. We will mention only two interesting cases here. The Australian platypodine Austroplatypus incompertus lives in large families in live Eucalyptus trees. The families display many features of true eusociality, most notably overlapping generations inside one colony, and a division of labor to the extent than some females seem to forego reproduction (Kent & Simpson, 1982).

The other interesting social system is subsociality observed in some Xyleborini. This is a topic that Peter Biedermann’s research group is studying at the University of Würzburg. In Xyleborinus saxeseni, females sometimes stay in their natal galleries and keep laying eggs, which results in generational overlap. Beetles also share various tasks, for example when juveniles help clean and guard the gallery.

References

Jordal, B., Normark, B.B., & Farrell, B.D. (2000) Evolutionary radiation of an inbreeding haplodiploid beetle lineage (Curculionidae, Scolytinae). Biological Journal of the Linnean Society, 71, 483-499.

Kent, D. S., and J. A. Simpson (1992). Eusociality in the beetle Austroplatypus incompertus (Coleoptera: Curculionidae). Naturwissenschaften 79:86-87.

Kirkendall, L.R. (1983) The evolution of mating systems in bark and ambrosia beetles (Coleoptera: Scolytidae and Platypodidae). Zoological Journal of the Linnean Society, 77, 293-352.