Origin of a haplodiploid beetle lineage

The beetle family Scolytidae includes several groups having regular sib-mat ing and extremely female-biased sex ratios. Two such groups are known to in clude haplodiploid species: (i) the tribe Xyleborini and (ii) Coccotrypes a nd related genera within the tribe Dryocoetini. Relationships of these grou ps have been controversial. We analysed elongation factor 1-alpha (852 bp) and cytochrome oxidase 1 (1179 bp) sequences for 40 species. The most-parsi monious trees imply a single origin of haplodiploidy uniting Xyleborini (ap proximately 1200 species) and sib-mating Dryocoetini (approximately 160 spe cies). The sister-group of the haplodiploid clade is the outcrossing genus Dryocoetes. The controversial genus Premnobius is outside the haplodiploid clade. Most haplodiploid scolytids exploit novel resources, ambrosia fungi or seeds, but a few have the ancestral habit of feeding on phloem. Thus, sc olytids provide the dearest example of W. D. Hamilton's scenario for the ev olution of haplodiploidy (life under bark leading to inbreeding and hence t o female-biased sex ratios through haplodiploidy) and now constitute a uniq ue opportunity to study diplo-diploid and haplodiploid sister-lineages in a shared ancestral habitat. There is some evidence of sex determination by m aternally inherited endosymbiotic bacteria, which may explain the consisten cy with which female-biased sex ratios and close inbreeding have been maint ained.