Recombination, chromosome number and eusociality in the

colony size; eusociality; Hymenoptera; karyotype; recombination; relatedness. Extraordinarily high rates of recombination have been observed in some eusocial species. The most popular explanation is that increased recombina-tion increases genetic variation among workers, which in turn increases col-ony performance, for example by increasing parasite resistance. However, support for the generality of higher recombination rates among eusocial organisms remains weak, due to low sample size and a lack of phylogenetic independence of observations. Recombination rate, although difficult to measure directly, is correlated with chromosome number. As predicted, sev-eral authors have noted that chromosome numbers are higher among the eusocial species of Hymenoptera (ants, bees and wasps). Here, we present a formal comparative analysis of karyotype data from 1567 species of Hyme-noptera. Contrary to earlier studies, we find no evidence for an absolute dif-ference between chromosome number in eusocial and solitary species of Hymenoptera. However, we find support for an increased rate of chromo-some number change in eusocial taxa. We show that among eusocial taxa colony size is able to explain some of the variation in chromosome number: intermediate-sized colonies have more chromosomes than those that are either very small or very large. However, we were unable to detect effects of a number of other colony characteristics predicted to affect recombination rate – including colony relatedness and caste number. Taken together, our results support the view that a eusocial lifestyle has led to variable selection pressure for increased recombination rates, but that identifying the factors contributing to this variable selection will require further theoretical and empirical effort