HERITABLE ALLOMETRIC VARIATION IN BUMBLE BEES - OPPORTUNITIES FOR COLONY-LEVEL SELECTION OF FORAGING ABILITY

Different characters of an organism may be correlated if genes control the allometric relationship between them. If genetic variation exists for such genes then the allometric relation itself is potentially sub ject to change by selection. In social insects allometric relations re present colony-level characters. If colonies differ in these relations and this variation leads to differential productivity among colonies, then selection on allometric relations can operate at the level of th e colony. We assessed the extent of heritable, between-colony variatio n for the allometric coefficients relating proboscis (=glossa) length to wing length for two bumble bee species (Bombus huntii and B. occide ntalis). We found that in both species colonies did not differ signifi cantly in slope (b) but did differ significantly in intercept (a) of t he regression of glossa length on wing length. Within-colony variation of the intercept was estimated by randomly constituting groups of fiv e workers from each colony and calculating the regression for each gro up. The intraclass correlation was then calculated from the between- a nd within-colony mean squares. We found significant intraclass correla tions in both species, giving heritabilities of 0.5 +/- 0.3 in B. hunt i and 0.7 +/- 0.3 in B. occidentalis. If this allometric relation affe cts colony foraging success and foraging environments vary geographica lly, then the intercept should exhibit corresponding geographic variat ion. We tested this prediction by comparing intercepts calculated usin g wild-caught B. vagans workers from Alberta, Ontario and Maine. We fo und that the intercepts did differ significantly between sites, with t he bees from Alberta having a significantly smaller intercept than the bees from eastern North America. Our results illustrate the opportuni ty for selection on an allometric relation that directly affects the f oraging success of individual bumble bee colonies.