Genetic influence on caste in the ant Camponotus consobrinus

Genetic influences on polyethism within social insect colonies are well kno wn, suggesting that the determination of caste (soldiers and minor workers) may also be genetically mediated. The Australian sugar ant Camponotus cons obrinus is suitable for such a study, having soldiers and minor workers tha t follow a complex allometry. Further, although most C. consobrinus colonie s are monogynous, 13 of 42 surveyed using microsatellites were found to be polygynous. Thus, although a minority of colonies were polygynous, the grea t majority of queens live in polygynous colonies. From the 29 monogynous co lonies studied, we inferred that the queens are monandrous. Ants from four polygynous colonies were assigned to families on the basis of microsatellit e genotypes, after measurements had been taken of head width and scape leng th. These measurements reflect a complex allometry interpretable as soldier and minor worker growth curves with a large changeover zone. Genetic influ ence on caste determination was examined by testing for differences between families within colonies in the distribution of scape lengths, residuals f rom the overall colony allometric curve, and proportions of soldiers and mi nor workers (as determined by head width falling above or below the inflect ion point of the overall colony allometric curve). Families in all four col onies differed significantly in caste proportions and in head-width distrib utions, and three of the four colonies showed significant differences betwe en families in residuals from the overall colony growth curve. Nested ANOVA s using head widths and scape-length residuals showed that when the effect of family is removed, intercolony differences ill allometry are negligible. This evidence indicates genetic rather than environmental causes for the o bserved differences between families. We speculate that this variation may reflect some selective advantage to within-colony heterogeneity between fam ilies or that selective differences are few between a wide array of family growth patterns.