Diacamma ceylonense

Distribution based on Regional Taxon Lists
Oriental Region: India, Sri Lanka.

Biology
In Diacamma, the future gamergate is very aggressive towards infertile workers during the first days of her adult life. Overt aggression disappears at about three weeks in D. ceylonense, when the future gamergate begins to lay male-destined eggs and is ready to mate. Over the same period, her cuticular hydrocarbon profile alters, changing from a chemical signature similar to that of a sterile worker towards that of a gamergate (Cuvillier-Hot et al. 2002). In nature, these behavioural and chemical changes coincide with a reduction in conflict within the nest: faced with a virgin future gamergate, infertile workers have an interest in producing male-destined eggs; however, once the gamergate produces female eggs, they have an interest in rearing her offspring. This is a clear demonstration of a shift from physical inhibition to chemical signalling.

In queenless ants, dominance interactions are highly directional, suggesting that olfactory recognition occurs. In Diacamma ceylonense, the cuticular hydrocarbons (C25–C35) of nestmate workers (same colony) vary in their proportions according to age and fertility. Newly eclosed adults (‘callows’) initially have the same cuticular profile, but with time this changes to that typical of foragers (Cuvillier-Hot et al. 2001). In contrast, workers that begin to produce eggs develop a different cuticular profile. Several substances (n-C29 and some methyl C25 and C27) discriminate these different social categories (callows, foragers and egg-layers). Inter-colony variation of the cuticular hydrocarbons was much lower than intra-colony variation. We also found qualitative differences between the sexes, with males having a clearly different profile with much more alkanes.

In several populations from south India, referred to as ‘nilgiri’, gamergates do not mutilate their nestmates but monopolize reproduction using dominance interactions. Various lines of evidence indicate that ‘nilgiri’ populations are closely related to the neighboring species D. ceylonense. To determine whether this important behavioural difference between ‘nilgiri’ and D. ceylonense is associated with significant genetic differentiation, Baudry et al. (2003) used microsatellite and mitochondrial markers to examine genetic variation within and between ‘nilgiri’ and D. ceylonense. We found a very high genetic differentiation between the two forms, which suggests a lack of gene flow. There was an unexpected pattern of mitochondrial variation, because all ‘nilgiri’ populations show identical or very closely related COII sequences except one population with a very different haplotype. This divergent haplotype is genetically much more distant from the other ‘nilgiri’ haplotypes than are D. ceylonense haplotypes. This pattern is not observed at the nuclear level, which suggests that introgression of mitochondrial DNA probably occurred in some ‘nilgiri’ populations.

Nomenclature

 *  ceylonense. Diacamma ceylonense Emery, 1897b: 159 (w.) SRI LANKA. Subspecies of rugosum: Forel, 1900d: 318; Emery, 1911d: 66. Revived status as species: Bingham, 1903: 79; Santschi, 1932b: 14. Current subspecies: nominal plus orbiculatum.