Not only did some of the genes responsible for ant smell become lost over time, but as a result, the ants also showed a reduced size in the olfactory lobes in their brains when microCT scans were performed.
“This is no surprise because ants predominantly communicate via chemical cues and have once been described as chemical factories,” explains Rabeling. “So, the loss of olfactory genes is correlated with an extreme transition of extensive morphological and behavioral changes.”
This includes the reduction or complete loss of the worker caste system, simplified mouthparts, antennae and integuments, loss of certain hormonal glands, and a nervous system of reduced complexity likely associated with a drastically narrowed behavioral repertoire.
Micro CT scans show the relative olfactory lobe (OL) size of the hosts and inquilines. The phylogram is an ancestral state reconstruction of OL volumes relative to total brain volumes across the social parasites (A. insinuator, A. charruanus and P. argentina) and their hosts (A. echinatior, and A. heyeri). Barplots show ratios of OL volume to total brain volume in inquiline parasites (in orange) relative to their hosts (in blue). Circles inserted at the tips of bars are proportional to the measured total brain volumes, while the smaller contained circles represent the measured volumes of the right and left OLs. On average, Panamanian species have larger brains than Uruguayan species (2-sample t-test, pt-test = 0.005, df = 2.97, t = ?7.74, n = 5). Relative OL volumes became reduced (pt-test = 0.059, df =2, t = ?2.65, n = 5) as inquiline social parasites evolved their different degrees of specialization along the gradient of inquiline adaptations known as the inquiline syndrome27. Shown below are 3D surface reconstructions of the brains (with the OLs highlighted in yellow) and of the head capsules of A. heyeri, A. charruanus, and P. argentina (from top to bottom). Credit: Arizona State University
From their comparative analysis, they could also put these changes into the larger perspective of evolutionary time. They were also able to date the origins of social parasitism within the leaf-cutting ant family tree.
Two independent origins of social parasitism occurred in the ant genus Acromyrmex. Within this genus, A. heyeri, a social ant, is the host species of both A. charruanus and P. argentina parasitic species.
First, a South American lineage of social ants (A. heyeri) separated from the last common (thought to be socially parasitic) ancestor of A. charruanus and P. argentina before the two social parasites diverged. Second, a Central American speciation event occurred when A. insinuator diverged from its host A. echinatior.
Both origins of social parasitism are evolutionarily recent, estimated to be about 2.5 million years ago for the divergence between A. heyeri and the last common ancestor of A. charruanus and P. argentina, and about 1 million years ago for the divergence between A. insinuator and A. echinatior.
“We infer that relaxed natural selection accelerated general genome erosion in social parasites and alleviated evolutionary constraints, which facilitated rapid adaptive evolution of specific traits associated with a socially parasitic lifestyle,” said Rabeling.