AsianScientist (Feb. 22, 2017) – Using a species of ‘queenless’ ants, researchers have identified genes that play a role in maintaining ant caste systems. Their findings have been published in Molecular Ecology.
Social insects—such as ants, bees and wasps—display an organizational complexity called eusociality, where individual members of a colony act more like parts of a whole rather than independent organisms. In eusocial colonies, each individual performs specific tasks based on which caste they belong to: either the reproductive caste or the worker caste.
Remarkably, although ants, bees and wasps all evolved eusociality separately, all of their societies have the same distinction between the reproductive and the worker castes, raising the question of whether common genes gave rise to this differentiation.
One way to answer this question is to look at exceptions to the general rule. In many ant species, queen ants are much larger than their worker counterparts. However, in species such as the native Okinawan Diacamma ants, there are few differences in appearance between the worker caste and reproductive caste. These species where reproductive and worker castes look the same are called ‘queenless’ ants.
The lack of physiological differences between the different castes makes Diacamma a particular intriguing ant to study, said Professor Alexander Mikheyev, leader of the Ecology and Evolution Unit at the Okinawa Institute of Science and Technology Graduate University (OIST).
“Unlike most ant species, where the ability to reproduce is determined during early development, Diacamma display an unusual biology in that every ant has the potential to become part of the reproductive caste until early adulthood,” Mikheyev explained.
When adult female Diacamma ants first emerge from their pupal case, they have a pair of wing-like structures called gemmae. Every female who retains her gemmae can reproduce. However, the reigning reproductive female, called a gamergate, typically attacks young female ants and mutilates the gemmae. This violent mutilation prevents the young females from developing ovaries, causing irreversible sterility. The sterile females then become part of the worker caste. However, if no gamergate is present, young females retain their gemmae and become part of the reproductive caste.
The scientists wanted to determine how this mutilation affects the ants at the molecular level. To do this, they collaborated with researchers at the University of the Ryukyus and the University of Tokyo to collect Diacamma colonies from different locations on the island of Okinawa and study their gene expression patterns.
“We wanted to compare gene expression between the mutilated and non-mutilated ants at the same stage in development to see which genes determined caste differentiation.” Mikheyev explained. “From there, we could compare the gene expression patterns between different castes in Diacamma ants to those of other species to elucidate the evolutionary origins of eusociality.”
The research revealed that only a small number of genes differentiated the reproductive caste from the worker caste, primarily genes related to nutrition. This hints that increased energy requirement of reproduction may be a factor in the evolution of eusociality, the researchers said.
Some of these genes have also been found to affect caste determination in other species, such as bees, wasps and other ants, though typically during development. Based on these findings, the researchers plan to further investigate the evolutionary origins of eusociality.
“We are in the process of conducting a follow-up study, comparing developmental profiles of gene expression in ants and honey bees, to explore parallels in how castes repeatedly evolved,” Mikheyev concluded.
Source: Okinawa Institute of Science and Technology Graduate University; Photo: Alexander Mikheyev.
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