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Learn From social to social Parasite

The unusual social parasite transition is important to understand because the genomes of ants have evolved for more than 100 million years. A single major transition occurred to introduce the novel “superorganism” level of social organizational structure with queen-worker caste segregation and unconditional altruism. This superorganism was so successful, it produced a biodiversity of 17 subfamilies, 338 genera and more than 13,900 living species.

“It is therefore no surprise that parallel shifts to a highly specialized socially parasitic behavior and lifestyle abandoning this fundamental ancestral condition, usually based on outbreeding and larger effective populations, leave significant genomic footprints,” said Rabeling. “The results of our analyses of just three of these species confirm that ant social parasites offer important study systems for identifying hallmarks of cooperative social colony life.

And in doing so, their analyses have confirmed that over a time span of about a million and a half years, these ant species have each found independent, separate ways to evolve and become social parasites. The signatures of genome-wide and trait-specific genetic erosion were found to be most extreme in social parasite ants.

Divergence estimates for Acromyrmex host and inquiline parasite species. ime-calibrated phylogeny of the fungus-growing ants for which genomes have been sequenced, including the three inquiline social parasite species and their two host species. The two origins of social parasitism in Acromyrmex (orange dots and boxes) occurred ca. 0.96 Ma ago for A. insinuator (1) and ca. 2.50 Ma ago when the ancestor of A. heyeri diverged from the stem group representative of Pseudoatta argentina (2) and A. charruanus (3). Credit: Arizona State University

Think of how it would start. A group of queen ants wants to just live in a colony without doing the work. And not work on the nest anymore. Next, the queen ants focus on solely producing new queens and males, and this small population size of social parasites would start frequent inbreeding to survive. This immediately reduces their genomic diversity over time. Then, over a blink in evolutionary time, due to natural selection and an increase in the prevalence of genetic drift, it would enhance the rates by which ancestral traits were lost while also slowing down the rates by which new, more adaptive traits could emerge.

It’s almost like a ‘snooze and lose it’ phenomena occurred within the parasitic ant DNA to trigger the genome erosion.

To prove this effect within the ant genome, the research team investigated the overall genomic structure and the individual genes that may be affected by this genomic decay. First, they found widespread evidence of genomic rearrangements and inversions that are hallmarks of instability and decay. Then, within gene networks, they identified 233 genes that showed evidence of relaxed selection in at least one of the social parasite branches and signatures of intensified selection in 102 genes. “Our analysis showed that gene family evolution at three of the four social parasite nodes is indeed largely characterized by gene losses,” said Rabeling.

The genome losses and reductions most affected were in the social parasite ants’ sense of smell and to a lesser degree taste.

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