The Diversification of Brain Structure in Ants

Abstract

Okinawa Institute of Science and Technology Graduate UniversityDoctor of PhilosophyAnts, as an example of complex social cooperation and ecological dominance, represent one of the most successful groups among animals. Their complex eusociality, cooperative behaviors, and adaptability to diverse environments is inextricably linked to the evolution of their organ systems. The brain, as a pivotal organ mediating organism-environment interactions, plays a critical role in these adaptive strategies. While the basic neuroanatomical scheme is conserved, natural selection has shaped relative sizes of neuropils and the overall brain size to generate diversity across the ants. While a rich body of literature has advanced our understanding of the vertebrate brain size evolution, broad scaled comparative neuroanatomy for insects has only gained more traction in recent decades. Here, I take the comparative neuroanatomical approach to unravel the general principles and processes driving the macroevolution of the ant brain. To do so, in the first chapter I take a broader phylogenetic view and explore the evolution of odorant receptor (OR) repertoires across a broad range of hymenopteran species, revealing that in contrast to a widely held hypothesis, eusociality did not drive an expansion in the OR repertoire. Instead, my results suggested that factors such as the loss of flight may have played a role in shaping some of this variation. Subsequently, in the second chapter I examined brain and neuropil size variation of 9 distinct neuropils from 75 ant species, uncovering divergent body size scaling patterns of different brain regions. While visual neuropils displayed hyper-allometric scaling, olfactory regions showed more constrained isometry. Additionally, I showed that the evolution of miniaturized sterile workers was associated with dramatic reductions in brain regions forming a “visual module”, leading to the reduction in the overall brain size, contributing to an energy-efficient "cheaper worker" phenotype. Together, these findings elucidate the intricate interplay between sensory, social, and ecological factors that make the brain of an ant “the most marvellous atoms of matter in the world…” (Darwin, “The descent of man”; 1871, p.145)doctoral thesi