Abstract
Studies of the genetic basis and evolution of complex social behavior emphasize
either conserved or novel genes. To begin to reconcile these perspectives, we studied
how the evolutionary conservation of genes associated with social behavior depends on
regulatory context, and whether genes associated with social behavior exist in
distinct regulatory and evolutionary contexts. We identified modules of co-expressed
genes associated with age-based division of labor between nurses and foragers in the
ant Monomorium pharaonis, and we studied the relationship between
molecular evolution, connectivity, and expression. Highly connected and expressed
genes were more evolutionarily conserved, as expected. However, compared to the rest
of the genome, forager-upregulated genes were much more highly connected and
conserved, while nurse-upregulated genes were less connected and more evolutionarily
labile. Our results indicate that the genetic architecture of social behavior
includes both highly connected and conserved components as well as loosely connected
and evolutionarily labile components.
DOI:http://dx.doi.org/10.7554/eLife.04775.001
Animal species vary widely in their degree of social behavior. Some species live
solitarily, and others, such as ants and humans, form large societies. Many
researchers have tried to understand the genetic changes underlying the evolution of
social behavior. Some researchers suggest that it involves recycling existing genes
that also have other conserved functions. Others propose that the evolution of social
behavior involves completely new genes that are not found in related but solitary
species.
Ants are one of the best-studied social animals. An established colony can contain
many 1000s of individuals that live and work together and perform different roles.
The queen's job is to lay eggs, while the worker ants do everything else,
including collecting food, caring for the young, and protecting the colony. In some
species of ant—including the pharaoh ant—a worker's role changes
as it ages. Younger workers tend to stay in the nest and nurse the brood, while older
workers tend to leave the nest and forage for food.
Mikheyev and Linksvayer asked: which genes are responsible for this age-based
division of labor? And how did this aspect of social behavior evolve? First, after
observing pharaoh ants from two colonies set up in the laboratory, they confirmed
that workers nursing the brood were on average almost a week younger than those seen
collecting food. Next Mikheyev and Linksvayer identified which genes were expressed
in ants of different ages, or ants engaged in different tasks. Specific sets of genes
were expressed more (or ‘up-regulated’) in nurse workers, while others
were up-regulated in foraging workers.
Mikheyev and Linksvayer then investigated how rapidly these genes had evolved by
comparing them to related genes found in other social insects (fire ants and honey
bees). They also determined the ‘connectivity’ of these genes by asking
how many other genes showed similar expression patterns. In many organisms, how
rapidly a gene evolves depends on how tightly connected its expression is to the
expression of other genes; highly connected genes evolve more slowly.
The genes that were expressed more in the older foraging workers were both more
highly connected and more evolutionarily conserved in the other social insects. Genes
that were up-regulated in the younger nurse workers were more loosely connected and
rapidly evolving.
Mikheyev and Linksvayer's findings show that the evolution of social behavior
in animals involves both new genes, which tend to be loosely connected, and conserved
genes, which tend to be more highly connected.
DOI:http://dx.doi.org/10.7554/eLife.04775.002
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