Findings reported this week reveal how an evolutionary innovation that involves sharing genes between two ant species has created a deep-seated dependency between them for the survival of both species populations. The new work illustrates how genetic exchange through cross-breeding between two species can lead to a system of interdependence at a high level of biological organization — in this case, the production of worker ants for both species.
Millions of years before the first modern humans evolved, ants adopted many of the social innovations we consider our own: division of labor, agriculture, and even slavery. In fact, these traits have been pushed to the extreme in many ant species, such as the case of slave-maker ants, which have become so specialized in looting food from colonies of other ants that they can no longer feed themselves or raise their younger siblings. Recent work on ants suggests we may need to add genetic engineering to the list of innovations ants have developed to use. Populations have been discovered in two species of harvester ants in which queens mate with males of another species to produce genetically new hybrid workers. In a new study, Dr. Sara Helms Cahan and colleagues show that both species involved have effectively given up the ability to produce pure species workers in favor of the hybrids, making them completely dependent on each other for survival.
Female ants generally come in two forms: reproductive queens and sterile workers. The role, or caste, of an individual is determined for life at a certain stage of her development. In virtually all ant species, it is the environment in which a female is raised, rather than genetic predisposition, that determines which caste she will adopt. However, in two harvester ant populations in southern New Mexico, queens and workers from the same colonies are genetically very different; in both species at the site, only the queens are genetically derived from a pure species-specific lineage, while all workers are hybrids that possess a combination of genes from the two species in a single individual. It’s not currently known whether the ants benefit from having hybrids do the job, but, as evidenced by the researchers’ own efforts at selective breeding and genetic engineering, combining genomes is an easy way to produce new traits that can be very beneficial for growth, environmental tolerance or disease resistance. Regardless of the specific benefits, however, it is clear that these ants are committed to the hybrid workforce strategy. When the researchers prevented queens from mating with males of the other species, few succeeded in making workers, a handicap that would lead to some population failure in the field. The new findings suggest that specialization that relies on interspecific hybrid workers has not allowed these species to survive independently.
Sara Helms Cahan, Glennis E. Julian, Steven W. Rissing, Tanja Schwander, Joel D. Parker, and Laurent Keller: “Loss of phenotypic plasticity generates genotype-caste association in harvest ants”
The other members of the research team are Glennis E. Julian of the University of Texas, Austin; Steven W. Rissing of Ohio State University; and Tanja Schwander, Joel D. Parker and Laurent Keller of the University of Lausanne. This work was supported by grants from the Durfee Foundation (administered by the Earthwatch Institute), the Swiss Society of Naturalists (ASSN), and the Swiss National Science Foundation.
Publication in Current Biology, Volume 14, Number 24, December 14, 2004, pages 2277-2282. www.current-biology.com
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