The Tsutsui Lab

An Argentine ant worker is dwarfed by a major worker of a native species it displaces (Camponotus dumetorum). Photo by Marc Dantzker.

The Argentine Ant (Linepithema humile):

Population Genetics and Molecular Ecology: A central focus of our research is the application of population genetic tools to questions regarding the behavior of Argentine ant workers, the organization of their colonies (and how this differs between the native and introduced ranges), and reconstructing the history of global introductions. Using microsatellite markers, we have shown that introduced Argentine ants have experienced one of the most severe genetic bottlenecks known for any organism, and have identified regions around the southern Rio Parana (Argentina), near the city of Rosario, as the most likely source of most introduced populations. In several different studies, we have also used genetic tools to show how the unicolonial social structure of introduced populations likely arises from the dearth of genetic diversity in the introduced range.

Chemical Ecology: The formation of massive supercolonies of introduced Argentine ants is ultimately a product of individual workers behaviors, specifically, the widespread acceptance of other Argentine ant workers as nestmates. We have been studying the chemicals that these ants use to distinguish members of their own colony from ants that belong to foreign colonies. These chemicals are largely cuticular hydrocarbons, waxy substances that are present on the exoskeleton. We have identified a number of these that appear to be highly correlated with aggressive behavior among Argentine ant workers. In collaboration with Dr. Ken Shea's lab at UC-Irvine, we have been synthesizing and testing individual hydrocarbons for their ability to trigger aggression among (normally friendly) nestmates.

The Evolution of Nestmate Recognition: The formation and persistence of social groups relies upon the ability of group members to discriminate among different individuals. Consequently, a wide variety of recognition systems have evolved built on different sensory modalities, that use a diverse array of signals and decision rules, and regulate behaviors ranging from intense aggression to life-long pair bonds.

Because these recognition systems may evolve over long time spans (thousands of generations), rapid changes in the social environment can outpace rates of evolutionary response. When such perturbations occur, social behaviors may be expressed without regard to fitness, radically altering the social structure of the species and, in turn, producing novel forms of selection on individuals and the recognition system itself.

The human-mediated introduction of Argentine ants to many Mediterranean-type ecosystems has created a rare and remarkable opportunity to examine how recognition systems evolve and function in different social contexts. In their native range, Argentine ant populations often consist of numerous genetically diverse and differentiated, mutually aggressive, highly territorial colonies within small areas (hundreds or thousands of meters). In marked contrast, introduced populations of Argentine ants are genetically homogeneous and depauperate, and possess a social structure characterized by the absence of aggression among conspecifics over hundreds or thousands of kilometers.

Our research combines behavioral and genetic data from both the field and the lab to explore how selection has shaped this system and how recent, non-adaptive genetic changes have altered the patterns of cooperation and conflict within and among social groups. These studies examine how changes in the social environment have contributed the Argentine ant's altered social structure, and how these changes influence the form of selection on the recognition system. Our studies have provided insights into the behavioral and genetic processes that have allowed the Argentine ant to become a successful invader. Moreover, because many other invasive ant species are also unicolonial, these findings may be applicable to ant invasions in general.

Honeybees (Apis mellifera): Genetics and genomics of honeybees in their native (Old World) and introduced (New World) ranges. In collaboration with Charlie Whitfield (University of Illinois-Urbana Champaign) and others, we have conducted a global genetic analysis of honeybees, from both their native and introduced ranges. Using a large number of single nucleotide polymorphisms (SNPs), we have reconstructed the history of range expansions and introductions, and have shown that introduced populations are an intriguing amalgamation of various different subspecies that humans have managed (and dispersed) for centuries.

Host-Parasite Co-evolution: Between slave-making ants (Polyergus breviceps) and their slaves (Formica spp.), phorid flies (Pseudacteon) and their ant hosts (Solenopsis spp.), and the bacterium Wolbachia pipiens and various insect hosts.

Conservation Genetics: Studies of genetic diversity and gene flow in fragmented habitats.

The Western Whiptail lizard (Cnemidophorus tigris): Gene Flow Across a Hybrid Zone in Baja California, Mexico

Solenosteira macrospira: The Evolution of Male Parental Care