Estimation of the number of founders of an invasive pest insect population: the fire ant Solenopsis incivta in the United States

Authors: ROSS, KENNETH - DEPT ENTOMOLOGY- UNIV GA; Shoemaker, David

Submitted to: Proceedings of the Royal Society of London B
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/19/2008
Publication Date: 6/24/2008
Citation: Ross, K.G., Shoemaker, D.D. 2008. Estimation of the number of founders of an invasive pest insect population: the fire ant Solenopsis incivta in the United States. Proceedings of the Royal Society of London B.2231-2240.

Interpretive Summary: Fire ants are considered significant ecological, agricultural, and public health pest throughout their invasive range in the U.S.A. A scientist at the Center for Medical, Agricultural, and Veterinary Entomology, USDA-ARS, Gainesville, Florida and a scientist from the University of Georgia describe here the results of a study aimed at inferring the makeup of the original founder population of fire ants in the USA. Determination of the number of founders responsible for the establishment of invasive plant and animal populations is important for developing biologically based management practices, predicting the invasive potential of species, and making inferences about basic ecological and evolutionary processes. Genotypic data from different classes of markers are largely congruent in suggesting that 9-20 mated fire ant queens likely comprised the initial founder group to colonize the USA at Mobile, Alabama in the mid-1930s. These results suggest that the rapid spread and massive population buildup of introduced fire ants occurred despite loss of substantial genetic variation associated with the relatively small number of founders, a pattern especially surprising in light of the substantial genetic load imposed by loss of variation at the sex-determination locus. Among other purposes, our results can be used to reconstruct likely scenarios for the introduction of this and other ants with similar habits into other parts of the world, as well as to help provide a general foundation for understanding how the loss of genetic diversity often associated with founder events may influence the evolution of invasive populations.

Technical Abstract: Determination of the number of founders responsible for the establishment of invasive plant and animal populations is important for developing biologically based management practices, predicting the invasive potential of species, and making inferences about basic ecological and evolutionary processes. The red imported fire ant, Solenopsis invicta, is an major invasive pest insect first introduced into the USA from its native South American range 70 years ago. We use extensive data from diverse genetic markers surveyed in the putative source population and relevant introduced populations to estimate the number of founders of the USA population. Genotypic data from different classes of nuclear markers (microsatellites, allozymes, sex-determination locus) and sequence data from the mitochondrial DNA are largely congruent in suggesting that 9-20 mated queens likely comprised the initial founder group to colonize the USA at Mobile, Alabama in the mid-1930s. Estimates of founder group size based on expanded samples from throughout the southern USA were marginally higher than this, consistent with the hypothesis that one or more secondary introductions of the ant into the USA occurred.