Mitochondrial DNA implicates the Paraná “invasion cradle” in another ant introduction-the South American big-headed ant, Pheidole obscurithorax

Pheidole obscurithorax is a non-native ant that has been spreading slowly along the U.S. gulf coast for the past few decades. Evidence of a sudden recent increase in the rate of spread (King & Tschinkel 2007) has spurred interest in learning more about this species. In the present study we examine the native distribution in an attempt to locate the region of origin for the introduced populations. We sampled non-native populations from Mississippi, Alabama, and Florida, and native populations of P. obscurithorax and related species across Paraguay and northern Argentina. Morphology and COI mitochondrial sequence data are concordant in delimiting the boundaries of P. obscurithorax, indicating a native distribution for the species that stretches from the Andean foothills in the west to the Uruguay River in the east. A single invariant haplotype was recorded from the introduced populations, suggesting a single introduction. While we did not find an exact match among native haplotypes to the introduced populations, the gulf coast samples nested within a clade from the middle Paraná of Argentina, a stretch along the Paraná River from Santa Fe to Resistencia. Perhaps not coincidentally, this is the same broad region of origin for introductions of the fire ant Solenopsis invicta and the Argentine ant Linepithema humile.