The evolution of colony structure in the ant genus Linepithema

The Argentine ant, Linepithema humile is one of the most prolific ant invaders worldwide. The success of this invader has been attributed to specific biological characteristics including its expansive colony structure and extreme polygyny; individual nests can contain hundreds of queens and colonies made up of interconnected nests can extend over large areas. While the Argentine ant is well studied, it is unknown whether these same colony traits also occur in other members of the genus Linepithema. This information is essential to test hypotheses about why some species become invasive while their close relatives do not. To examine the evolution of colony characteristics in the genus Linepithema, we examined nest number and dispersion, and estimate queen number for colonies of eight species in the genus from Argentina, Ecuador and Brazil. Our observations revealed significant variation in colony size, nest number, and estimated queen number both within and between the eight Linepithema species. However, L. humile was the only species to have colonies with nests dispersed over 250 meters. Polydomy occurs at larger distances in a stepwise fashion within the ‘humile' clade, suggesting species have incrementally increased their colony size sequentially, rather than an abrupt change in colony structure evolving solely in L. humile. Queen number estimates from field data and Microsatellite analysis suggest a similar pattern; many species within the ‘humile' clade have colonies with multiple queens, but none possess as many egg laying queens as often seen in L. humile colonies.