Cold adaptation in the invasive ant Wasmannia auropuncta: Key factor in its geographical expansion?

MUÑOZ, IGNACIO; CALCATERRA, L. A.; SCHILMAN, P. E.

CABA

Simposio; International Symposium of Environmental Physiology of Ectotherms and Plants; 2019

In insects, cold tolerance generally increases with latitude, due to adaptation or acclimation, particularly for species undergoing range expansions or invasions. Originally from the tropic, the little fire ant, Wasmannia auropunctata, has expanded its distribution to subtropical and temperate regions of South America. The current southern limit of its distribution is in Buenos Aires, Argentina, and genetic analyses showed that this population is the source for the recent introduction in Israel. The aim of this study is to analyze if the thermo-physiological adaptation of W. auropunctata can explain its current and potential distribution limits. We used colonies across a latitudinal thermal gradient from Jujuy, Argentina (S24°31´) to CABA, Argentina (S34°54´). These ants were acclimated to constant temperatures of 15, 25°C or a fluctuating cycle of 12h at 35ºC (light) and 12h at 15ºC (dark), then maximum and minimum critical temperatures (CTmax and CTmin) were measured. Results show that a colony from low latitude reaches a significant lower CTmax (43.9ºC) than the other colonies (44.1-44.5ºC), and a colony from high latitudes has a significant lower CTmin (6.1ºC) than other colony from this same latitude (6.9ºC), and to colonies from a same low latitude, but different longitude (7.3-7.4ºC). In addition, no correlation between (1) CTmax and the average maximum temperature of the warmer month of the colony location, or (2) CTmin and the minimum temperatures of the coldest month were found for colonies keep at 25°C. However, the latter correlation was significant when colonies were acclimated at 15°C for two weeks. Noteworthy, latter results showed populations cluster into 3 groups, which correspond to three genetically distant COI haplotypes. No clear differences on CTs were found for colonies exposed to temperature cycles. Results support the cold adaptation hypothesis in the southern limit of W. auropunctata distribution, which would allow to this invasive ant to colonize temperate regions as Israel.