Altitudinal distribution of two sibling species of the Drosophila tripunctata group in a preserved tropical forest and their male sterility thermal thresholds.

Variation of ecophysiological traits may help to explain geographic distribution patterns of Drosophila sibling species. Many traits in ectotherms have optimal performance within specific temperature ranges. Altitudinal gradients are potentially informative for characterizing differences of sibling species distributions. We collected two sibling species of the tripunctata group - Drosophila mediopunctata (MPT) and D. unipunctata (UNI) - at eight altitudes (ranging from 593 to 1185m above sea level) located at a continuous Atlantic Rainforest reserve in consecutive years (2009-2011), with two collections at the hot-rainy season and two at the cold-dry season. Mean altitude was significantly different between species and seasons. Their distributions showed a consistent pattern with MPT always occurring at higher altitudes than UNI. A significant correlation was found between altitude and species relative abundance. We characterized the thermal range of fertility, an important fitness component, for each species and found evidence for differential thermal adaptation. Our results suggest that the two species altitudinal distributions and seasonal relative abundances are consistent with their differential thermal adaptations: MPT seems to be adapted to lower temperatures, occupies higher altitudes and occurs at higher relative abundances in the cold-dry season; while UNI tolerates higher temperatures and occurs at lower altitudes and higher relative abundances in the hot-rainy season. However, their thermal ranges overlap at most temperatures, suggesting that additional variables (e.g. habitat choice, competition, differential survival etc.) may also play a role to determine their distribution in the field.