We compared mortality and heat shock protein 70 (HSP70) levels between two introduced leaf-cutter bee species: Megachile apicalis and M. rotundata. Both are introduced species that have established populations in the western USA. Previous studies demonstrate that both species appear to occupy marginal, xeric habitats found along riparian zones in the Central Valley of California more frequently than native megachilid species, although M. apicalis appears to be more abundant. Samples of prepupae from both species were stored in a refrigerator before being acclimated to room temperature for a four-day period. Brood cells of both species were incubated together at each of nine three-hour temperature exposures: 2.5 oC increments from 35 to 55 oC. After one hour, a subsample of bees was stored at -80 oC while the remainder was reared at 30 oC for a one-month period. Heads of prepupae were later homogenized and assayed for HSP70 using SDS-PAGE and ELISA techniques. Megachile apicalis showed higher survivorship after 50 oC than did M. rotundata. Megachile rotundata had significantly higher levels of HSP70 than M. apicalis at all temperature regimes. Megachile apicalis also showed a later HSP70 peak at 45 oC relative to a peak of 42.5 oC for M. rotundata. This pattern of mortality and HSP70 accumulation in the two species suggest that M. apicalis is better adapted to the warmer climate of the Central Valley of California than M. rotundata. These results are consistent with other studies that show species differences among closely related congeners that occupy thermally distinct habitats or microhabitats.
Species 1: Hymenoptera Megachilidae Megachile rotundata (alfalfa leaf-cutter bee, alfalfa leafcutting bee)
Species 2: Hymenoptera Megachilidae Megachile apicalis
Keywords: stress proteins, introduced species
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