Variations in thermal history lead to dyssynchronous diapause development

The alfalfa leafcutting bee, Megachile rotundata, is the world’s most intensively managed solitary bee and the primary pollinator for alfalfa seed production.  Managed bees are subjected to thermal regimes for overwintering and subsequent adult emergence in time for alfalfa bloom.  In nature, first generation larval development is typically completed by mid-July when a portion of bees undergo diapause, a period of suppressed metabolism and development, and overwinter as prepupa.  However, a proportion of the population will avert diapause to produce a second generation.  Management practices during the prepupal stage affect adult bee physiology and have implications for overwintering survival.  Therefore, an understanding of diapause regulation will help improve management practices and enable use in additional agricultural markets.

Diapausing prepupae produced early and late in the season were divided between two management groups, those kept at a constant 4-5°C and those kept outdoors, exposed to natural temperature fluctuations.  Samples from four time points (November, January, March and May) were chosen for Illumina sequencing to span the diapause maintenance, termination and post-diapause quiescence stages of development.  Within month comparisons of differentially regulated genes indicates all four treatment groups represent distinct populations, demonstrating the plasticity of the bee’s phenotype in response to their environment.  Additionally, while the diapause process synchronizes spring emergence, it does not synchronize the physiology of bees oviposited at different points in the season.  These data confirm results from other species showing that laboratory studies are necessary, but not sufficient to explain diapause physiology under field conditions.