We show that multiple suboptimal splice sites underlie the thermal-sensitive splicing of the period (per) 3′-terminal intron (dmpi8) from D. melanogaster, enabling this species to prolong its midday "siesta," a mechanism that likely diminishes the deleterious effects of heat during the longer summer days in temperate climates. In D. yakuba and D. santomea, which have a more ancestral distribution indigenous to Afro-equatorial regions wherein day length and temperature exhibit little fluctuation throughout the year, the splicing efficiencies of their per 3′-terminal introns do not exhibit thermal calibration, consistent with the little effect of temperature on the daily distribution of activity in these species. We propose that the weak splice sites on dmpi8 underlie a mechanism that facilitated the acclimation of the widely colonized D. melanogaster (and possibly D. simulans) to temperate climates and that natural selection operating at the level of splicing signals plays an important role in the thermal adaptation of life forms.
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We thank Doug Pike for excellent technical assistance. We are indebted to D. Coyne (University of Chicago) for providing D. santomea and D. yakuba flies, and the Tuscon Stock Center for D. yakuba and D. simulans strains. This work was supported by a grant from the National Institutes of Health (NINDS42088) to I.E.
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