Natural variation in Drosophila melanogaster diapause due to the insulin-regulated PI3-kinase

Abstract

This study links natural variation in a Drosophila melanogaster overwintering strategy, diapause, to the insulin-regulated phosphatidylinositol 3-kinase (PI3-kinase) gene, Dp110. Variation in diapause, a reproductive arrest, was associated with Dp110 by using Dp110 deletions and genomic rescue fragments in transgenic flies. Deletions of Dp110 increased the proportion of individuals in diapause, whereas expression of Dp110 in the nervous system, but not including the visual system, decreased it. The roles of phosphatidylinositol 3-kinase for both diapause in D. melanogaster and dauer formation in Caenorhabditis elegans suggest a conserved role for this kinase in both reproductive and developmental arrests in response to environmental stresses.

Fig. 1.

Diapause natural variants map to Dp110. Wild-type W (filled circles) or C (open squares) flies were crossed to two deletions, which remove Dp110{yw;P[ry+,gH],Dp110^A/TM3,Ser,y⁺ (A) and yw;P[ry⁺,gH], Dp110^B/TM3,Ser,y⁺ (C)} and to two rescue lines each bearing a deletion plus a genomic Dp110 rescue fragment [yw;P{w⁺,g(Dp110⁹⁸)},Dp110^A/TM3,Ser,y⁺ (B) and yw;P{g(Dp110¹⁰¹)};Dp110^B/TM6B,Tb,Sb (D)]. From each of the four crosses, two populations of flies were recovered: one in which C or W was heterozygous with the deletion (or deletion plus rescue fragment) and one in which C or W was heterozygous with the balancer chromosome. The interaction plots show the mean proportion of flies in diapause (±SEM) for the progeny of each cross. There were significant statistical interactions between the proportions in diapause for deletion and the balancer [Dp110^A: F_(1,24) = 4.53, P < 0.04; Dp110^B: F_(1,24) = 30.47, P < 0.0001]. The lack of parallel lines found for the crosses to the deletion strains Dp110^A and Dp110^B indicate statistical interaction, demonstrating positive quantitative complementation (13). The interactions disappeared when the Dp110 genomic rescue fragments were present, indicating restoration of the phenotype with the genomic fragments [P{g(Dp110⁹⁸)} F_(1,17) = 0.03, P = 0.86; P{g(Dp110¹⁰¹)} F_(1,15) = 0.00, P = 0.95].

Fig. 2.

Expression of Dp110. (A) Expression of Dp110 in the nervous system decreases diapause. Independently generated homozygous viable transgenic lines UAS-Dp110^II and Dp110^III with inserts on chromosomes-2 and -3, respectively, are used here. We first confirm that the two UAS-Dp110 lines generated had a photoperiodic response. We measure diapause under SD (light gray bars) and LD (dark gray bars) conditions, an accepted test of photoperiodic diapause (5). Our expectation that under SD conditions more flies would diapause than under LD conditions was met for both the UAS-Dp110 lines [ANOVA F_(1,6) = 10.7, P = 0.02 and F_(1,9) = 12.9, P = 0.006]. (B) Pan-neuronal expression of UAS-Dp110^II and UAS-Dp110^III by using the elav-GAL4 driver under SD conditions (open bars) significantly lowered the proportion of flies in diapause by comparison with the two control excision lines generated from UAS-Dp110^IIex and UAS-Dp110^IIIex and crossed to elav-GAL4 (filled bars) [two-way ANOVA: F_(1,17) = 5.59, P < 0.03]. (C) Expressing UAS-Dp110^II and UAS-Dp110^III by using a GMR-GAL4 driver under SD conditions (white hatched bars) did not significantly affect the proportion of flies in diapause when compared with the two control excision lines (dark gray hatched bars) [two-way ANOVA: F_(1,31) = 0.68, P = 0.41].