The Correlation of Climate and Host Plant Morphology with a Geographic Gradient of an Inversion Polymorphism in Drosophila pachea

A latitudinal gradient in frequency of a single inversion in chromosome 7 of Drosophila pachea is described. Correlations were made with long-term monthly means of precipitation and temperature according to seasons and according to time of collection from 15 weather stations. These data suggest that the 7+ gene arrangement is wet adapted and warm, but not hot adapted and that the fall season should be the time of greatest selection pressure from seasonal weather. Correlations were also made with the shape of the stem of the host plant. A boundary effect at approximately 28⚬N. latitude is described showing the coincidence of a break in the surface-volume ratio cline in the host plant and a shift in the biota of the region with the lower limit of appreciable heterozygosity of the inversion. Moreover, high chromosomal heterozygosity in this vicinity corresponds with high variability of heterozygote frequency at the Acph locus. Laboratory tests should specify the relative effects of climatic means, extremes and host plant morphology on inversion viability.