THE GENETICS OF CENTRAL AND MARGINAL POPULATIONS OF DROSOPHILA-SERRATA .1. GENETIC-VARIATION FOR STRESS RESISTANCE AND SPECIES BORDERS

A selection experiment was used to determine if levels of genetic vari ance in an ecologically important trait, desiccation resistance, were different in central and marginal populations. Four populations of Dro sophila serrata were sampled from central and marginal areas of its di stribution, along a 3000-km stretch of Australia's east coast. Rainfal l patterns along this stretch of coastline change from a tropical cycl e in the north to a temperate cycle in the south. Replicate lines from the four populations underwent selection for desiccation resistance f or 14 generations. Realized heritabilities calculated after 10 and 14 generations of selection indicated that the four populations differed significantly in the level of genetic variation for desiccation resist ance available to selection. Populations from the more southern margin al areas had lower realized heritabilities than more northern central populations. However, a corresponding increase in mean desiccation res istance toward the margin was not found. A mechanism by which D. serra ta seemed to have responded to selection was a reduction in the extent that metabolic rate was increased when flies were exposed to low humi dity. This response indicates genetic variation for the control of met abolic rate. In contrast, increased desiccation resistance was not ass ociated with lipid or glycogen levels. Increased resistance to desicca tion was accompanied by increased starvation resistance, but radiation resistance was not affected. Selection did not affect the degree that replicate lines or populations had diverged.