Resistance to temperature extremes between and within life cycle stages inDrosophila serrata, D-birchii and their hybrids: intraspecific and interspecific comparisons

Intraspecific Drosophila studies suggest that resistance to heal and cold s tresses are largely independent and that correlations across life cycle sta ges are low whereas comparisons of Drosophila species indicate correlations between heat and cold resistance as well as between resistance levels in d ifferent life cycle stages. These inconsistent results may reflect differen ces in associations among traits at the interspecific and intraspecific lev els or interspecific correlations arising because of correlated selection p ressures. These alternatives were tested using Drosophila serrata, D. birch ii and hybrids derived from these species. Variation among hybrid lines and families was used to test associations at the interspecific level while in traspecific variation was examined using isofemale lines of D. serrata. The re was a significant association between adult heat knockdown time at 38 de greesC and adult cold resistance in one set of hybrid lines. An association between female knockdown resistance to heat and larval heat resistance was also evident in one set of hybrids. Resistance to heat anti cold at the la rval stage were not correlated at either the intraspecific or interspecific levels. At the intraspecific level, lan al heat resistance and two measure s of adult heat resistance were uncorrelated. Moreover, adult and larval co ld resistance measures were not correlated at either the intraspecific or i nterspecific levels. These results suggest that there are no associations b etween resistance to heat and cold extremes and that extreme temperature re sistance is largely independent across life cycle stages at both the intras pecific and interspecific levels. Species associations may therefore arise from correlated selection pressures rather than trait correlations. (C) 200 0 The Linnean Society of London.