Reaction norms across and genetic parameters at different temperatures forthorax and wing size traits in Drosophila aldrichi and D-buzzatii

Reaction norms across seven constant and one fluctuating temperature of dev elopment were measured for thorax length and several wing size traits for u p to 10 isofemale lines of each of the cactophilic Drosophila species, D. a ldrichi and D. buzzatii, originating from the same locality. Maximum thorax length was reached at different low to intermediate temperatures for the t wo species, whereas wing length was highest at the lowest temperature in bo th species. Various ratio parameters showed pronounced species differences. The reaction norm for the wing loading index (wing length/thorax length) d ecreased monotonically with temperature in both species, but was much steep er and spanned a wider range in D. aldrichi than in D. buzzatii, suggesting either that wing loading is not a good characterization of night capacity or, more likely, that night optimization does not occur in the same manner in both species. The vein ratio (distal length/proximal length of the third vein) increased with temperature in D. buzzatii but decreased in D. aldric hi. Wing development in the two species thus is very different, with the pr oximal part of the wing in D. buzzatii more closely allied to the thorax th an td the distal part. Among line variation was significant for all traits in both species, and most pronounced for thorax length and the ratio parame ters. Coefficients of variation were significantly different between the sp ecies for all traits, with those in D. aldrichi higher than in D. buzzatii. Genetic variance in plasticity was significant for all traits in D. buzzat ii, but only for seven out of 12 in D. aldrichi. Additive genetic variances for all traits in both species were significantly larger than zero. Geneti c correlations between thorax length and several wing length parameters, an d between these and wing area, were positive and generally significant in b oth species. The genetic correlation between the distal and the proximal le ngth of the third vein was not significantly different from zero in D. aldr ichi, but negative and significant in D. buzzatii. Heritabilites varied sig nificantly among temperatures for almost all traits in both species. Phenot ypic variances were generally higher in D. aldrichi than in D. buzzatii, an d commonly highest at the extreme temperatures in the former species. At th e high temperature the genetic variances also were usually highest in D. al drichi. The data clearly suggest that the process of thermal adaptation is species specific and caution against generalizations based on the study of single species.