GENETIC-VARIATION AND PLASTICITY OF THORAX LENGTH AND WING LENGTH IN DROSOPHILA-ALDRICHI AND DROSOPHILA-BUZZATII

Reaction norms across three temperatures of development were measured for thorax length, wing length and wing length/thorax length ratio for ten isofemale lines from each of two populations of Drosophila aldric hi and D. buzzatii. Means for thorax and wing length in both species w ere larger at 24 degrees C than at either 18 degrees C or 31 degrees C , with the reduction in size at 18 degrees C most likely due to a nutr itional constraint. Although females were larger than males, the sexes were not different for wing length/thorax length ratio. The plasticit y of the traits differed between species and between populations of ea ch species, with genetic variation in plasticity similar for the two s pecies from one locality, but much higher for D. aldrichi from the oth er. Estimates of heritabilities for D. aldrichi generally were higher at 18 degrees C and 24 degrees C than at 31 degrees C, but for D. buzz atii they were highest at 31 degrees C, although heritabilities were n ot significantly different between species at any temperature. Additiv e genetic variances for D. aldrichi showed trends similar to that for heritability, being highest at 18 degrees C and decreasing as temperat ure increased. For D. buzzatii, however, additive genetic variances we re lowest at 24 degrees C. These results are suggestive that genetic v ariation for body size characters is increased in more stressful envir onments. Thorax and wing lengths showed significant genetic correlatio ns that were not different between the species, but the genetic correl ations between each of these traits and their ratio were significantly different. For D. aldrichi, genetic variation in the wing length/thor ax length ratio was due primarily to variation in thorax length, while for D. buzzatii, it was due primarily to variation in wing length. Th e wing length/thorax length ratio, which is the inverse of wing loadin g, decreased linearly as temperature increased, and it is suggested th at this ratio may be of greater adaptive significance than either of i ts components.