Behavioral differentiation in oviposition activity in Drosophila buzzatii from highland and lowland populations in Argentina: Plasticity or thermal adaptation?

Highland populations of several Drosophila species in Argentina were active early in the afternoon in the held as opposed to populations from a much w armer lowland site, where flies were mainly active in the early evening pri or to sunset. For one of these species, Drosophila buzzatii, we tested for a genetic component of activity differences by carrying out crosses within and between populations and measuring oviposition activity of the progeny i n the laboratory. We found that activity in the highland population exceede d that in the lowland one during the midafternoon whereas activity in the l owland population exceeded that in the highland one prior to the beginning of the dark period. Oviposition activity for the period corresponding to th e field observations was regressed on the proportion of the genome derived from the highland population. This variable significantly predicted oviposi tion activity between 1400 and 1600 and between 2000 and 2200 h. Activity o f both reciprocal crosses was intermediate and not significantly different from each other, suggesting that nuclear genetic, rather than cytoplasmic f actors contribute to differences in oviposition activity between the popula tions. Two morphological, one genetic, and one stress resistance trait were also scored to examine whether temperature differences between environment s were associated with other differences between populations. Wing length o f wild-caught and laboratory reared flies from the highland population sign ificantly exceeded that in the lowland. Thorax length of laboratory-reared flies from the highland population also significantly exceeded that from th e lowland. Chromosomal inversion frequencies differed significantly between the two populations with a fivefold reduction in the frequency of arrangem ent 2st in the highland as compared to the lowland population. This arrange ment is known for its negative dose effect on size, and thus, the highland population has experienced a genetic change, perhaps as a result of adaptat ion to the colder environment, where body size and the frequency of arrange ment 2st have changed in concert. Finally, a heat knockdown test revealed t hat the lowland population was significantly more resistant to high tempera ture than the highland one. In conclusion, we suggest that temperature has been an important selective agent causing adaptive differentiation between these two populations. We also suggest that the activity rhythms of the two populations have diverged as a consequence of behavioral evolution, that i s, through avoidance of stressful temperatures as a mean of thermal adaptat ion.