Different mechanisms underlie phenotypic plasticity and interspecific variation for a reproductive character in drosophilids (Insecta : Diptera)

The insect ovary is a modular structure, the functional unit of which is th e ovariole. Ovariole number is positively correlated with potential reprodu ctive output. Among drosophilids (Insecta: Diptera), ovariole number shows both phenotypic plasticity and substantial interspecific and interpopulatio nal variation. Here we examine the mechanistic connection between phenotypi c plasticity and genetically fixed variation in ovariole number within the melanogaster species group. When a laboratory population of Drosophila mela nogaster was reared under reduced food conditions, differences in ovariole number were entirely due to alterations in cell differentiation during the wandering stage at the very end of larval development. Cell growth and cell death were not affected. When these same flies were reared under a variety of temperatures, ovariole number differences arose during the latter half of the third (final) larval instar. Cell differentiation was affected, alth ough cell number was not, and ovariole number differences were established before metamorphosis. Ln contrast, genetically fixed, interspecific and int erpopulational variability in ovariole number was caused by alterations in the dynamics of cell differentiation and by cell number differences. Furthe rmore, the stages affected were different in different species and populati ons in the melanogaster species group, ranging from the first (D. sechellia ) through the middle of the third (D. simulans and D. mauritiana) larval st age. Therefore, the mechanistic bases for plasticity-based variability are largely distinct from the mechanistic bases for interspecific and interpopu lational variability. Our results suggest that phenotypic plasticity indica tes evolutionary flexibility in underlying ontogenetic processes.