Correlated response in reproductive and life history traits to selection on testis length in Drosophila hydei.

Flies in the genus Drosophila have undergone striking evolutionary divergence in the size and number of sperm produced. Based on comparative studies of sperm length, testis length, and other reproductive and life history traits, including body size, age at first reproduction, and the number of sperm produced, macroevolutionary trade-offs resulting from the need to produce high-investment testes have been postulated. To understand better the microevolutionary processes underlying these interspecific patterns, we imposed replicated bidirectional selection for testis length for 11-12 generations on D. hydei, a species with 23.5 mm-long sperm and 30 mm-long testes. Testis length exhibited realized heritabilities ranging from 0.45 to 0.72. Following selection, traits were assayed for correlated responses. Thorax length, testis mass, sperm length, egg-to-adult development time, and posteclosion maturation time showed consistent positive correlated responses. Numbers of sperm produced and transferred to females, male longevity, female egg productivity, and seminal receptacle length did not show consistent correlated responses to selection on testis length. The pattern of correlated responses to testis length reveal the potential for the evolution of reproductive strategies to alter important life history attributes.