Virility does not Imply Immensity: Testis size, Accessory Gland Size and Ejaculate depletion pattern do not Evolve in Response to Experimental Manipulation of Sex Ratio in Drosophila melanogaster

Sperm competition theory predicts that increase in sperm competition should result in bigger male reproductive organ(s) or higher ejaculate investment. We test this idea using experimental evolution in Drosophila melanogaster. We maintained replicate populations of Drosophila under male (M) and female (F) biased sex ratio regimes for more than a hundred generations. Males in M regime have evolved higher sperm competitive ability than those in F. We measured the testes and the accessory gland size of virgin and singly mated males from the M and F populations. The M and F males do not differ in either testis or accessory gland size. Additionally, ejaculate investment is not different in the M and F males, as measured by reduction in testis and accessory gland sizes. Thus, contrary to theoretical prediction and evidence from other species, evolved differences in sperm competitive ability is not due to evolution of testis/accessory gland size or strategic ejaculate investment in these populations.