INVESTMENT IN TESTES AND THE COST OF MAKING LONG SPERM IN DROSOPHILA

Relationships among body mass, testis mass, sperm length, and the numb er of sperm produced were examined among 11 Drosophila species, after controlling for phylogenetic effects. This is the first study to exami ne many of these relationships comparatively in an invertebrate taxon; patterns observed among these variables were fundamentally different from those consistently reported in studies of vertebrates. In regress ion analyses, testis mass increased with body mass with an exponent gr eater than one, which indicates that males of larger-bodied Drosophila species make a proportionately greater energetic investment in testes than do males of smaller-bodied species. The positive allometry of te stis mass is hypothesized to be a combined consequence of the unusual positive relationship between body mass and sperm length and the posit ive relationship between sperm length and testis mass. Interspecific v ariation in testis mass was found to be a function of variation in spe rm length rather than variation in the number of sperm produced. Signi ficant trade-offs were identified between sperm length and the number of sperm produced and transferred per copulation. Results are discusse d in terms of the costs of producing longer sperm, the correlated evol ution of sperm length and body size, the relationship between breeding system and sperm production patterns, and the nature of differences b etween vertebrates and invertebrates in sperm production and the size of testes.