Investment in testes and the cost of making long sperm in Drosophila

Relationships among body mass, testis mass, sperm length, and the number of sperm produced were examined among II Drosophila species, after controlling for phylogenetic effects. This is the first study to examine 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 regression analyses, testis mass increased with body mass with an exponent greater 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 testis mass is hypothesized to be a combined consequence of the unusual positive relationship between body mass and sperm length and the positive relationship between sperm length and testis mass. Interspecific variation in testis mass was found to be a function of variation in sperm length rather than variation in the number of sperm produced. Significant trade-offs were identified between sperm length and the number of sperm produced and transferred per copulation. Results are discussed in terms of the costs of producing longer sperm, the correlated evolution of sperm length and body size, the relationship between breeding system and sperm production patterns, and the nature of differences between vertebrates and invertebrates in sperm production and the size of testes.