Sperm length is not influenced by haploid gene expression in the flies Drosophila melanogaster and Scathophaga stercoraria.

Recent theoretical models have postulated a role for haploid-diploid conflict and for kin selection favouring sperm cooperation and altruism in the diversification and specialization of sperm form. A critical assumption of these models-that haploid gene expression contributes to variation in sperm form-has never been demonstrated and remains contentious. By quantifying within-male variation in sperm length using crosses between males and females from populations that had been subjected to divergent experimental selection, we demonstrate that haploid gene expression does not contribute to variation in sperm length in both Drosophila melanogaster and Scathophaga stercoraria. This finding casts doubt on the importance of haploid-diploid conflict and kin selection as evolutionary influences of sperm phenotypes.