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Characterizing the role of Ecdysone induced protein 74EF in the length of sperm in males and seminal receptacle in females of D. Open Access

Sperm of D. melanogaster are among the longest known and are evolving in concert with the female's long, coiled sperm storage organ, the seminal receptacle (SR). During sperm competition, long sperm outcompete short sperm but primarily in long SRs, providing a post-copulatory analog to male trait-female preference coevolution, commonly modeled in pre-copulatory sexual selection. Sperm and SRs are also likely coevolving via Fisherian runaway sexual selection, as evidenced by a recently discovered genetic correlation between these two traits. We previously identified a number of candidate genes influencing sperm length using a RAD QTL sequencing approach and have prioritized Ecdysone induced protein 74EF (Eip74EF) due to its role in autophagy during development, post-meiotic expression during spermatogenesis (when spermatid elongation occurs), the presence of multiple SNP markers within the gene, and signatures of rapid evolution under positive selection among the 12 Drosophila genomes. We have found that knockout mutant males have shorter sperm, suggesting that Eip74EF acts on the positive regulation of sperm length. We are also investigating female mutants to determine if a similar pattern exists for SR length. If so, Eip74EF may have pleiotropic effects on both sperm length and SR length and may therefore be a key to the molecular mechanism of Fisherian runaway sexual selection. Both male and female mutants also have decreased fertility relative to control flies, and males have reduced sperm competitive success, consistent with the expectation that shorter sperm are weaker competitors.

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