Male accessory gland size depends on genotypes but not larval density conditions in nutrition-rich environments

Accessory glands (AGs), responsible for producing seminal fluid, are critical for male reproductive success, especially under sperm competition. In Drosophila melanogaster, AG size is often used as a proxy for reproductive investment, but its variation in response to developmental environments, as well as distinct genotypes remain unclear. Here, we investigated how larval density without nutritional limitation per individual, genotype, and genotype-by-environment interactions (GEIs) influence AG size using 27 isogenic lines from the Drosophila Genetic Reference Panel (DGRP). While larval density significantly affected body size, with higher density producing smaller males even though their nutritional source is rich, we could not detect any effect on AG size, either in absolute terms or relative to body size. In contrast, AG size varied significantly among genotypes, with only minor GEI effects. Our findings suggest that AG size might be genetically determined and relatively insensitive to larval density cues under nutrient-rich conditions. These results highlight the limitations of using AG size as a sole proxy for reproductive investment and the importance of genotype in shaping male reproductive traits.