Variation in sperm motility and seminal plasma protein expression is shaped by pre- and post-mating sexual selection in the mouthbrooding cichlid ( <i>Ophthalmotilapia ventralis</i> )

Sexual selection is a key driver of reproductive strategy evolution; however, its molecular mechanisms remain poorly understood. In the mouthbrooding Tanganyikan cichlid (Ophthalmotilapia ventralis), fertilization occurs in the female mouth cavity, where sperm competition and post-mating female choice arise because females collect sperm from multiple territorial males. Seminal plasma glycoprotein 120 (SPP120) is involved in sperm immobilization and aggregation, which may contribute to prolonged storage and use of sperm in the female buccal cavity. In this study, we examined how factors related to pre- and post-mating sexual selection affect sperm traits and SPP120 expression in O. ventralis. Field observations and physiological and molecular analyses showed positive correlations between sperm longevity of territorial males and encounter rate with floating males (ERFM) as a measure of sperm competition or bower (spawning site) density related to sperm competition and between SPP120 expression of territorial males and ERFM or courtship success as a measure of pre- and post-mating female mate choice. Furthermore, males with higher SPP120 expression levels achieved greater mating success. These findings highlight the complex interplay between sexual selection and molecular adaptation, providing new insights into the evolution of animal reproductive strategies.