by Lauren Closs
Faculty mentor: Dr. Dianne Baker
Mate guarding, when two males compete for one female, is a reproductive strategy seen across a variety of vertebrate species. This often leads to hierarchical relationships, in which one male exerts dominance over other, subordinate males. However, the physiological mechanisms that promote dominance or subordinance in males remain largely unexplored. This study investigates the reproductive success and endocrine signals of these reproductive strategies in Japanese medaka (Oryzias latipes). To identify dominant and subordinate males, triads consisting of two males of different genotypes and one female were observed repeatedly for 5 days. Male reproductive success was determined by genotyping embryos from each female. We found that the number of eggs fertilized by dominants and subordinates did not differ (p=0.29), indicating that dominant behavior does not guarantee reproductive success and that subordinate males may successfully fertilize eggs using sneaker male tactics. We hypothesized that these behaviors are linked to activity in the reproductive endocrine axis. To test this hypothesis, we quantified pituitary luteinizing hormone (LH) and follicle stimulating hormone (FSH) in dominant and subordinate males using ELISAs. While FSH did not differ between the groups, LH was unexpectedly higher in subordinate males (p=0.047). This indicates that either LH production is stimulated, or its pituitary release is inhibited in subordinates. To investigate these opposing explanations, we measured mRNA levels of LH, FSH, and GnRH receptors in the pituitary, and GnRH and AVT in the brain of dominant and subordinate males using qPCR. Mean differences between dominants and subordinates were not significant for any gene. Dominant fish expressed higher lhb in 8/12 tanks, indicating that LH production is not stimulated in subordinates, but as the transcripts for GnRH and its receptors also did not differ, further studies are needed to determine the mechanism by which LH release may be inhibited.