Meeting Abstract
Mangrove rivulus fish exist predominantly as self-fertilizing hermaphrodites but males, which result from hermaphrodite sex change, occur in varying abundances across their geographical range. Levels of heterozygosity and frequencies of outcrossing between hermaphrodites and males also differ among populations, raising the question of why male abundance varies. We hypothesized that heterozygous progeny derived from outcrossing would have higher fitness than homozygous progeny derived from selfing, especially under stressful conditions. We predicted that increased heterozygosity would correspond with lower mortality, higher growth, and greater reproductive success, trends that would be amplified in suboptimal conditions. To test this hypothesis, fish with varying levels of heterozygosity experienced control conditions or stressors common to their native habitat (high/low salinity, tidal flux). High salinity animals showed greater mortality, lower fecundity, and lower rates of sex change than those in low salinity or controls. Tidal animals were consistently smaller and less fecund than controls but mortality was unaffected. There were no treatment-dependent effects of heterozygosity but fecundity decreased as heterozygosity increased. Sex explained a significant proportion of phenotypic variation even prior to sexual maturation. Animals that eventually transitioned to male were significantly larger during the pre-treatment period than those that remained hermaphrodite. Our results suggest that ecologically relevant stressors influence fitness in a heterozygosity-independent manner and, contrary to prediction, heterozygosity confers no fitness advantage. The adaptive advantages of sex change and outcrossing and, ultimately, which factors govern variation in male abundance thus remain open questions.
