Sex and the Evolution of a Double Hermaphrodite

Abstract

The Kryptolebias marmoratus species complex contains the only known self-fertile hermaphroditic vertebrates. There are three species in this clade and all three live in the mangrove forests across the tropical Americas. All three have individuals with both testis and ovarian tissue in their gonads with two using self-fertility as their main mode of reproduction, and all three have apparent different sex determination and sexual modes. In this dissertation, I explore aspects of sex in these species. K. marmoratus is the androdiecious and self-fertile member of the species complex with sequential hermaphroditism. In this species, the control of sex change from hermaphrodite to male is poorly understood. Individuals that were believed to be genetically identical could be raised in the same environment and change sex at drastically different times or not at all. Small fluctuations and variance in the hormonal profiles of individuals was thought to be a potential cause and while androgen dosing can lead to masculinization of both the gonad and the soma, it was not enough to maintain a permanent transition like what is seen in nature. In K. ocellatus, the obligate outcrosser of the K. marmoratus species complex, it was believed that they were using genetic sex determination to differentiate between males and the females that had hermaphroditic gonads. While we found strong evidence against heteromorphic sex chromosomes, all tests for homomorphic sex chromosomes came back inconclusive due to apparent K. hermaphroditus DNA contaminating the dataset. K. hermaphroditus, the self-fertile hermaphrodite species with exceptionally rare males, appears to be extending its range further and further south and/or hybridizing with K. ocellatus at rates previously underappreciated. The hermaphrodites of the Kryptolebias genus still hold many evolutionary and physiological secrets but can potentially be revolutionary to the understanding of vertebrate sexual development and evolution.