Sexual antagonism and sex determination in three syngnathid species alongside male pregnancy gradient and varying sex roles

Teleost fish show an enormous diversity of sex determination systems, varying from environmental sex determination to full sex chromosomes. Traditionally sex chromosomes are considered within a framework of sexually antagonistic relationships and are viewed not only as sex determination systems but also as a means to resolve sexual conflict by confining conflicting genes to one sex. However, the relationship between emergence sexually-antagonistic loci, resolution of sexual conflict and sex determination is not fully understood. In this study we take a look at genomic and transcriptomic signatures of sexual antagonism, and relate them to putative sex determination systems in three species from the family Syngnathidae: Nerophis ophidion, Syngnathus typhle and Hippocampus erectus. The family is famous for their extreme form of parental care - male pregnancy. Male pregnancy forms a parental investment gradient among species of the family. We selected species alongside this gradient to investigate the effects male pregnancy has on sexual conflict in syngnathids. We found that both S.typhle and N.ophidion seem to have environmental sex determination and that sexual conflict is primarily resolved via differential gene and allele expression. On the other hand H.erectus possess an XY sex chromosome system. Strikingly, the identified homomorphic Y chromosome is not homologous to the Y chromosome of H.erectus identified in an independent study. We hypothesize that both sex-linked chromosomes evolved due to captive breeding and represent transition states between sex determination systems. Furthermore, we propose that the master sex determination gene is located outside of these sex-linked chromosomes.