Can Epigenetic Regulation of Sex-Ratios Explain Natural Variation in Self-Fertilization Rates? - Kryptolebias marmoratus

The proportion of males and females in fish populations often differs as a consequence of environmental factors (e.g. temperature, sexual selection, selective exploitation). Imbalance in sex-ratios is problematic when reproduction requires both sexes, as a lack of suitable mates can negatively impact population growth and viability, particularly if fishing selectively favours one of the sexes. Fish have the most diverse array of sex determining mechanisms, yet the most common sex determining environmental variable is temperature. Temperature-dependant sex determination (TSD) has so far been reported in over 50 fish species from 13 different families and has evolved independently multiple times. Whether all of these have pure TSD mechanisms or actually possess genetic sex differentiation (GSD) influenced by temperature, remains a subject of debate. We used a unique self-fertilising mixed-mating fish (Kryptolebias marmoratus) with TSD to investigate the relative roles of genetics and epigenetics in sex determination, by combining methylation sensitive-AFLP (MS-AFLP) markers with Illumina sequencing. We identified a total of 54000 Differentially Methylated Regions between males and hermaphrodites of which 18,225 were considered significant at p<0.001. The former included six loci homologous that have been implicated in regulating sexual identity in other fishes. These DMRs were finally validated by bisulfite sequencing.  We conclude that in K. marmoratus an epigenetic mechanism regulated by temperature controls sexual identity, providing a widespread mechanism by which environmental change may influence selfing rates in mixed-mating species.