Evolution of Sex Determination in Amniotes: Did Stress and Sequential Hermaphroditism Produce Environmental Determination?

• Published in: BioEssays 2020-10, Vol.42 (10), p.e2000050-n/a
• Main Authors: Straková, Barbora, Rovatsos, Michail, Kubička, Lukáš, Kratochvíl, Lukáš
• Format: Article
• Language: English
• Subjects: amniotes; Chromosomes; Embryos; environmental sex determination; Gender aspects; Genomes; Hermaphroditism; heterochronies; Homology; Phylogenetics; Phylogeny; Reptiles; sequential hermaphrodites; Sex; sex changes; Sex chromosomes; Sex determination; Sex differences; stress
• ISSN: 0265-9247; 1521-1878
• DOI: 10.1002/bies.202000050

Frequent independent origins of environmental sex determination (ESD) are assumed within amniotes. However, the phylogenetic distribution of sex‐determining modes suggests that ESD is likely very ancient and may be homologous across ESD groups. Sex chromosomes are demonstrated to be old and stable in endothermic (mammals and birds) and many ectothermic (non‐avian reptiles) lineages, but they are mostly non‐homologous between individual amniote lineages. The phylogenetic pattern may be explained by ancestral ESD with multiple transitions to later evolutionary stable genotypic sex determination. It is pointed out here that amniote ESD shares several key aspects with sequential hermaphroditism of fishes such as a lack of sex differences in genomes, biased population sex ratios, and potentially also molecular mechanism related to general stress responses. Here, it is speculated that ESD evolves via a heterochronic shift of the sensitive period of sex change from the adult to the embryonic stage in a hermaphroditic amniote ancestor. Also see the video here https://youtu.be/q2mjtlCefu4. It is speculated here that environmental sex determination in amniotes evolves via a heterochronic shift of sex change from the adult to the embryonic stage in a hermaphroditic ancestor. Subsequently, the loss of responsiveness to environmental stimuli (stress) led to genotypic sex determination, where the sex is decided at conception.