Developmental expression patterns of six6: a gene linked with spawning ecotypes in Atlantic salmon

The Atlantic salmon has been studied extensively, particularly as a model for understanding the genetic and environmental contributions to the evolution and development of life history traits. Expression pattern analysis in situ, however, is mostly lacking in salmon. We examine the embryonic developmental expression of six6, a candidate gene previously identified to be associated with spawning ecotypes and age at sexual maturity, in Atlantic salmon. Six6 is a member of the sine oculis homeobox family of transcription factors and is known to regulate eye and brain development in other vertebrates. We assay the expression of this gene in embryonic Atlantic salmon Salmo salar by whole-mount in situ hybridization. In line with earlier studies in other vertebrate species, we find conserved expression in the developing brain and sensory organs, including optic and olfactory primordia. However, we also find previously unreported domains of expression that suggest additional roles in axial and appendicular development, cardiovascular, intestinal, and sensory organogenesis. Each of these systems are important in the sensory ecology of Atlantic salmon, suggesting it is plausible that six6 may have pleiotropic roles in this complex phenotype.