Lateralized expression of left-right axis formation genes is shared by adult brains of lefty and righty scale-eating cichlids.

Variation in the laterality often exists within species and can be maintained by frequency-dependent selection. Although the molecular developmental mechanisms underlying the left-right axis formation have been investigated, the genomic mechanisms underlying variation in laterality remain largely unknown. The scale-eating cichlid Perissodus microlepis in Lake Tanganyika exhibit lateralized predation; lefty individuals with the mouth opening toward the right preferentially attack on the prey's left trunk, while righty individuals with the opposite opening attacks on the right trunk. Here, we performed RNA-sequencing and subsequent confirmation with quantitative-PCR in the telencephalon, optic tectum, and hindbrain of the cichlid and identified five genes (pkd1b, ntn1b, ansn, pde6g, and rbp4l1) that were differentially expressed between the hemispheres regardless of the laterality. Surprisingly, pkd1b and ntn1b are involved in nodal and netrin signalling, respectively, which are important for left–right asymmetry formation during early embryogenesis. This result indicates that nodal- and netrin-related signals may also play important roles in the maintenance of asymmetry in adult brain. By contrast, no genes showed reversal of lateral differences between lefty and righty individuals in any brain regions examined, suggesting that laterality in the scale-eating cichlid does not simply result from inversion of the left–right asymmetry of gene expression.