A panel of medaka isogenic lines suggests individual, seasonal, and sexual genetic variation of bdnf gene expression in the brain

Introduction: Once recognized only in humans, variance in the cognitive phenotype is now acknowledged in a range of vertebrate species. However, our understanding of its underlying causes is still incomplete. Brain-derived neurotrophic factor (BDNF) is an essential protein for brain functioning and plays a key role in cognitive processes such as learning and memory, including interindividual variation. Environmental factors influence BDNF abundance in the brain, and so do genetic polymorphisms in humans and mice. Methods: Using the Medaka Inbred Kiyosu-Karlsruhe (MIKK) panel of near-isogenic medaka lines, which captures a wide range of natural genetic variation in this species, we investigated the potential quantitative genetic variation in bdnf gene expression in the brain. Results: Our findings show significant variation in bdnf mRNA expression levels across MIKK lines, with a two-fold difference between the lines exhibiting lower and higher expression. Seasonal variation was also observed, with higher average bdnf levels in summer. However, a tentative analysis suggested that this average effect was not consistent across the lines, with some lines even showing significantly greater expression in winter. Similarly, across the entire sample, males and females did not differ in bdnf expression overall, although some lines displayed sex differences greater than expected by chance. Discussion: These results suggest that quantitative genetic differences, in concert with environmental influences, contribute to bdnf expression variability.