The number of anal fin-rays is decided by two heritable traits, anteroposterior length of the anal fin and interval between the anal fin-rays

The number of median fin-rays is a readily quantifiable trait that has been previously studied due to its interspecific variation. I observed in F2 progeny from two inbred lines of medaka with differing anal fin-ray numbers (Kaga and Hd-rRII1) that individuals exhibited the same ray number but differed in the anteroposterior length of the anal fin and the interval between the anal fin-rays. Inducing vertebral fusion to shorten the anal fin anteroposterior length resulted in a decrease in ray number. Given reports of an increased ray number during low-temperature development, I reared Hd-rRII1 fry at 17°C post-hatching, which resulted in an increase in the number of rays. At this temperature, the anal fin anteroposterior length and the interval between the anal fin-rays were also slightly reduced. Since the posterior boundary of the anal fin is determined by Hox genes, I hypothesize that a temperature-sensitive signaling pathway exists downstream of these genes. Collectively, our results suggest that the medaka anal fin-ray number is determined by at least two genetic traits: the anal fin anteroposterior length and the interval between the anal fin-rays.