Genetic analysis of isometric growth control mechanisms in the zebrafish caudal Fin.

The body and fins of the zebrafish grow rapidly as juveniles and slower as they reach maturation. Throughout their lives, the fins grow isometrically with respect to the body. Growth of individual fin rays is achieved by the distal addition of bony segments. We have investigated the genetic control of mechanisms that initiate new segments or control size of newly initiated segments. We find that both segment initiation and segment length are regulated during fin growth in wild-type fish. We examined the growth properties of lof and sof fin length mutants for effects on the number and length of fin ray segments. Fins of lof mutants continue to grow rapidly even after wild-type fin growth slows, resulting in positive allometric growth and additional fin ray segments. We suggest that lof mutants bypass mechanisms that limit segment initiation. Isometric growth is retained in sof mutants, resulting in shorter fins one-half the length of wild-type fins. The primary defect in sof mutants is that fin ray segments are shorter than wild-type segments, although segment number is also diminished. Double mutants for sof;lof reveal that segment length and segment number are controlled in different pathways. Our findings suggest that the lof gene product regulates segment initiation and the sof gene product regulates segment length.