CROTWELL, P.L.*; MABEE, P.M.; Univ. of South Dakota, Vermillion; Univ. of South Dakota, Vermillion: Co-option of genetic regulatory mechanisms? From fish segments to mouse joints.

We have hypothesized that the mechanisms underlying radial segmentation in fishes may have been co-opted over several million years of evolution to give rise to tetrapod synovial joints. Radials are cartilaginous rods that support the dorsal and anal fins, and that undergo segmentation during fin development. A clear understanding of gene expression patterns associated with fish radial segmentation is necessary for fish fin to tetrapod joint comparisons. I examined gene expression patterns for Gdf5, noggin, Collagen II, Bmp2, and Bmp4 in zebrafish median fins. Expression patterns are dynamic, changing in both extent and intensity during development and segmentation of the radials. Both similarities (Gdf5, noggin, Collagen II) and differences (Bmp2, Bmp4) in segment vs. joint development were observed. In the course of testing our hypothesis, several general questions have been raised: How is a hypothesis of co-option best tested? At what point are data sufficient to support or reject such a hypothesis? Is there a continuum between co-option of a preexisting gene network vs. construction of a novel network that may or may not use some of the same genes? Comparisons of skeletal segmentation mechanisms within an evolutionary framework serve here as an example for evaluating hypotheses of co-option.