Meeting Abstract
Caudal fin morphology varies extensively among fish species, yet we know very little about the developmental programs underlying such diversity. Further, the molecular pathways that create proximal versus distal morphological characteristics in the fins are poorly understood. In zebrafish, the caudal fin skeleton is composed of bony fin rays made up of individual segments, which taper and shorten distally, and form a primary branch at about half of the fin’s length at adult stage. Mutations in various ion channels are known to cause overall scaling of the entire fin, but the morphological features along the proximal-distal axis are maintained in proportion. In contrast to these proportionally scaled changes, we identify a novel role for thyroid hormone in patterning the proximo-distal morphology of the fin rays. While thyroid hormone is a well-known endocrine regulator of skeletal development and is generally considered to promote bone mineralization, the role as a proximo-distal patterning factor is novel. We show that the hormone mediates both the proximo-distal morphology of the ray segments as well as the position of the fin ray branches, but not overall fin size. Thus, thyroid hormone acts independently of the bioelectricity-mediated pathways that regulate fin growth. Sonic hedgehog signaling is known to be essential in fin ray branching, and we show that thyroid hormone acts upstream of this pathway. Further, our expression analyses show differences in the transcriptomes of proximal versus distal regions of the regenerating fin, suggesting target pathways that create proximal versus distal morphologies; our data are consistent with thyroid hormone regulation of these proximo-distal expression patterns. In all, our results provide new insights into the mechanisms underlying proximo-distal identity as well as adaptations in the fin skeleton.
