Meeting Abstract

S2-1.3  Thursday, Jan. 3  Segmenting the Mosaic Head KIMMEL, C. B.*; EBERHART, J. K.; MOENS, C. B.; University of Oregon, Eugene; University of Oregon, Eugene; Fred Hutchinson Cancer Center, Seattle kimmel@uoneuro.uoregon.edu

Segmental patterning provides for evolvability, and hence is of great evo devo interest. Two prominent series of segments lie almost in register in the same region of the head, the hindbrain rhombomeres and the pharyngeal branchiomeres. The two are directly connected by early neural crest migration as well as by later hookups by cranial motor and sensory nerves. The ancient idea that these two series are separate parts of a single �primary� process of head segmentation cannot be fully decided until we learn more about the initial iteration-generating mechanism(s). However, the primary segmentation model has received little or no support in recent studies. In contrast, the new work supports an alternative hypothesis that the head is assembled in a piecemeal, mosaic fashion with the two series having perhaps nearly independent origins. Furthermore, specializations and elaborations of particular segments may have misled past workers as to understanding how many segments are present in a particular series, and where in the head a series actually begins and ends. We can illustrate the controversies, and provide some resolution, with studies of branchiomere patterning in zebrafish. We argue that understanding gene expression patterns, fate maps of specific skeletal elements, patterning roles of endoderm and oral ectoderm, and the phenotypes of embryos bearing mutations in developmental regulatory genes all provide new insights into the old issues of segmentation in the head and its developmental basis.