The clock and wavefront mechanisms underlying segmentation in vertebrates and the genetic hierarchy regulating segmentation in Drosophila seemingly imply independent origins of metameric development. However, comparative studies in non-drosophilid insects and other arthropods provide increasing molecular evidence for a common ancestry. In Drosophila, segments are patterned simultaneously. In most other insects including the red flour beetle Tribolium castaneum, other arthropods and vertebrates, segmentation is a longer process that occurs progressively from anterior to posterior. To understand the molecular mechanisms driving sequential metamerism and identify new genes important to this process, we are studying the genetic regulation of segmentation in Tribolium. In Tribolium, abdominal segments arise from a posterior growth zone during germband elongation, and RNAi with several early patterning genes truncates segmentation from the growth zone, suggesting this is an important regulation point. We have identified a gene circuit of primary pair-rule genes that functions to generate segments sequentially. Defects in its components disrupt the gene circuit and truncate the segmentation process. Periodic expression of two genes in this circuit initiates as twin spots in the posterior growth zone and may be regulated by early patterning genes. In addition, we have found that TcWnt8 is expressed in twin spots in the posterior growth zone, and TcWnt8 RNAi embryos are truncated in the thorax. We hypothesize that segmentation and elongation may be regulated by posterior signals that control components of the pair-rule gene circuit and/or proliferation. We will combine genetic and molecular approaches to analyze the relationship of cell proliferation, convergent extension and posterior signaling to segmentation, all of which are fundamental to embryonic elongation in vertebrates and arthropods. The genetic and genomic tools we have developed, combined with the Tribolium genome sequence, provide a unique opportunity to identify, regulatory genes and molecular processes which may represent presently unrecognized, developmentally significant mammalian counterparts important to segmentation. Our studies have the potential to provide new insight into human birth defects in spinal development and cancers related to misregulation of signaling mechanisms.
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