Meeting Abstract
Although segmentation has been studied in great detail in Drosophila melanogaster, the simultaneous formation of segments (long germ system), as found in this fly, and the underlying mechanism appears to be a derived character. In most animals, the metameric units (segments, somites) form mainly by sequential posterior addition (short germ systems). The currently prevalent models of these processes in vertebrates and short germ arthropods rely on successive patterning of large pools of undifferentiated cells (presomitic mesoderm, posterior growth zone) by anteriorly travelling waves of gene expression. By analysing gene expression and cell proliferation, we show that posterior segmentation in the spider Parasteatoda tepidariorum does not involve a large pool of undifferentiated cells. Instead segmentation and cell proliferation are tightly coupled. Each new segment is made in a two-phase process of tissue generation and segment patterning. This sequence repeats itself for every new segment being formed. Furthermore, we found that the segment polarity gene Pt-hedgehog is one of the earliest genes involved in this process. To our knowledge, this is the first report of a role of hedgehog in segmentation upstream of its highly conserved role as a segment polarity gene. Expression analysis of pair-rule gene and gap gene orthologues further supports that segments are patterned individually at the place of their tissue origin in a process of expression domain refinement. We further demonstrate that the gap gene orthologues Pt-krüppel1 is involved in this process. Posterior segmentation in spiders thus represents an alternative way to the canonical model of short germ segmentation at the base of the arthropods.
