A putative serpentine receptor gene tasA required for normal morphogenesis of primary stalk and branch structure in Polysphondylium pallidum.

The fruiting body of Polysphondylium pallidum is composed of whorls of branches along the axis of a primary stalk. In the course of fruiting body formation, the interval between neighboring whorls and the number and the spacing of branches in a whorl are highly regulated. In this study, using restriction enzyme mediated integration mutagenesis, we have obtained a mutant (strain M6226) with thicker and aberrant primary stalk. The gene responsible for the mutant phenotype, confirmed by homologous recombination, encodes an open reading frame with 383 aa residues (46.3 kDa) and was named thick and aberrant stalk A (tasA). TasA is highly homologous to Dictyostelium discoideum cyclic adenosine 3',5'-monophosphate receptors. A tasA transcript is expressed strictly at the late aggregation stage. Cells expressing a tasA::gfp fusion DNA are localized at the posterior region of the primary sorogen where secondary sorogens and branches originate. This result indicates the existence of 'prebranch' and 'pretrunk' regions in P. pallidum instead of the prespore and prestalk regions in D. discoideum. The analyzes of the gene disruptant and chimeric fruiting bodies also suggests that TasA affects the normal morphogenesis of the primary stalk and the process of cell differentiation into prebranch cells, but not into spore or stalk cells directly.