NOVEL INVENTION OF SPORE INDUCTION IN A SISTER SPECIES TO GROUP 4 DICTYOSTELIA

Abstract

Background Dictyostelia are soil amoebas that aggregate to form fruiting bodies with spores and stalk cells in response to starvation. Where known, species across the dictyostelid phylogeny use secreted cAMP, detected by cAMP receptors (cARs) to induce the differentiation of spores and to organize fruiting body construction. However, recent deletion of the single cAR of Polyspondylium violaceum (Pvio) left both its fruiting bodies and spores intact. Methods To investigate whether Pvio sporulation can occur in the absence of secreted cAMP and to explore alternative inducers in a bioassay, three prespore genes were identified and gene fusions of their promoters with the LacZ reporter gene were transformed into Pvio cells. After assessing the spatial expression pattern of the genes and the stage at which prespore gene expression initiated, the effect of cAMP and other Dictyostelium discoideum (Ddis) signal molecules were tested on prespore gene expression in vitro. Results Pvio genes g4562 (psp1), g2696 (psp2) and g2380 (psp3) were identified as homologs of Ddis spore coat genes. They were first expressed around 4 h of starvation in aggregation centres and later in the posterior 4/5th of emerging sorogens and the spore head of early fruiting bodies. Cells from dissociated 4 h aggregates and shaken in suspension for 6 h increased prespore-LacZ reporter activity 4-fold for psp1 and 6-fold for psp2, but this increase was at least 5-fold higher when cells were plated on solid substratum for 6 h to develop normally. cAMP had no effect on prespore gene induction and neither had the Pvio chemoattractant glorin nor the Ddis chemoattractants and differentiation inducers folate, c-di-GMP, DIF-1, GABA, cGMP and 8Br-cAMP. Conclusions The Pvio lineage uniquely evolved a novel genetic network for synthesis, detection and processing of the signal that triggers its main survival strategy.