‘Rejected’ vs. ‘rejecting’ transcriptomes in allogeneic challenged colonial urochordates

In botryllid ascidians, allogeneic contacts between histoincompatible colonies lead to inflammatory rejection responses, which eventually separate the interacting colonies. In order to elucidate the molecular background of allogeneic rejection in the colonial ascidian Botryllus schlosseri, we performed microarray assays verified by qPCR, and employed bioinformatic analyses of the results, revealing disparate transcription profiles of the rejecting partners. While only minor expression changes were documented during rejection when both interacting genotypes were pooled together, analyses performed on each genotype separately portrayed disparate transcriptome responses. Allogeneic interacting genotypes that developed the morphological markers of rejection (points of rejection; PORs), termed 'rejected' genotypes, showed transcription inhibition of key functional gene groups, including protein biosynthesis, cell structure and motility and stress response genes. In contrast, the allogeneic partners that did not show PORs, termed 'rejecting' genotypes, showed minor expression changes that were different from those of the 'rejected' genotypes. This data demonstrates that the observed morphological changes in the 'rejected' genotypes are not due to active transcriptional response to the immune challenge but reflect transcription inhibition of response elements. Based on the morphological and molecular outcomes we suggest that the 'rejected' colony activates an injurious self-destructive mechanism in order to disconnect itself from its histoincompatible neighboring colony.