Genotype-specific Expression of Uncle Fester Suggests a Role in Allorecognition Education in a Basal Chordate

Abstract

Histocompatibility is the ability to discriminate between self and non-self tissues, and has been described in species throughout the metazoa. Despite its universal presence, histocompatibility genes utilized by different phyla are unique-those found in sponges, cnidarians, ascidians, and vertebrates are not orthologous. Thus, the origins of these sophisticated recognition systems, and any potential functional commonalities between them, are not understood. We are studying histocompatibility in the botryllid ascidians, members of the chordate subphylum, Tunicata, which provide a powerful model to understand both the origins and functional aspects of this process. Histocompatibility in the botryllids occurs at the tips of an extracorporeal vasculature that come into contact when two individuals grow into proximity. If compatible, the vessels will fuse, forming a parabiosis between the two individuals. If incompatible, the two vessels will reject-an inflammatory reaction that results in melanin scar formation at the point of contact, blocking anastomosis. Compatibility is determined by a single, highly polymorphic locus called the fuhc with the following rules: individuals that share one or both fuhc alleles will fuse, while those who share neither will reject. The fuhc locus encodes at least six proteins with known roles in allorecognition. One of these genes, called uncle fester, is necessary and sufficient to initiate the rejection response. Here, we report the existence of genotype-specific expression levels of uncle fester, differing by up to eight-fold at the mRNA-level, and that these expression levels are constant and maintained for the lifetime of an individual. We also found that these differences had functional consequences: the expression level of uncle fester correlated with the speed and severity of the rejection response. These findings support previous conclusions that uncle fester levels modulate the rejection response, and may be responsible for controlling the variation observed in the timing and intensity of the reaction. The maintenance of genotype specific expression of uncle fester is also evidence of an education process reminiscent of that which occurs in mammalian Natural Killer cells. In turn, this suggests that while histocompatibility receptors and ligands evolve via convergent evolution, they may utilize conserved intracellular machinery to interpret binding events at the cell surface.

Fig. 1

(A) A Botryllus colony is shown. The center is occupied by the zooids, which are feeding and asexually reproducing. Zooids are connected to a common vascular network, which runs throughout the colony. At the colony edge the vascular network terminates in closed, finger shaped protrusions called ampullae, which surround the colony, and are the sight of allorecognition. The dotted line shows where ampullae are surgically separated from the colony. The vascular network quickly heals, and the ampullae regenerate within 72 h. This ablation surgery can be done repeatedly. Scale Bar 5 mm. (B) Examples of equal and preferential rejection. Interface of two incompatible individuals during a rejection response. Ampullae from top colony are clear, ampullae from bottom colony are dark. Equivalent points of rejection (POR) are highlighted with black arrows, where the two ampullae are in contact and POR forming between them. Preferential POR are highlighted with white arrows, where POR are forming from ampullae of only one of the colonies, either top (*), or bottom colony (**). Scale Bar 500 µm.

Fig. 2

DE-Seq analysis of uncle fester gene expression levels in nine B. schlosseri genotypes. Each DE-Seq analysis was compared against SB825FEA with the normalized ratio plotted. Asterisks represent a statistical difference (padj < 0.05).

Fig. 3

(A) Graphical representation of histocompatibility outcome amongst different lines used in this study. fuhc alleles are denoted below the genetic line. (B) Relative qPCR analysis across several genotypes. Each sample was normalized ratio against SB825FEA was plotted with standard deviation as the error bars. Plot represents a minimum of three qPCR runs. (C) Graphical representation of the predicted promoter region of uncle fester with identified polymorphisms from each genotype listed below. Annotated positions are relative to the TSS (+1). TRR = tandem repeat region; TATA = TATA box; TSS = transcriptional start site; N.D. = not determined.