Species specificity in allorecognition loci in an invertebrate chordate group

Abstract

In the context of the many marine species that are colonial, competition for space necessitates complex interactions within and between species. Allorecognition is the process of discriminating between self and non-self tissues in the same species, while xenorecognition occurs in different species. In botryllid ascidians (Class Ascidiacea, Family Styelidae), interactions between two colonies of different species can result in no response or rejection by one or both of the interacting colonies. The cellular processes of rejection in xenorecognition and allorecognition are very similar in these species, leading to the hypothesis that the same mechanisms are involved in xenorecognition and allorecognition rejection. We sequenced two allorecognition loci (FuHC-sec and FuHC-tm) from two Botryllus and four Botrylloides species and created pairwise alignments of all combinations of species within each genus. We then calculated the percentage of species-specific amino acids at each exon. For FuHC-sec, Exons 1, 2, 7, and 8 in Botrylloides and Exons 5 and 6 in Botryllus were labelled as high specificity. Exons 5 and 6 were also identified as under selection in a population-genetic study of Botryllus schlosseri , and may therefore be involved in both allorecognition and xenorecognition. For FuHC-tm, Exons 2 and 11 were labelled as high specificity across both genera. However, the exons under selection intra-specifically in Botryllus schlosseri (allorecognition) are not the same exons differentiating the two species (xenorecognition). Protein domains associated with high-specificity exons in FuHC-sec and FuHC-tm are β-sandwich, epidermal growth factor-like, Ig-like, and signal peptide.