UC Santa Barbara

Sessile colonial marine invertebrates can experience a natural histocompatibility process similar to tissue and organ transplantation observed in humans. The colonial ascidian Botryllus schlosseri has been used for about a century to study this allorecognition phenomenon. When two B. schlosseri colonies enter into contact using their extracorporeal vasculature systems, they can fuse, establishing a chimeric colony or reject, developing an inflammation-like process that prevents communication between the colonies. Allorecognition in B. schlosseri is genetically controlled by a single genomic region, the fuhc (fusion/histocompatibility) locus. Six allorecognition genes have been described within this locus; the fuhc-sec, fuhc-tm, fester, uncle fester, hsp40l and bhf genes.

Tunicates can be classified into three main groups: Appendicularia, Thaliacea and Ascidia. However, it is unclear which species within these groups exhibit the allorecognition genes. To address this knowledge gap, I explored the emergence and evolution of B. schlosseri allorecognition genes. I searched for the allorecognition genes in the available transcriptomes and genomes of tunicates and discovered that the allorecognition genes appeared in different groups. Importantly, the fuhc-sec gene (a putative determinant in this allorecognition system) is present in species of the Styelidae family. At the same time, I established that the fester and uncle fester genes are indeed a gene family which is present in the colonial ascidians of the Styelidae family. Furthermore, we found that the allorecognition genes are located in a single genomic region in Botrylloides diegensis as in the genome of B. schlosseri. However, these genes are located on two different chromosomes (2 and 14) in the solitary species Styela clava. The presence of the fuhc-sec, fuhc-tm, bhf and hsp40l genes in solitary tunicates is intriguing. Although the functions of these genes are unknown, my work suggests that the allorecognition genes were co-opted from solitary species to perform a histocompatibility function in the colonial species of the Styelidae family.

At the same time, my analysis of the fester family genes (the putative receptors of this allorecognition system) revealed different mechanisms to generate variability, such as haplotype variation, alternative splicing and polymorphism. I found that the fester family has 30-40 and 19 members in B. schlosseri and B. diegensis, respectively. However, each individual has in its genome just a subset of fester genes, for instance 3-10 fester genes for B. schlosseri, which are different compared to the fester gene set of other individuals. Furthermore, we detected that the fester receptors have two regions, one extracellular and another intracellular, which exhibit alternative splicing. Moreover, we found that several fester genes (FFBs1 and FFBd1) show polymorphism. This is consistent with their proposed role in allorecognition.

In conclusion, I described the evolution of B. schlosseri allorecognition genes, generating new questions as to the presence and function of these genes in solitary species. Further, my analysis of the fester gene revealed that it is a multigene family with high variability and unique distribution across individuals. The predicted structure of the gene product is consistent with its proposed role as a receptor, though the ligand(s) remains unknown. Collectively, this work sets the stage for important questions that must be addressed to understand the evolution and mechanism of histocompatibility in B. schlosseri.