Allorecognition elements on a urochordate histocompatibility locus indicate unprecedented extensive polymorphism BARUCH RINKEVICH1, RAM PORAT 1 a n d MENACHEM GOREN 2 1 Israel Oceanographic & Limnological R e s e a r c h ,National Institute of P.0. Box 80 2 Department of Zoology, Tel-Aviv University, Ramat Aviv, Tel-Aviv 69978, Israel SUMMARY Tissue contacts between different colonies of the cosmopolitan urochordate Botryllus schlosseri resulted either in fusion between allogenic blood vessels or in the formation of rejections, characterized by necrotic areas. This colony specificity is manifested by a single, highly polymorphic mendelian locus, with codominantly expressed alleles, called the tunicate fusibility/histocompatibility (Fu/HC) haplotype. Two colonies sharing even one allele at the Fu/H C locus may fuse; rejecting colonies share no alleles. Previous studies have revealed up to 100 alleles on the Fu/HC locus in different natural populations. We studied Fu/HC polymorphism in three populations (> 1000 colonies) along the Mediterranean coast of Israel, 12-36 km apart. By using the protocol of colony allorecognition assays (caas ) on 444 tests within localities, 58—306 alleles per locality were calculated. We also estimate a total of 479-560 Fu/H C alleles in the three Israeli populations. An additional 103 interpopulation caas resulted in zero fusion, an outcome which contradicts previous findings showing 4.4—12.0% fusions between more remote B. schlosseri populations such as those from Israel versus Monterey, California, and California versus Japan assays. We propose that the recorded unprecedented Fu/HC polymorphism is maintained through overdominant selection, which is promoted by the gregarious settlement of Fu/HC-compatible larvae and by the subsequent threat of germ cell parasitism following colony fusion. 1. IN T R O D U C T IO N Botryllus schlosseri is a worldwide-distributed, encrusting subtidal ascidian, an inhabitant of natural and artificial hard-bottomed substrates. Colonies have been re- corded from Europe, the eastern and western con- tinental shelves of North America, and Asia (Japan, Korea), raging from Norway in the north to the Great Barrier Reef, Australia, in the south (Berrill 1950; Tokioka 1953; Plough 1978; Kott 1985; Boyd et al. 1990; Rinkevich & Weissman 1991). A typical colony is composed of a few to more than 1000 modular units (zooids) which are connected to each other via a ramified blood system. All zooids within a single colony are genetically identical, having derived from a single founder zooid by a complex but highly synchronized weekly developmental cycle called blastogenesis (Ber- rill 1950). Zooids and blood vessels are embedded within a translucent organic matrix (tunic) which bears, in the colony’s periphery, sausage-like termini of blood vessels (ampullae). Allogenic fusion and rejection events take place where ampullae of two colonies meet (Bancroft 1903). Allorecognition of B. schlosseri, like that in other botryllid ascidians, exhibits several types of colony specificity. Pairs of colonies that meet naturally in the wild, or are forced into contact under laboratory Proc. R. Soc. Lond. B (1995) 259 , 319-324 Printed in Great Britain conditions, either fuse their contacting peripheral ampullae to form a vascular parabiont (cytomictical chimera) or develop cytotoxic lesions (points of rejection) in the contact zone (reviewed in Rinkevich 1992). This colony specificity is genetically controlled by a single, highly polymorphic fusibility/histocom- patibility locus (Fu/HC; Weissman al. 1990) with multiple codominantly expressed alleles (Oka & Watanabe 1960; Sabbadin 1962; Scofield et al. 1982); the degree of polymorphism resembles that of some genes of the vertebrate major histocompatibility com- plex (Scofield et al. 1982). A wild Botryllus colony is usually heterozygotic at the Fu/HG locus. Such a colony has the capacity to fuse with any other colony carrying at least one of its two Fu/H C alleles. Rejecting colonies share no Fu/H C allele. Several studies have revealed the number of alleles residing in the Fu/HC locus in accordance with fusion frequencies ( s e n s u Cu rtis et 1982; Grosberg 1988) within a limited number of populations. Using B. schlosseri from Woods Hole, Massachusetts, U.S.A., Karakashian & Milkman (1967) found 78 fusions in 1262 contiguous borders of colonies in nature (6.2%), which reflects the existence of roughly 80 alleles on the Fu/HC locus. Scofield & Nagashima (1983) calculated 50 100 histocompatibility alleles in B. schlosseri in this region, and Crosberg & Quinn (1986) recorded 22 319 © 1995 The Royal Society Downloaded from https://royalsocietypublishing.org/ on 18 January 2022