535 INTRODUCTION Invertebrates employ non-adaptive, germline-encoded immunity that efficiently identifies allogeneic and xenogeneic attributes through the expression of a multiplicity of cellular and morphological phenomena (Rinkevich, 1999). The literature provides ample evidence for the crucial role of invertebrates’ innate immunity in manifesting these highly specified arrays of effector mechanisms (Loker et al., 2004) and the importance of high polymorphism for their efficient maintenance and expression (Rinkevich, 2004; Cadavid et al., 2004). While allorecognition is one of the major characteristics of invertebrate immunity, its qualities and the events expressed morphologically by the effector arms vary fundamentally between different taxa, although all share the hallmark nature of precise discriminatory capability between ‘self’ and ‘non-self’, even between closely related conspecifics (Grosberg, 1988; Leddy and Green, 1991; Rinkevich, 1996; Rinkevich, 1999; Schwarz et al., 2007). Historecognition of ‘self’ versus ‘non-self’, however, may represent two separate avenues for immunity, either by detecting the presence or absence of attributes that define self or by detecting the presence or absence of non-self attributes (Neigel, 1988). As in other invertebrates, it is above dispute that self/non-self recognition is hallmark to cnidarian immunity (Neigel, 1988; Leddy and Green, 1991; Rinkevich, 1996; Rinkevich, 1999), albeit without being able to distinguish between the two different immunological routes. Literature on cnidarian immunity documents that allorecognition and xenorecognition are naturally expressed phenomena that result in either fusion between contacting allogeneic partners in a wide array of histoincompatible outcomes or culminating in various ‘rejection’ phenomena (Rinkevich and Loya, 1983; Hidaka, 1985; Chadwick-Furman and Rinkevich, 1994; Rinkevich, 1996; Rinkevich, 1999; Frank et al., 1997; Hidaka et al., 1997; Amar et al., 2008). All cnidarian’s immune characteristics implicate innate immunity parameters as no true adaptive components have been identified in these innate systems (Rinkevich, 1999; Loker et al., 2004; Dunn, 2009), although elements suggesting memory and specificity were documented in several cases (Rinkevich, 1996; Rinkevich, 1999). However, as in other invertebrate taxa (Magor et al., 1999), the major obstacle for finding a true evolutionary relationship is that homologous molecules operating in non-identical systems may have different constraints on structural conservation and, therefore, may display distinct patterns of activities. Working on hard and soft corals’ immunity, definitive studies (Hidaka, 1985; Frank et al., 1997; Hidaka et al., 1997; Barki et al., 2002) showed that high proportions of allogeneic interactions between young partners culminated in fusions, an outcome not documented when branches of adult colonies were paired. This is of special interest because allorecognition is thought to reduce costly tissue fusion with individuals other than self (Rinkevich, 1999). Fusion between conspecifics is not restricted to corals and is commonly found even in hydrozoans (Frank and Rinkevich, 1994; Cadavid et al., 2004). Fusion between juveniles of scleractinian corals (the formation of chimeric entities) were first detailed by Hidaka (Hidaka, 1985) in Pocillopora damicornis and then in other pocilloporid corals like Stylophora pistillata (Frank et al., 1997), Seriatopora caliendrum and Seriatopora hystrix (Nozawa and Loya, 2005). Histocompatible The Journal of Experimental Biology 213, 535-540 © 2010. Published by The Company of Biologists Ltd doi:10.1242/jeb.039529 Mounting of erratic histoincompatible responses in hermatypic corals: a multi-year interval comparison K.-O. Amar* and B. Rinkevich Israel Oceanographic and Limnological Research, National Institute of Oceanography,Tel-Shikmona, PO Box 8030, Haifa 31080, Israel *Author for correspondence (sunshine@ocean.org.il) Accepted 29 October 2009 SUMMARY Studies on allorecognition in the phylum Cnidaria have disclosed complex arrays of effector mechanisms, specificity and competency to distinguish precisely between self and non-self attributes, and have revealed the existence of allogeneic maturity. Here we studied allo-responses between young Stylophora pistillata colonies by following 517 allogeneic interactions between naturally settled kin aggregates and by establishing 417 forced allogeneic and autogeneic assays made of solitarily settled spat that were cut into two similar size subclones, of which one had been challenged allogeneically. Fused assays were exposed to a second allorecognition challenge, made of three allogeneic types. Whereas about half of the kin allogeneic interactions led to tissue fusions and chimera formations, none of the 83 non-sibling pair combinations were histocompatible. In contrast to previous results we recorded rejections between siblings at the age of less than two months. More challenging, we documented cases of fusions between interacting siblings at ages older than one-year-old partners, all differing from a previous study made on the same coral population more than a decade ago. Similar erratic histoincompatible responses were recorded in other pocilloporid species. We suggest that these results reflect reduced genetic heterogeneity caused by chronic anthropogenic impacts on shallow water coral populations where planulae originating from the same mother colony or from different mother colonies that are genetically related share increasing parts of their genomes. Offspring born to related parents may also reveal an increase in genomic homozygosity, and altogether impose erratic alloimmunity. Key words: allorecognition, corals, histocompatibility, fusion, rejection. THE JOURNAL OF EXPERIMENTAL BIOLOGY