Ontogenetic changes in fusibility in the colonial hydroid Hydractinia symbiolongicarpus

Abstract

Invertebrate fusion and rejection responses are typically assumed to be mutually exclusive and temporally stable states. We here describe a segregant class appearing in an F1 cross of the colonial hydroid, Hydractinia symbiolongicarpus, that displays an ontogenetic change in fusibility. Reproductively immature offspring fuse to parental strains and have been observed to remain continuously fused for up to 76 days. By contrast, reproductively mature offspring initially fused with parental strains, only to separate into distinct colonies after a period of 3–21 days postfusion. Experimental manipulations demonstrate that (1) ontogenetic changes in fusibility cannot be removed by resetting a colony to a reproductively immature state, and (2) secondary contact does not result in an accelerated response.

EVOLUTIONARY GENETICS OF ALLORECOGNITION IN THE COLONIAL HYDROID HYDRACTINIA SYMBIOLONGICARPUS

“…obs.). Second, Shenk and Buss (1991) confirmed von Hauenschild's (1954) observation of a third class of intergenotypic interaction in H. symbiolongicarpus, transitory fusion, in which two colonies, either parents and their offspring or a pair of full sibs, initially fuse, and subsequently unilaterally or bilaterally reject each another. Most recently, Mokady and Buss (1996), based on a series of incrosses and backcrosses derived from a single mated pair (one member of which was highly inbred) argued that, as in the ascidian genus Botryllus, a single, highly polymorphic locus with multiple codominant alleles controls allorecognition in H. symbiolongicarpus…”

“…Preliminary data from a number of studies (e.g., Teissier 1929;Crowell 1950;von Hauenschild 1954von Hauenschild , 1956MUller 1964;Ivker 1972) revealed that close relatives fused considerably more often than distantly related individuals, suggesting that allorecognition specificity had a genetic basis. Later studies (Yund et al 1987;Shenk and Buss 1991) confirmed these observations, and extended them in several fundamental ways. First, it is now clear that in natural populations, virtually all (i.e., >95%) allogeneic interactions produce an incompatibility response, involving the unilateral or bilateral deployment of hyperplastic stolons (Yund et al 1987;our unpubl.…”

“…In a subsequent paper (von Hauenschild 1956), he scored transitory fusions as compatible interactions. Shenk and Buss (1991) described the morphological features of transitory compatibility in H. symbiolongicarpus and showed that in at least one set of parent-offspring interactions, the onset of the rejection phase of the interaction was associated with the attainment of sexual maturity by the offspring. We did not consistently find such an association (unpubl.…”

“…Fusion may also allow partners to monopolize a shell, and prevent the subsequent colonization by competitors, more rapidly than a single col-ony could. However, there may also be substantial costs to intergenotypic fusion in terms of intraspecific parasitism, pathogen transmission, and developmental instability (Mukai 1976;Sabbadin and Zaniolo 1979;Buss 1982Buss , 1987Sabbadin and Astorri 1988;Shenk and Buss 1991;Grosberg 1992;Rinkevich and Weissman 1992a,b). Similarly, agonistic behavior is one of the principal ways that clonal cnidarians can acquire space occupied by other organisms; however, the production and deployment of structures that mediate aggressive interactions can exact substantial costs in terms of future growth and reproductive potential (Buss 1990;Buss and Grosberg 1990;Grosberg 1995, 1996).…”

“…From ecological, morphological, cytological, and developmental perspectives, the mechanisms governing the nature and outcome of intraspecific competitive interactions between colonies of hydroids in the genus Hydractinia are now better understood than in perhaps any other clonal animal (reviewed in Ivker 1972;Buss et al 1984;Buss and Grosberg 1990;Shenk and Buss 1991;MUller et al 1987). Hydractinia symbiolongicarpus Buss and Yund, like many other members of the Family Hydractiniidae, colonizes gastropod shells inhabited by hermit crabs (Yund et al 1987;Buss and Yund 1989).…”