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The ability to distinguish among these genetic alternatives, and to circumscribe the rules governing compatibility, determines how the expression of allorecognition-dependent behaviors influences the evolution of allotypic variation.
Our data show that, as in botryllid ascidians, allotypic identity is not a prerequisite for compatibility in H.
This study also generally confirms prior studies of the inheritance of allotypic specificity in Hydractinia, in showing that the likelihood of tissue fusion declines according to the relatedness of interacting pairs (e.
symbiolongicarpus, several lines of evidence argue that along the continuum of genetic models that could confer allotypic specificity, relatively few loci with a moderate number of alleles govern allotypic specificity in this species.
simulations), an outcome consistent with the existence of distinguishable allotypic groupings within sibships.
If five loci carrying six equal frequent alleles per locus governed allotypic specificity in H.
Such high levels of allotypic diversity could, at least in part, represent a balance between the generation of new allotypic variants by mutation and their loss via drift (Neigel 1988; Brown and Eklund 1994).
In the case of cnidarians such as Hydractinia, in which allorecognition systems determine whether individuals will fuse or fight, frequency-dependent selection acting at the level of the individual can favor the accumulation of allotypic variation when the fitness costs of intergenotypic fusion exceed the benefits of aggression (Grosberg and Quinn 1988).
symbiolongicarpus has attributes that imply that both individual and kin selection play important roles in the evolution of allotypic specificity.
The combination of high allotypic diversity and the requirement that to be compatible individuals share a substantial proportion of allorecognition alleles virtually guarantees that unrelated individuals will be incompatible, and suggests that self/nonself discrimination is an important component of the evolution of allotypic specificity in H.
From an ecological perspective, the incidence of multiply colonized hermit crab shells and the probabilities of interacting with kin versus unrelated individuals (along with the costs and benefits of fusion and aggression) will dictate the strength of selection acting to promote the evolution of allotypic diversity, and whether selection favors discrimination among different classes of kin or self from nonself (Hamilton 1964).