Differential effect of allorecognition loci on phenotype in Hydractinia symbiolongicarpus (Cnidaria: Hydrozoa).

The allorecognition complex of Hydractinia symbiolongicarpus is a chromosomal interval containing two loci, alr1 and alr2, that controls fusion between genetically distinct colonies. Recombination between these two loci has been associated with a heterogeneous class of phenotypes called transitory fusion. A large-scale backcross was performed to generate a population of colonies (N = 106) with recombination breakpoints within the allorecognition complex. Two distinct forms of transitory fusion were correlated with reciprocal recombination products, suggesting that alr1 and alr2 contributed differentially to the allorecognition response. Specifically, type I transitory fusion is associated with rapid and persistent separation of allogeneic tissues, whereas type II transitory fusion generates a patchwork of continuously fusing and separating tissues.