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

The allorecognition complex of Hydractinia is a chromosomal interval containing two loci, alr1 and alr-2, 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 alr-1 and alr-2 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.