Allorecognition in Colonial Marine Invertebrates: Does Selection Favor Fusion with Kin of Fusion with Self?

Previous analyses of the selective forces operating on allorecognition systems in colonial marine invertebrates have suggested that advantages to fusion with kin have selected for the ability to recognize and fuse with related colonies. While this explanation is compatible with the observation of aggregated settlement of fusible larvae in an ascidian species, it is not compatible with two other prominent features of allorecognition systems--the extensive allorecognition allele polymorphism commonly observed in natural populations and the recently reported instability of chimeric colonies. We suggest that selection for fusion with self rather than fusion with kin, oglers a more parsimonious explanation for the two features listed above. Consequently, self fusion may be a major selective force acting on allorecognition systems in colonial invertebrates.