On the occurrence of intracolonial genotypic variability in highly clonal populations of the hydrocoral Millepora platyphylla at Moorea (French Polynesia).

Intracolonial genotypic variability is described in many colonial organisms and arises from mosaicism (somatic mutation) and/or chimerism (allogenic fusion). Both processes provide an additional source of genotypic variation in natural populations and raise questions on the biological significance of colonies having more than one genotype. Using fifteen microsatellite markers, we screened for potential genetic heterogeneity within Millepora platyphylla colonies, a hydrocoral species known for its extensive morphological plasticity among reef habitats. We aimed to determine whether mosaicism and chimerism were related to specific reef habitats and/or colony morphologies. Our results show that intracolonial genotypic variability was common (31.4%) in M. platyphylla at Moorea, French Polynesia, with important variations in its frequency among habitats (0-60%), while no effect of morphology was observed. Mosaicism seemed responsible for most of the genetic heterogeneity (87.5%), while chimerism was rarer. Some mosaics were shared among fire coral clones indicating that mutations could be spread via colony fragmentation. Further, the genotypic variability among clones suggests that colonies produced asexually through fragmentation have the potential to accumulate their own mutations over time. Such mutation dynamics might have important implications for the adaptive potential of long-lived reef-builder populations that are predominantly sustained through asexual reproduction.