Chimerism following fusion in a clonal ascidian (Urochordata)

Many marine invertebrates bud vegetatively to produce a modular colony of individuals derived from a single zygote. Fusion of different colonies to produce a genetically composite entity (a chimera) is known from experiments on sponges, hydroids, corals, bryozoans and ascidians – groups which together dominate sessile faunas on marine hard substrates. Random amplified polymorphic DNA–polymerase chain reaction (RAPD–PCR) analysis was applied to individual modules (zooids) dissected from colonies of a colonial ascidian, Diplosoma listerianum (Milne Edwards), to investigate the presence and extent of chimerism. The technique revealed chimerism in wild material. In total, 288 colonies from eight different natural populations were analysed. Chimeric colonies were present in all populations, at frequencies of up to 61%, with up to six different genotypes present in some colonies. Zooids of different genetic origin often intermingled within a chimeric colony to produce a zooidal mosaic. Although fusion of colonies has been observed directly in the laboratory, an unknown proportion of the chimerism detected in wild populations might have arisen through somatic mutation. To assess this possibility, tissue of 12 clones in culture was sampled repeatedly over a period of 3 years and subjected to RAPD–PCR analysis. RAPD banding patterns were generally very stable; the changes noted mostly involved minor bands that would not, on their own, have been taken as evidence for chimerism under the conservative criteria adopted for the study of wild populations. It was concluded that a large proportion of natural chimerism is attributable to colony fusion. © 2003 The Linnean Society of London. Biological Journal of the Linnean Society , 2003, 79 , 183–192.