CHIMERAS VS GENETICALLY HOMOGENEOUS INDIVIDUALS - POTENTIAL FITNESS COSTS AND BENEFITS

Colonies of the compound tunicate Botryllus schlosseri show the capacity for colony specificity resulting either in vascular fusion or in rejection between genetically distinct colonies. Cosettlement of kin colonies increases the probability of fusion between allogeneic colonies. However, laboratory studies revealed that fusion terminates in the resorption of one partner, and that fusion raises the threat of somatic/germ cell parasitism. Here we tested in a laboratory study, done with 88 oozooids from 4 gravid colonies, the suggestion that colony fusion between relatives may be beneficial to one or both members of a chimera by increasing the size, the fecundity and/or survivorship. The results show, however, that these parameters were not augmented in chimeras when compared to isolated colonies or when compared to rejecting pairs of kin and non-kin colonies. These results are discussed along with two hypotheses. The first refers to the suggestion that the formation of chimerism is ecologically advantageous, since any chimera may express a higher variety of physiological attributes than either colony component. In the second hypothesis, we suggest that allorecognition elements in Botryllus are used for screening the relative heterozygosity/homozygosity of the whole genomes of both partners within a single chimera. In that view, the less heterozygous colony, hence the less adapted to environmental changes, may be eliminated through the process of colony resorption. Both hypotheses indicate that the costs in fitness incurred by the fusion, the possible resorption of the less vigorous colony, and the threat of germ cell parasitism are more beneficial than other costs incurred by alternative processes of intraspecific competition for settlement space.