Morphological consequences for multi-partner chimerism in Botrylloides, a colonial urochordate

Studies on multi-partner chimeras (MC) of the colonial urochordate Botryllus schlosseri have revealed that these chimeras form more stable and vigorous entities than bi-partner chimeras (BC). This outcome has been further studied here on another botryllid species. Botrylloides leachi subpopulation 1, by analysing the morphological consequences of BC, chimeras comprising three partners, tri-chimeras (TC) and chimeras comprising six partners each, hexa-chimeras (HC). For each chimerical type, five different genotypic combinations with three replicates per combination were established. Chimeras were observed for up to 10 months, at which point, all had died. While life spans of BC, TC and HC were the same, the average maximum sizes of HC were higher resulting from more than three times greater daily growth rate. BC and TC reached a maximal chimeric size at a much earlier age. Some morphological resorptions were also expressed differently in HC as compared to BC. When comparing chimerical parameters of bi- vs. multi-chimerism between Botryllus and Botrylloides, these two genera differ in nine out of 13 characteristics tested. However, it seems that irrespective to the species studied and the different characteristics expressed, MC in botryllid ascidians is a commonly developed phenomenon shaped by evolutionary pressures that interact on a 'group' level instead of each individual partner. MC reveal modified entities in which contradicting intraspecific interactions are alleviated.