Alloimmune Hierarchies and Stress-Induced Reversals in the Resorption of Chimeric Protochordate Colonies

In mixed-blood cell chimeras of the cosmopolitan protochordate Botryllus schlosseri, one of the genotypes in each chimera is often morphologically resorbed. Here we studied elements in the proposed heritable nature of the resorption by establishing lines of offspring from two sexually matured colonies. Mother colonies always resorb self-crossed offspring. Linear hierarchies emerged between offspring. When more than 20 chimeras were made from each of two specific pairs of genotypes, reversals in resorption directionality were documented in 2-3 cases. To elucidate further a possible effect of stress conditions on resorption directionality, we established 14 chimeras between one mother colony and its self-crossed offspring. A mild stress (bud puncturing) was then used on the `winner' mother colony subclones. In eight (57.1%) cases, the mother colony subclones were completely or partly resorbed by the `subordinate' offspring (compared with 0% in 39 control chimeras). In the other six cases, disconnections were recorded without any sign of resorption to either partner. Colony resorption usually occurs during the takeover process of blastogenesis, a cyclic, genetically controlled apoptotic process. We propose that nonprogrammed, induced cell deaths, characterized morphologically by the same pathways and features, are elicited by perturbations to the developing buds. These stress conditions, when developed on the `winner' partner in the chimera, may induce reversals in the resorption directionality.