Not like Botryllus: indiscriminate post–metamorphic fusion in a compound ascidian

Abstract

Fusion trials between metamorphs of the aplousobranch compound ascidian Diplosoma listerianum indicated that chimera formation was not dependent on relatedness. Similar, high rates of union were observed between full siblings, half siblings, unrelated individuals from the same population, and individuals from two geographically distant localities. This is in contrast to the well-studied ascidian genus Botryllus, in which a highly polymorphic allorecognition system governing the fusion^non-fusion reaction (colony speci¢city) largely limits fusion to close relatives. Fusion in Botryllus establishes a vascular chimera throughout which stem cells may circulate, promoting cell lineage competition between the fusion partners. The restriction of fusion to close kin in Botryllus is thought to reduce the inclusive ¢tness costs of competitive interactions between cell lineages within the chimera. In contrast to Botryllus, modules (zooids) of a D. listerianum colony are not interlinked by blood vessels, seemingly precluding the exchange of stem cells. The apparent absence of strict colony speci¢city in D. listerianum is thus in keeping with the predictions of the Botryllus model for the maintenance of allorecognition polymorphism.However, colony speci¢city has been reported in other species of aplousobranch ascidian that also lack a common vascular system. In these, the threat of migrating blood-borne stem cells cannot be responsible for the presence of colony speci¢city. One possibility, requiring experimental investigation, is that stem cells could perhaps migrate between zooids by another route, such as through the matrix of the colonial tunic. Even in the absence of stem cell exchange, cheating on the costs of colony maintenance and defence could also produce selective forces favouring colony speci¢city. In compound ascidians, this could involve unequal contribution to extrazooidal structures, principally the tunic and related tissues. This consideration seems potentially relevant to the lack of discrimination during fusion in D. listerianum, since extrazooidal somatic investment in this species appears minimal, severely limiting the scope for this other form of cheating.The various possible modes of exploitative interaction between fused colonies are not mutually exclusive, and o¡er fundamentally similar explanations for colony speci¢city. If none of them can be shown to occur in non-botryllid species possessing colony speci¢city, the generality of the Botryllus model may require re-evaluation.

Bottlenecks and loss of genetic diversity: spatio-temporal patterns of genetic structure in an ascidian recently introduced in Europe

“…Fusion has been described in a number of colonial ascidians (see Bishop & Sommerfeldt 1999 for a compilation of earlier published records; Ben-Shlomo et al 2001, Sommerfeldt et al 2003, including Perophora japonica (Koyama & Watanabe 1981, 1986, and can result in the formation of chimeras, i.e. genetically composite entities (Sommerfeldt et al 2003, Rinkevich 2005.…”

“…genetically composite entities (Sommerfeldt et al 2003, Rinkevich 2005. In Botryllinae (Family Styelidae) and Perophoridae, zooids within colonies are linked by common vascular systems (Bishop & Sommerfeldt 1999, Pérez-Portela et al 2009) that fuse when chimeras are formed, thereby allowing haemolymph cells from different genotypes to circulate between fusion partners (Watanabe & Taneda 1982, Koyama & Watanabe 1986). In P. japonica fusion of different stolons of the same colony has been documented during the growth of reared colonies (Koyama & Watanabe 1981, 1986) and presumably results in the lattice-like array of interconnected stolons seen in wild colonies.…”

Lessons in modularity: the evolutionary ecology of colonial invertebrates

“…In the absence of such competition, discrimination against allogeneic fusion is not predicted. Accordingly perhaps, allogeneic fusion occurs frequently among unrelated colonies of the ascidian Diplosoma listerianum (Bishop and Sommerfeldt, 1999). Zooids of D. listerianum retain physical independence, shifting and mingling within the growing chimera.…”

Larval activity levels and delayed metamorphosis affect post-larval performance in the colonial ascidian Diplosoma listerianum

“…It is widely distributed in temperate ports and harbours and has an encrusting growth form, with zooids embedded in a fragile tunic. The zooids bud and reproduce independently of each other (Bishop & Sommerfeldt 1999) and this growth form allows colonies to grow very rapidly. The lecithotrophic larvae are readily obtained by exposing adult colonies to light, and most are competent to metamorphose within minutes of their release from the parent.…”