Evolution of Allorecognition in the Tunicata

Abstract

Allorecognition, the ability to distinguish self or kin from unrelated conspecifics, plays several important biological roles in invertebrate animals. Two of these roles include negotiating limited benthic space for colonial invertebrates, and inbreeding avoidance through self-incompatibility systems. Subphylum Tunicata (Phylum Chordata), the sister group to the vertebrates, is a promising group in which to study allorecognition. Coloniality has evolved many times independently in the tunicates, and the best known invertebrate self-incompatibility systems are in tunicates. Recent phylogenomic studies have coalesced around a phylogeny of the Tunicata as well as the Order Stolidobranchia within the Tunicata, providing a path forward for the study of allorecognition in this group.

Keywords: Aplousobranchia; Phlebobranchia; Stolidobranchia; ascidian; colonial invertebrate; fusion; self-incompatibility; tunicate.

Figure 1

Phylogeny of the Subphylum Tunicata, based on [31,32]. The tree includes Subphylum Vertebrata as the outgroup. Red text denotes a group where all species have solitary growth forms, blue text denotes a group where all species have colonial growth forms, and purple text denotes a group with both solitary and colonial growth forms. Vertical bars represent independent evolutions of the colonial growth form. (a) Phlebobranchia paraphyletic, (b) Phlebobranchia monophyletic.

Figure 2

Phylogeny of the Order Stolidobranchia, based on [36]. The order contains three families: Molgulidae, Pyuridae, and Styelidae. Red text denotes a group where all species have solitary growth forms, Blue text denotes a group where all species have colonial growth forms.

Figure 3

Phylogeny of the Order Aplousobranchia, based on [30,63,65]. The tree includes family Perophoridae (Order Phlebobranchia) as the outgroup. Vertical bars represent budding types. 1: stolonic; 2: inner bud vesicles form by epidermal constriction of the abdomen; 3: inner bud vesicles formed by epidermal constriction of the post-abdomen; 4: epicardial budding with no constriction of the abdomen or post-abdomen.