Tunic cell populations during fusion events in the colonial ascidian Didemnum vexillum (Tunicata)

Abstract

We documented changes in the abundance and distribution patterns of tunic cells involved in the allorecognition response of the colonial aplousobranch Didemnum vexillum, whose zooids do not share a common vascular system. A histological examination of the fusion zone of isogeneic (CIAs) and allogeneic (CAAs) fused colony assays revealed that tunic cuticles were rapidly regenerated. The underlying tunic matrix fused readily in all assays and controls. We identified four different types of tunic cells. Phagocytic cells represented the most abundant cell type in allogeneic fusions, followed by morula cells. These cells were more abundant at the immediate fusion junction than at 120 lm or 240 lm from the junction, most likely because they mediate the allorecognition reaction. Elongated filopodial cells also were present, although only at very low abundances, and a layer of bladder cells was located immediately below the cuticle. Our results provide quantitative evidence for the involvement of tunic cells in the allorecognition response of a highly invasive ascidian.

Efficient dispersal and substrate acquisition traits in a marine invasive species via transient chimerism and colony mobility

“…In common with many other colonial tunicates, adjacent D. vexillum colonies can fuse, at least transiently, to form chimeric colonies ( Smith et al, 2012 ; Rinkevich & Fidler, 2014 ; Sellers, Fagerberg & Litvaitis, 2013 , this study). While in botryllid ascidians, allorecognition between colonies is strongly influenced by highly polymorphic genetic loci ( Rinkevich, 2004 ; Rinkevich, 2011 ; Rinkevich & Weissman, 1992 ; Rinkevich & Weissman, 1987 ; Rinkevich, 1996 ), for D. vexillum itself little is known on genetic mechanisms associated with chimeric colony formation ( Smith et al, 2012 ; Rinkevich & Fidler, 2014 ; Sellers, Fagerberg & Litvaitis, 2013 , this study). Moreover, fusion in botryllid ascidians begins via vascular anastomoses (‘cytomictical chimerism’; sensu ( Rinkevich & Weissman, 1987 )), which enables stem cells circulating throughout the chimeric colony to initiate what is, in effect, ‘cell lineage competition’ between the two genotypes in the chimera.…”

Genotypic variability following fusion in the invasive colonial tunicate Didemnum vexillum

“…Hemocytes in the tunic have been identified as effector cells of allorecognition in aplousobranch ascidians (Ishii et al, ; Sellers et al, ) and present a possible mechanism for the genetic changes we observed. Phagocytes and morula cells are abundant at the fusion junction between allogeneic colonies of D. vexillum (Frey, ; Sellers et al, ), with morula cells proposed as effector cells in allorecognition in this species (Sellers et al, ). Long‐term studies that track the movement of these cell types over time throughout fused colonies will be informative in determining the extent to which they are involved in the genotypic changes we saw in chimeric colonies of D. vexillum .…”

“…Previous studies on fusion in aplousobranch ascidians have shown that interactions occur between mobile tunic cells at allogeneic contact sites (Dias & Yokoyama, ; Ishii et al, ; Sellers et al, ). Allogeneic zooids in chimeric colonies of D. listerianum may also intermingle, leading to changes in the distribution of genotypes in fused colonies (Bishop & Sommerfeldt, ; Sommerfeldt et al, ), a process that has not yet been investigated in D. vexillum .…”

“…Didemnid tunicates (Order Aplousobranchia) lack a common blood vascular system through which allogeneic cells can be exchanged following fusion, so interactions between allogeneic cells and tissues in chimeric didemnid colonies are therefore expected to differ from those in botryllid colonies (Bishop & Sommerfeldt, ; Clancy, ; Dias & Yokoyama, ; Sellers, Fagerberg, & Litvaitis, ). However, like the botryllids, certain didemnid tunicates, including the cosmopolitan species Didemnum vexillum , have been shown to express colony specificity (Clancy, ; Ishii, Hirose, & Taneda, ; Mukai & Watanabe, ; Oka & Watanabe, ; Smith et al, ).…”

“…The thick tunic of D. vexillum consists of shared structures that require energetic investment from individual zooids (Kott, ; López‐Legentil, Erwin, Velasco, & Turon, ; Morris & Carman, ; Stefaniak & Whitlatch, ) and provides a medium in which mobile cells may interact (Frey, ; Ishii et al, ; Sellers et al, ). When visually distinguishable colonies of D. vexillum fuse, distinct color differences on either side of the fusion line may be visible.…”