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Chimeras |
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Research study 1 |
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 In Monterey Bay, California and elsewhere colonies of Botryllus schlosseri are found growing on the surfaces of pilings, floats, rocks, and other objects in low intertidal and shallow subtidal areas characterised by calm  water. Colonies may be 8cm across, 1-1.5mm thick, and be orange, red-brown, or dark blue in colour. A colony comprises many groups of zooids (5-15 in number) arranged in oval- or star-shapes within a common tunic. The periphery of each colony is fringed with sausage-shaped vascular ampullae. These are filled with blood and are part of the circulatory system of the colony. They may be involved with wound repair and possibly defense. Boyd et al. 1990 Biol Bull 178: 239. Photograph courtesy Kevin Lee, Fullerton, California diverkevin.
Colonial tunicate Botryllus schlosseri. The peculiar
shapeof the colony is a result of it having to grow
around other more toxic invertebrates. These
include a sponge in the upper Left and several
cup
corals Balanophyllia sp. scattered throughout 0.5X
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Research study 2 |
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Interestingly, when two colonies of Botryllus schlosseri butt against one another during growth, they may fuse with one another to form chimeras. During this process, the many ampullae in one colony extend to those in the othe colony. The tunics then fuse and the ampullae grow to touch those of the second colony, not at their distal regions but at the proximal parts, and fusion of these unites the two circulatory systems. Contact between colonies may also lead to rejection, whereupon blood cells gather, infiltrate the region around the ampullar tips, and the ampullae withdraw. The process of fusion/rejection in Botryllus has generated much interesting research. Boyd et al. 1990 Biol Bull 178: 239.
NOTE lit. “monster” G. Many papers have been written on the formation, ecology, and genetics of B. schlosseri chimeras. Major centres of research are in Italy, Israel, Woods Hole (Massachusetts), and Pacific Grove (California). Only a brief summary of the work is presented in the ODYSSEY |
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Examples of contact between 2 colonies of the colonial tunicate Botryllus schlosseri leading to fusion (top row) or rejection (bottom row). The study shows that not only are colonies of Botryllus from Monterey Bay, California and Woods Hole, Massachusetts morphologically similar, but fertile crosses are obtained from interpopulation crosses. The authors conclude, despite some slight differences in allorecognition reactions (i.e., colony specificity: see illustration), that the 2 populations belong to the same species |
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Research study 3 |
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 Similar responses are obtained when oozooids1 from different colonies of Botryllus schlosseri are placed together immediately upon initiation of metamorphosis. In all cases, tunic fusion occurs within 12h2. After 2d some oozooids undergo vascular fusion (Left pair of zooids), while other pairs undergo rejection (Right pair). In the latter case, rejection occurs at the ampulla level, and necrotic elements appear and ampullae are sometimes damaged. The elements that allow fusion, or cause rejection, are humoral3 and cellular entities in the blood. Scofield & Nagashima 1983 Biol Bull 165: 733.
NOTE1 during embryogenesis in a colonial tunicate the fertilised egg hatches to a tadpole larva that settles and metamorphoses into the first unit of the colony, the oozooid. Subsequent growth is by asexual division to produce blastozooids
NOTE2 the work, done in Woods Hole, Massachusetts, uses laboratory seawater at an unspecified temperature
NOTE3 these are antibodies in vertebrates and similarly-functioning hemagglutinins in invertebrates |
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Research study 4 |
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Fusibility in Botryllus schlosseri is determined by a simiple 1-locus system with multiple co-dominant alleles. Colonies that match at one or both alleles fuse, while colonies that fail to match at both alleles reject. Interestingly, larvae not only settle near to already established colonies, but they can in some way recognise kin on the basis of these shared alleles and settle preferentially near to them. This kin recognition system, along with limited powers of larval dispersion, therefore promotes co-settlement of histocompatible individuals. The function of this is unclear, but it must serve to “jump-start” colony growth in areas suitable for B. schlosseri of certain genetic makeups. Grosberg & Quinn 1986 Nature 322: 456; see also Buss 1990 Trends Ecol Evol 5: 352 for brief review. |
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Research study 5 |
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| On contact, histocompatible colonies may not simply fuse; rather, one colony may actually resorb the tissues of another, leaving behind an empty tunic. The larger or faster-growing colony may dominate in such interactions, but mostly the outcome is unpredictable. This is shown graphically in studies of Botryllus schlosseri grown in the laboratory over a 7-mo period. The sequence begins 5d after fusion between a larger orange colony (240 zooids) and a smaller blue colony 72 zooids). Other outcomes of colony fusion include 1) establishment of a stable chimera, 2) both participants die, 3) chimeric partners become separated, and 4) one partner dies or degenerates, sometimes killing the other partner as well. Such lop-sided and unpredictable outcomes lead the authors to question whether fusion between closely related colonies is actually evoutionarily beneficial. Rinkevich & Weissman 1987 J Zool, Lond 213: 717; Rinkevich & Weissman 1987 Biol Bull 173: 474; Rinkevich & Weissman 1989 Bull Mar Sci 45: 213. |
2wk later, the blue colony has grown rapidly and the orange one shows evidence of partial resorption. |
5wk after fusion the orange colony is mostly resorbed and is reduced to 80 zooids. |
By 7wk orange is split into two parts, one part still joined with blue (58 and 7 zooids, respectively). |
By 10wk the blue colony is growing rapidly and the orange one is now in 3 parts. |
At 12wk the orange colony exists only as a satellite colony of 16 zooids. |
By 17wk orange, now on its own, has rebounded marginally to 22 zooids. |
At 22wk post-fusion, the blue colony has re-fused with the orange remnant. |
At 31wk orange is just a small region of empty tunic and a few zooids, while blue consists of 560 zooids. |
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Research study 6 |
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Colonies of Botryllus schlosseri that meet, then, may either fuse or reject, as shown in the photos on the Left. Fusion can be amicable, or can lead to a phenomenon known as germ-cell parasitism.  Recall that the circulatory systems of fused colonies intercommunicate and cells, both somatic and germ, freely intermingle in both participating colonies (tests with Monterey Bay colonies show that complete intermingling takes 4-14d from initial fusion). In the example shown on the Right, a blue colony has fused with an orange colony and, within each, somatic and germ cells wander and compete for sites of growth. In this example, ultimately the orange somatic cells win out over the blue, and the new zooids are genetic clones of the orange colony. Independently, however, the blue germ cells have outcompeted the orange germ cells for sites of spermiogenesis, and thus the testes in the new zooids are derived from the blue colony. The testes, then, represent blue germ-cell parasites in the new orange colony. From a review by Magor et al. 1999 Immunol Rev 167: 69; photos and diagram courtesy B. Magor, Stanford University, California, and colleagues
NOTE these wandering cells are actually embryonic stem cells and, in the case of germ-stem cells, may have the capability of forming either testes or ovaries; however, it may also be possible that different germ cells are involved in forming either testes or ovaries in Botryllus |
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Research study 7 |
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Each cycle of growth of zooids in a colony of Botryllus schlosseri culminates in a phase of programmed zooid death (or takeover), in which all zooids in a single colony die and are replaced by a new generation of zooids. Interestingly, experiments using chimeras from colonies from Monterey, California provide evidence for a programmed life span. Thus, when segments (called ramets) of one partner in a chimera are grown separate from the chimera, they die at the same time as the parent colony. This occurs whether the ramets are kept in the same aquarium tank or in different tanks. If a tank contains mixed parent colonies and ramets from them (i.e., different genotypes), only particular parental colonies and their respective ramets undergo senescence and die within a defined period of time. The authors note that their findings provide evidence for a heritable basis underlying mortality in these protochordates, unlinked to other life-history traits. Rinkevich et al. 1992 Proc Natl Acad Sci USA 89: 3546.
NOTE also known as allogenic resorption
NOTE originally a botanical term referring to a cutting grown from vegetative material collected from a parent plant; hence, a clone |
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Research study 8 |
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An investigation of life-history plasticity in the colonial tunicate Botryllus schlosseri, conducted at Pacific Grove, California, involves comparing chimeras maintained for several weeks in the sea with ones maintained in the laboratory for the same length of time. In the field, chimeras grow rapidly, attain large size, and produce many eggs. The genotypic compositions of these colonies remain stable throughout their lifespan, and the chimaeric partners senesce and die synchronously. In the laboratory, however, the genotypic replicates to the ones in the field grow slowly, then shrink and fragment. Egg production is less in the lab than in the field, and absorption of one genotype by the other is much more common than in the field. Life spans are longer in the lab than in the field (18-19wk vs. 8-9wk). The authors conclude that chimeras of B. schlosseri have “extremely plastic life histories”. Chadwick-Furman & Weissman 1995 Proc Roy Soc Lond B 262: 157.
NOTE chimeras are created by freeing up fragments of fusible colonies (each containing 9-17 zooids), and allowing them to attach to glass plates in the lab with the edge of one fragment positioned 1-2mm from the other. Within a week the fragments fuse with their partners and firmly attach to the plates |
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Research study 9 |
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Do chimeric colonial tunicates form under natural conditions in the sea? A survey of 309 colonies of Botryllus schlosseri in Monterey Marina, California indicates that about 20% are in allogenic1 contact with conspecifics and thus have the potential to fuse into chimeras (no data are provided for actual frequency of fusion). Interestingly, a comparison2 of chimeras with “normal” colonies shows that in several life-history traits the chimeras have reduced fitness. Thus, in terms of growth3, age/size at first reproduction,and especially fecundity, chimeras do more poorly than colonies in isolation or colonies in allogeneic contact that have failed to fuse with their neighbours. The authors note that this is the first documentation in a tunicate that allogenic contact4 leading to both fusion and rejection come at a cost to life-history processes of growth and reproduction. The authors also briefly discuss the pros and cons of fusion for a colonial tunicate. Chadwick-Furman & Weissman 2003 Biol Bull 205: 133.
 NOTE1 lit. “different origin” G. In addition, the authors note that 28% of the 309 colonies are in contact with encrusting bryozoans and 40% with other species of colonial and solitary ascidians. Only 16% of the colonies are isolated from contact with other sessile macroorganisms
NOTE2 the authors create 3 treatment groups for study on glass plates: isolated colonies, incompatible pairs that reject one another, and compatible pairs that fuse into chimeras. These are illustrated in the photos
NOTE3 colony age is expressed as asexual growth “cycles”. During a cycle, the zooids produce buds, then shrink, and are replaced by their buds. Cycle frequency varies with season but, based on data in an earlier study (see previous entry), a typical cycle in May in Monterey Bay takes about 1wk
NOTE4 the authors provide some data that xenogenic (“stranger” “origin”) contact also affects colony fecundity, but this is not as severe as allogenic contact. The species in question is another colonial tunicate Botrylloides violaceous |
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