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Figure 2. Schematic larval ascidian. Note the resemblance to the tadpole stage of a frog. ht –heart, in – intestine, lm – lateral muscle cells (left series omitted to show notochord), nc – nerve cord, nt – notochord, tr – trunk.

Figure 3. Diplosoma listerianum, an ascidian in its adult sessile stage. Note the four rows of gill slits in the branchial basket. is - incurrent siphon, gs – gill slit, st – stomach.

Figure 4. Two colonial ascidians. Aggregates of individual animals are created by asexual reproduction (= budding). A –Small colony of Diplosoma listerianum; B – Small colony of Botryllus schlosseri.

Figure 5. Regenerating arm of a brittle star, Amphiura filiformis. Note the slender tip (arrow) that developed after the arm broke off.

Figure 6. Fission, the splitting of an ‘individual’ animal, is a common process in echinoderms. A – Fission in an adult holothurian, Stichopus chloronotus (Uthicke 2001); B – Fission in a larval brittle star, Ophiopholis aculeata (Balser 1998).

 

Introduction

 

Professor Michael Thorndyke
Chair of Experimental Biology
The Royal Swedish Academy of Sciences

Projects

Budding and Development

Neuronal Regeneration

Neurogenomics

Hematopoesis

 

 

Recent years have witnessed rapid advances in molecular and genetic technologies that allowed complete sequencing of the genomes of several model organisms (Man (Homo sapiens), Fruit Fly (Drosophila melanogaster), Nematode (Caenorhabditis elegans), Mouse (Mus musculus)). Until now the power of modern genomics has only rarely been applied to marine organisms (Sea Urchin (Strongylocentrotus purpuratus), Sea Squirt (Ciona intestinalis)).
In our laboratory we seek to exploit these rapidly advancing technologies to understand the rich diversity of genetic processes present in marine organisms and their importance in the context of evolution, adaptation, and ecology. To this end, we study tunicates and echinoderms, which are exclusively marine animals and members of the deuterostomes, a phylogenetically distinct group that includes the chordates (see Fig. 1). Because of this phylogenetic position, they are of great interest as they include the closest extant invertebrate relative of vertebrates.
Our particular research focus is on developmental processes during budding and regeneration.

Figure 1. Phylogenetic hypotheses of higher deuterostome groups. A – Based on molecular analyses (Adoutte et al. 2000); B – Based on analysis of morphological data (Nielsen 2000).

Projects

In the Kristineberg’s Marine Genomics Group we study tunicates and echinoderms utilizing molecular and genetic technologies as well as histological techniques with a focus on developmental processes during budding and regeneration. Our strategy is to combine the power of modern genomic technology with the rich diversity of largely un-exploited models available in the marine environment.

Budding

During embryogenesis, development of cell types follows a well-described pattern for which many, highly conserved genetic pathways are known from studies on the classical model organisms. While general principles are common, modifications of mechanisms are likely to have evolved in phylogenetically distant groups.
The particular relevance of this to our research lies in the life history patterns found in many marine invertebrates. Most model organisms are direct developers, that is the embryo and larval stage are similar to small adults in terms of their body patterns and axes. In contrast, tunicates and echinoderms have clearly distinct larval phenotypes that undergo a drastic metamorphosis. Ascidians, members of the tunicates, have attracted particular attention in this respect, because their larva resembles what some researchers regard as an archetypal chordate with a characteristic morphology that includes a dorsal nerve cord, notochord and lateral muscles (see Fig. 2). This is in sharp contrast to the usually sessile filter-feeding adult (see Fig. 3). However, many adult tunicates bypass the larval stage and metamorphosis when generating offspring in a process called budding (see Fig. 4). One of our goals is to compare shared and disparate developmental patterns between asexual budding in adults and larval embryogenesis.

Key questions include:
· Is this adult developmental process simply a re-expression of the embryonic mechanism?
· Does it involve the re-programming of differentiated adult cells (de-differentiation and/or transdifferentiation)?
· What is the extent of biological diversity in the mechanisms employed in the different groups and what are the phylogenetic implications?

Budding and Development

Hematopoesis

Regeneration

Adult vertebrates, particularly mammals show a very limited capacity for neuronal replacement following trauma or disease. This limitation is one of the main driving forces that underpins the current high level of interest in those small areas of neuronal renewal now known to exist in the mammalian brain.
In dramatic contrast to this, both tunicates and echinoderms exhibit a remarkable plasticity as adults and are capable of extensive regeneration of large parts of their bodies following traumatic loss or damage (see Fig. 5). This potential has been exploited adaptively in both groups of animals as a mechanism for budding. Many adult tunicates produce asexual buds that develop into new and independent adults (see above), while some adult and larval echinoderms can undergo fission (see Fig. 6) and so produce two or more animals from a single original.
A central question here then is: what is the nature of the molecular regulatory pathways that facilitate this remarkable ability? Of particularly interest are questions regarding the presence and role of stem cells, especially neural stem cells. Given that these animals are closely related to vertebrates and share many genetic pathways with them (and thus with mammals, including man), it becomes a fascinating question to ask why can adult tunicates and echinoderms regenerate extensive parts of their anatomy, while most adult vertebrates and certainly mammals, cannot.

Key questions include:
· Are similar genetic pathways for neurogenesis employed in adult regeneration as are used in normal embryonic, larval and adult development?
· Does it involve embryonic stem cells that are conserved or re-instated in adults?
· Does it involve re-programming of existing adult stem cell lineages, for example the haematopoietic lineage?
· What is the adaptive value of the diverse patterns of adult regeneration and how are they regulated by environmental change and variation?

Neuronal Regeneration

Neurogenomics