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 –
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).
Professor Michael Thorndyke
Chair of Experimental Biology
The Royal Swedish Academy of Sciences
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
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
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.
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
Budding and Development
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
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