Research
interests
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Prologue
My very first
encounter with an arthropod specimen was, as far as I remember, when I was at
the age of three. It was a “disgusting” brownish-black hairy monster
with an
endless number of legs that scared me (almost) to death (Fig.
1). Well, I survived
this memorable experience but from that time on I was definitely
captivated by
the small spider that “attacked” me - and its countless small
relatives, that
some time later became my “friends” when I was playing outside. It was
mainly
catching them and studying their behaviour that fascinated me at that
time.
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Fig. 1 -
Tegenaria atrica
(the “monster”)
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Earlier work
Today, I am
mainly interested in development and evolution of arthropods and their
relatives. The euarthropods consist of at least one million described
species
and the number of individuals is countless. The extant euarthropods
(i.e.
insects, crustaceans, myriapods and chelicerates) have conquered
virtually
every habitat, a feat reflected in their high morphological diversity.
Very
important and common to all arthropods is their segmented body. It is
generally
believed that this feature was a main prerequisite for specialisation
and fast
adaptation of the arthropods to all kinds of habitats: A track record
starting
as early as 550 million years ago in the Cambrian.
The word “arthropod”
is actually an amalgamation of the Greek words for “joint”
and “foot”. And
this feature of jointed appendages is actually unique to all
arthropods.
Specialization of single segments very often goes hand in hand with the
specialization of appendages. |
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questions are: |
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1.
What is the
origin of segmentation?
My main interest is in the
origin of segmentation, Traditional phylogenies unite segmented
protostomes
(i.e. annelids and arthropods) into a group called “Articulata”. Since
newer
phylogenies based on molecular data now unite moulting animals (i.e.
“ecdysozoans”), separating them from the rest (i.e. “lophotrochozoans”)
the
question recurred, whether segmentation has evolved independently in
annelids
and arthropods, or whether the last common ancestor of those two
groups, the
urprotostome, was already segmented. In the latter case, loss of
segmentation
has to be considered for all protostomes that are not segmented (e.g.
brachiopods, molluscs, priapulids, nematodes). |
Fig. 2 - Glomeris
marginata; Photo: Ralf Janssen
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Fig. 3 - Cupiennius salei
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2.
What is the
archetype of arthropod segmentation?
In
order to contribute to
answering the fundamental question of the origin of segmentatin I
examined
segmentation in the pill millipede Glomeris
marginata (Fig.2) and the spider Cupiennius
salei (Fig.3) (Janssen et al.,
2004, 2006a, 2006b, 2008, subm.;
Janssen and Damen, 2006; Damen et al., 2005). |
3. What
is the
archetype of arthropod appendages?
In the
case of arthropod appendages it is thought that the diversity of all
different
appendage types has evolved from one ancestral type of appendages.
Trying to
unravel the nature of such an ancestral appendage and to explore the
developmental mechanisms underlying its development is also one of the
topics of
my current research (Prpic et al., 2003,
2005; Jansen et al., 2008).
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Current
work
At the moment I
am trying to study the orthologs of Drosophila
segmentation genes in the priapulid Priapulus
caudatus (Fig.4). The aim of this study is to reveal whether
remnants of
segmentation exist in the priapulid worm on the molecular level. The
priapulids
comprise only around 20 described species, but as the fossil record
reports,
priapulids showed a much higher diversity of species in the Paleozoic
era. Like
the arthropods, they belong to the ecdysozoans, and hence form a sister
group
to the arthropods. Priapulus caudatus
is one of the largest recent priapulids and therefore a good candidate
for
comparative studies. Although phylogenetic position and fossil record
make the
priapulids a group of high interest, relatively little work has been
done on
them in the past. Embryology for example is described rudimentarily and
is
currently in the progress of re-evaluation (Wennberg
et al., 2008, 2009).
If the
priapulids should show remnants of segmentation, then this would
strengthen the
idea of a segmented protostome ancestor. |
Fig. 4 - Priapulus
caudatus; Photo: Ralf Janssen
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List of publications
Shigenobu S, Bickel RD,
Brisson JA, Butts T, Chang C-C, Christiaens O, Davis GK, Duncan EJ,
Ferrier DEK, Iga M, Janssen R,
Lin G-W, Lu H-L, McGregor AP, Miura T, Smagghe G, Smith JM, van der Zee
M, Velarde R, Wilson MJ, Dearden PK, Stern DL. Comprehensive survey of
developmental genes in the pea aphid, Acyrthosiphon
pisum: frequent
lineage-specific duplications and losses of developmental genes. Insect
Mol. Biol. (in press)
Damen WGM, Prpic N-M and Janssen R.
Embryonic development and the understanding of the adult body plan in
myriapods. Soil Organisms. (in press)
Janssen R, Wennberg
SA and Budd GE 2009. The hatching larva of the priapulid worm Halicryptus spinulosus. Frontiers in Zoology 6:
8.
Wennberg
SA, Janssen R and Budd GE 2009.
Hatching and earliest larval stages of the priapulid worm Priapulus
caudatus. Inv. Biol. 157-171.
Janssen R, Budd GE, Damen WGM and
Prpic N-M
2008. Evidence for Wg-independent tergite boundary formation in the
millipede Glomeris marginata. Dev. Genes Evol. 218:
361-370.
Wennberg
SA, Janssen R and Budd GE 2008.
Early embryonic development of the priapulid worm, Priapulus
caudatus Lam. Evol. Dev. 10: 326-338.
Janssen R. and Damen WGM 2008.
Diverged and
conserved aspects of heart formation in a spider. Evol. Dev. 10:
155-165.
Janssen R, Damen WGM and Prpic
N-M, 2008. H15 and optomotor-blind in
the spiders Cupiennius salei, Tegenaria atrica and Achaearanea
tepidariorum (Chelicerata: Araneae) and the
dorso-ventral axis of
arthropod appendages. Evol. Dev. 10: 143-154.
Janssen R, Prpic N-M and Damen WGM
2006. A
review of the correlation of tergites, sternites, and leg pairs in
diplopods.
Frontiers in Zoology 3:2.
Janssen R and Damen WGM 2006b. The
ten Hox
genes of the millipede Glomeris marginata.
Dev. Genes Evol. 216: 451-465.
Janssen R, Prpic N-M and Damen
WGM. 2006a.
Dorso-ventral differences in gene expression in Glomeris
marginata Villers, 1789 (Myriapoda: Diplopoda). Norw. J.
Entomol. 53: 129-137.
Damen
WGM, Janssen R and Prpic N-M. 2005.
Pair rule gene orthologs in spider segmentation. Evol. Dev. 7: 618-628.
Prpic N-M, Janssen
R, Damen WGM and Tautz D.
(2005). Evolution of dorsal-ventral axis formation in arthropod
appendages: H15 and optomotor-blind/bifid-type
T-box genes in the millipede Glomeris marginata (Myriapoda:
Diplopoda). Evol. Dev. 7: 51-57.
Janssen R, Prpic N-M and Damen
WGM. 2004.
Gene expression suggests decoupled dorsal and ventral segmentation in
the
millipede Glomeris marginata (Myriapoda:
Diplopoda). Dev. Biol. 268: 89-114
Prpic
N-M, Janssen R, Wigand B, Klingler
M, Damen WGM 2003. Gene expression in spider appendages reveals
reversal of exd/hth spatial
specificity, altered leg gap gene dynamics, and suggests divergent
distal morphogen signaling. Dev. Biol. 264,
119-140.
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