Ralf Janssen

UPPSALA UNIVERSITET: Institutionen för geovetenskaper: Paleobiologi: Personal

Paleobiologi

Personal

Ralf Janssen

Researcher
Department of Earth Sciences
Palaeobiology
Villavägen 16
SE-752 36 Uppsala
SWEDEN

Phone: + 46 18 471 27 63

e-mail: ralf.janssen@geo.uu.se

Research interests

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.

Fig. 1 - Tegenaria atrica (the “monster”)


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.

The main questions are:

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

Cupiennius salei

Fig. 3 - Cupiennius salei

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).

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

List of publications