Abstract
Several alternative hypotheses on the relationships between the major arthropod groups are still being discussed. We reexamine here the chelicerate/myriapod relationship by comparing previously published morphological data on neurogenesis in the euarthropod groups and presenting data on an additional myriapod (Strigamia maritima). Although there are differences in the formation of neural precursors, most euarthropod species analyzed generate about 30 single neural precursors (insects/crustaceans) or precursor groups (chelicerates/myriapods) per hemisegment that are arranged in a regular pattern. The genetic network involved in recruitment and specification of neural precursors seems to be conserved among euarthropods. Furthermore, we show here that neural precursor identity seems to be achieved in a similar way. Besides these conserved features we found 2 characters that distinguish insects/crustaceans from myriapods/chelicerates. First, in insects and crustaceans the neuroectoderm gives rise to epidermal and neural cells, whereas in chelicerates and myriapods the central area of the neuroectoderm exclusively generates neural cells. Second, neural cells arise by stem-cell-like divisions of neuroblasts in insects and crustaceans, whereas groups of mainly postmitotic neural precursors are recruited for the neural fate in chelicerates and myriapods. We discuss whether these characteristics represent a sympleisiomorphy of myriapods and chelicerates that has been lost in the more derived Pancrustacea or whether these characteristics are a synapomorphy of myriapods and chelicerates, providing the first morphological support for the Myriochelata group.
Highlights
The relationships between and within the major arthropod groups have not been consistently resolved
We have presented comparative morphological and molecular data on neurogenesis in the euarthropod groups
There are differences in the formation of neural precursors, most arthropod species analyzed generate approximately 30 single neural precursors or precursor groups per hemisegment, which are arranged in regular rows
Summary
The relationships between and within the major arthropod groups have not been consistently resolved. Whereas the Atelocerata hypothesis is mainly supported by morphological evidence, the idea of Pancrustacea was initially based on the phylogenetic analysis of ribosomal-RNA sequence data in which crustaceans and insects grouped together to the exclusion of myriapods (Field and others 1988; Turberville and others 1991; Ballard and others 1992; Friedrich and Tautz 1995; Giribet and Ribera 1998). Several independent phylogenetic analyses based on molecular data support a chelicerate/ myriapod sister group relationship, the so-called Myriochelata hypothesis (Friedrich and Tautz 1995; Hwang and others 2001; Kusche and Burmester 2001; Nardi and others 2003; Mallatt and others 2004; Pisani and others 2004), a link that had never been considered by comparison of morphological structures. We present new data on the geophilomorph centipede Strigamia maritima (Myriapoda) and discuss the data in a phylogenetic context
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