Ultrastructure, Formation and Evolution of Chaetae in Annelids

Abstract

Annelid systematics has been quite unstable over the past century. Especially, with the introduction of molecular techniques to unravel phylogenetic relationships, previously established morphology-based phylogenetic trees were completely changed. This challenges earlier ideas on homology and autapomorphies. Annelid worms bear chaetae. These are chitinous extracellular structures that have an incredible structural diversity, but show significant constancy within species and supraspecific taxa. This thesis presents a series of studies that employ chaetal characters such as chaetogenesis and chaetal arrangement to test homology hypotheses and help us understand how annelid chaetae, which are critically important in the systematics of the group, evolved.<br /> One of the main aspects of this work is the evolution of hooked chaetae in annelids. These chaetae resemble tiny anchors and are mainly associated with tube-borrowing sedentary worms. A series of previously published papers in fact utilized the comparative study of chaetogeneses as a basis to establish a sound hypothesis on the homology of this type of chaeta and proposed a common ancestry of hook bearing taxa. In light of recent molecular phylogenies this assumption no longer holds true. In order to understand and explain the structural diversity and unity amongst the hooked chaetae of annelids the existing information on chaetal formation of hooked chaetae is extended and further taxa were included to provide a complete picture of the diversity; now covering all the major branches of the annelid tree. The formation of hooded hooks in errant eunicid species differs significantly from the superficially similar looking hooded hooks of Capitellidae and Spionidae. This indicates an independent evolution of the hooded hooks, which is consistent with inferences based on phylogenetic analyses. The study on the chaetation and the formation of hooked chaetae in maldanids, revealed striking similarities to their well established sister-taxon Arenicolidae and supports a homology of hooks at least for these sister-groups. In another study the chaetae of the sabellariid Sabellaria alveolata were investigated. The results of this investigation showed remarkable differences in chaetal formation to any other hooked chaetae studied so far, arguing against their homology and demonstrating that hooked chaetae formation is not as uniform as previously assumed.<br /> Chaetal arrangement and the position of the chaetal formative site is utilized to test the homology of chaetae in Echiura to the chaetae in remaining annelids. Echiuran worms have an extremely derived morphology and their position within annelids is established only recently. The study of echiuran chaetation revealed that a ventral pair of chaetae evolved in the stem lineage of Echiura by transforming neuropodial cappillary chaetae and shows that the hemi-circles of anal chaetae evolved once within Echiura as a derived condition. <br /> Furthermore, a more systematic approach was employed in another study to test hypotheses on the evoloution and radiation of chaetal types within a monophyletic taxon (Eunicida). The general hypothesis based on ancestral state reconstructions is that early annelids only possessed simple capillary chaetae. The results of the study on eunicid chaetae shows that chaetae diversify within a monophyletic taxon but also provides explanations for cases in which chaetation or chaetal diversity is secondarily lost. <br /> The series of studies and results presented herein show that chaetal structure and development, together with their topological arrangement provide a valuable set of characters that can be used as an instrument for testing homology hypotheses. Contrary to common belief that morphology is a rather descriptive discipline, this thesis demonstrates that morphology can be hypothesis driven. Especially when the interpretation of morphological characters can be corroborated with well-supported and robust phylogenies a better understanding of structural diversity and evolution can be achieved.