Chaetal type diversity increases during evolution of Eunicida (Annelida)
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ORIGINAL ARTICLE
Chaetal type diversity increases during evolution of Eunicida (Annelida) Ekin Tilic 1 & Thomas Bartolomaeus 1 & Greg W. Rouse 2
Received: 21 August 2015 / Accepted: 30 November 2015 / Published online: 15 December 2015 # Gesellschaft für Biologische Systematik 2015
Abstract Annelid chaetae are a superior diagnostic character on species and supraspecific levels, because of their structural variety and taxon specificity. A certain chaetal type, once evolved, must be passed on to descendants, to become characteristic for supraspecific taxa. Therefore, one would expect that chaetal diversity increases within a monophyletic group and that additional chaetae types largely result from transformation of plesiomorphic chaetae. In order to test these hypotheses and to explain potential losses of diversity, we take up a systematic approach in this paper and investigate chaetation in Eunicida. As a backbone for our analysis, we used a threegene (COI, 16S, 18S) molecular phylogeny of the studied eunicidan species. This phylogeny largely corresponds to previous assessments of the phylogeny of Eunicida. Presence or absence of chaetal types was coded for each species included into the molecular analysis and transformations for these characters were then estimated using the mK1 likelihood model. Our results show that chaetal type diversity does indeed increase within eunicids and provide possible explanations for the homology, convergence, and loss of chaetal types in eunicidan subtaxa.
Electronic supplementary material The online version of this article (doi:10.1007/s13127-015-0257-z) contains supplementary material, which is available to authorized users. * Ekin Tilic [email protected] 1
Institute of Evolutionary Biology and Animal Ecology, University of Bonn, An der Immenburg 1, 53121 Bonn, Germany
2
Scripps Institution of Oceanography, UCSD, 9500 Gilman Drive, La Jolla, CA 92093, USA
Keywords Chaetae . Molecular phylogeny . Eunicida . Systematics
Introduction Chaetae in annelids have attracted the interest of scientist for a very long time, making them one of the most studied, if not the most studied structures of annelids. This is partly due to the significance of chaetal features when identifying annelids, since chaetal structure and arrangement are highly constant in species and supraspecific taxa. Aside from being a valuable source for taxonomists, chaetae have also been the focus of many studies in functional ecology (Merz and Edwards 1998; Merz and Woodin 2000; Merz 2015; Pernet 2000; Woodin and Merz 1987). Furthermore, the cellular mechanisms behind the chaetal formation process described by Bouligand (1967) and O’Clair and Cloney (1974) have been an intriguing field of study. Chaetae are extracellular, chitinous structures formed within an ectodermal pouch, the so-called chaetal follicle. The basalmost cell within this follicle is the chaetoblast (Bartolomaeus 1998; Bouligand 1967; Hausen 2005; Schroeder 1984; Specht and Westheide 1988). This cell possesses apical microvilli which rele
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