The role of character loss in phylogenetic reconstruction as exemplified for the Annelida

Abstract

Annelid relationships are controversial, and molecular and morphological analyses provide incongruent estimates. Character loss is identified as a major confounding factor for phylogenetic analyses based on morphological data. A direct approach and an indirect approach for the identification of character loss are discussed. Character loss can frequently be found within annelids and examples of the loss of typical annelid characters, like chaetae, nuchal organs, coelomic cavities and other features, are given. A loss of segmentation is suggested for Sipuncula and Echiura; both are supported as annelid ingroups in molecular phylogenetic analyses. Moreover, character loss can be caused by some modes of heterochronic evolution (paedomorphosis) and, as shown for orbiniid and arenicolid polychaetes, paedomorphic taxa might be misplaced in phylogenies derived from morphology. Different approaches for dealing with character loss in cladistic analyses are discussed. Application of asymmetrical character state transformation costs or usage of a dynamic homology framework represents promising approaches. Identifying character loss prior to a phylogenetic analysis will help to refine morphological data matrices and improve phylogenetic analyses of annelid relationships.

Parallel evolution of segmentation by co‐option of ancestral gene regulatory networks

“…Whole body segmentation has been lost, complete with all segmented organ systems, several times within one phylum, namely, the annelids. (41,42) Two unsegmented groups, the sipunculids and the echiurans, previously regarded as phylum level taxa, have now been shown to be nested within polychaete annelids. (43) Both taxa retain transient vestiges of a segmented body plan during their development.…”

Systematics, evolution and phylogeny of Annelida – a morphological perspective

“…It is suggested that cladistic analyses using morphological data may fail to recognise absent characters as losses rather than as primary absences Bleidorn, 2007;see Fitzhugh, 2008). Thus, the sister-group relationship Polychaeta-Clitellata as found in Rouse and Fauchald (1997) may have been biased by the misinterpretation of a number of convergently lost characters.…”

Annelids in evolutionary developmental biology and comparative genomics

“…Such loss of characters is clearly a plausible and common phenomenon (e.g. Jenner 2004;Bleidorn 2007). At present we tend to then fall back on supposing that this rare retention of ancestral complexity can be revealed by the same complex genetic network being used in such supposed non-derived lineages.…”

“…The discussions on the reconstruction of the annelid and clitellate ancestors have been reviewed in greater detail elsewhere (e.g. Bartolomaeus et al, 2005;Bleidorn, 2007;Nielsen, 2001;Purschke, 2002;Rouse & Fauchald, 1997;Rouse & Pleijel, 2001;Westheide, 1997;Westheide et al, 1999). There is still plenty of debate about the form of these ancestors in addition to the processes outlined above.…”

“…However, a consensus that is rapidly gaining support is that the annelids now include the polychaetes (bristle worms) and clitellates (the leeches and oligochaetes), along with other groups that have had a rather nomadic phylogenetic history, including the Pogonophora/Siboglinidae (beard worms), the Echiura (spoon worms), the Sipuncula (peanut worms) and the Myzostomida (parasites and commensals of crinoid feather stars). The emerging consensus from the molecular data is that the polychaetes are a paraphyletic group with the Clitellata, Sipuncula, Echiura, Siboglinidae and Myzostomidae embedded within it (Bleidorn et al, 2007;Struck et al, 2007).…”