Novel implications for the basal internal relationships of Gastrotricha revealed by an analysis of morphological characters

Abstract

Novel implications for the basal internal relationships of Gastrotricha revealed by an analysis of morphological characters. -Zoologica Scripta , 37 , 429-460. A cladistic analysis of Gastrotricha based on morphological characters is presented. Unlike previous morphological analyses, our study uses species rather than higher level taxa, for which the ground pattern is often unknown. The analysis comprises 79 ingroup taxa, 4 outgroup taxa and 135 binary and multistate characters in total. Character coding is based on a careful assessment of original species descriptions. Characters included cover general body organization, internal and external features as, for example, data on the adhesive tubes, digestive tract or cuticle armament. Character systems such as many ultrastructural findings, for which it was problematic to obtain data for a large set of the included taxa, were not considered. To minimize a priori assumptions, all characters were treated with equal weight and left unordered. The four outgroup representatives were chosen in accordance with the current sister group hypotheses for Gastrotricha. Two search strategies, a heuristic search (maximum parsimony) and a parsimony ratchet search, reveal a comparable scenario. Gastrotricha split into two sister taxa. One group comprises genus Neodasys only, the sister group N.N.1 (Eutubulata nom. nov.) consists of all remaining Gastrotricha. Within Eutubulata, monophyletic Macrodasyida s. str. and N.N.2 (Abursata nom. nov.) are sister taxa of highest rank. Abursata consists of the 'freshwater macrodasyids' Marinellina and Redudasys as sister group of monophyletic Paucitubulatina. Some traditional families are supported by this analysis. We evaluate possible apomorphies for the most basal stem lineages and track the evolution of selected organs. Our findings reveal that secondary character loss may play an important role in the stem lineage of Abursata and further in Paucitubulatina. Moreover, according to this analysis there might have been a single invasion of the freshwater environment in the stem lineage of Abursata followed by several independent returns to marine habitats within the monophylum Paucitubulatina.

The nervous system of Neodasys chaetonotoideus (Gastrotricha: Neodasys) revealed by combining confocal laserscanning and transmission electron microscopy: evolutionary comparison of neuroanatomy within the Gastrotricha and basal Protostomia

“…Travis 1983;Ruppert 1991;Hochberg and Litvaitis 2000;Ax 2001). Kieneke et al (2008) concluded from a re-evaluation of morphological data that the genus Neodasys represents the sister taxon of all other gastrotrichs (Fig. 11e, f).…”

“…If the superordinate structure is not present, all following substructures have to be coded as inapplicable states (-). Most character states are linked to a certain taxon (species) to simplify comparability (Kieneke et al 2008 called this ''morphological type concept'').…”

“…The twelve gastrotrich species (plus the branch leading to the Paucitubulatina) that are included in our character matrix have been manually assembled to cladograms using the tree window of MacClade 4.0. These thirteen gastrotrich taxa were arranged according to the inferred systematic position of congeneric species (systematic position taken from Todaro et al 2006 andKieneke et al 2008) or the genus they belong to (systematic position taken from Hochberg and Litvaitis 2000). We regard this procedure legitimate since monophyly of macrodasyidan and most paucitubulatinan genera is much likely, i.e.…”

“…Hochberg 2005) and possibly is the sister group of one of the traditional high-level taxa of Gastrotricha, the Paucitubulatina, together forming the Chaetonotida (Hochberg and Litvaitis 2000). However, a recent cladistic analysis (Kieneke et al 2008) revealed Chaetonotida to be polyphyletic and Neodasys as the sister group to all other gastrotrichs united within a common monophylum. This putative basal position makes Neodasys a prior object for evolutionary-phylogenetic estimations, such as ground pattern reconstructions.…”

Checklist de Gastrotricha do Estado do Mato Grosso do Sul, Brasil

“…O táxon, com distribuição cosmopolita, é composto por mais de 760 espécies tradicionalmente dividido em duas grandes ordens: Chaetonotida e Macrodasyida (Hochberg & Litvaitis, 2000;Balsamo et al, 2008;2009;Todaro & Hummon, 2008;Hummon & Todaro 2010; mas veja Kieneke et al, 2008). O grupo Chaetonotida apresenta mais de 450 espécies, ou seja, compreende a maioria dos Gastrotricha, e possui representantes com uma morfologia externa que lembra o formato de um "pino de boliche".…”