A Comprehensive Molecular Phylogeny of Dalytyphloplanida (Platyhelminthes: Rhabdocoela) Reveals Multiple Escapes from the Marine Environment and Origins of Symbiotic Relationships

Abstract

In this study we elaborate the phylogeny of Dalytyphloplanida based on complete 18S rDNA (156 sequences) and partial 28S rDNA (125 sequences), using a Maximum Likelihood and a Bayesian Inference approach, in order to investigate the origin of a limnic or limnoterrestrial and of a symbiotic lifestyle in this large group of rhabditophoran flatworms. The results of our phylogenetic analyses and ancestral state reconstructions indicate that dalytyphloplanids have their origin in the marine environment and that there was one highly successful invasion of the freshwater environment, leading to a large radiation of limnic and limnoterrestrial dalytyphloplanids. This monophyletic freshwater clade, Limnotyphloplanida, comprises the taxa Dalyelliidae, Temnocephalida, and most Typhloplanidae. Temnocephalida can be considered ectosymbiotic Dalyelliidae as they are embedded within this group. Secondary returns to brackish water and marine environments occurred relatively frequently in several dalyeliid and typhloplanid taxa. Our phylogenies also show that, apart from the Limnotyphloplanida, there have been only few independent invasions of the limnic environment, and apparently these were not followed by spectacular speciation events. The distinct phylogenetic positions of the symbiotic taxa also suggest multiple origins of commensal and parasitic life strategies within Dalytyphloplanida. The previously established higher-level dalytyphloplanid clades are confirmed in our topologies, but many of the traditional families are not monophyletic. Alternative hypothesis testing constraining the monophyly of these families in the topologies and using the approximately unbiased test, also statistically rejects their monophyly.

Diceratocephala boschmai (Platyhelminthes: Temnocephalida) from crayfish farms in Thailand: investigation of the topographic surface and analysis of 18S ribosomal DNA sequences

“…Therefore, this result fills a gap, placing D. boschmai into the scheme of the phylogenetic relationships within the Rhabdocoela as classified by 18S rDNA sequences. Furthermore, it is congruent to the recent study of Van Steenkiste et al (2013), which clarified the dalytyphloplanid phylogeny based on complete 18S rDNA and partial 28S rDNA, in which C. truncate and M. rossi were positioned within Dalyellioida. However, G. buccinicala and Provortex spp.…”

Two new species of Syndesmis (Platyhelminthes, Rhabdocoela, Umagillidae) from the sea urchin Pseudechinus magellanicus (Echinodermata, Echinoidea) in the Southwestern Atlantic Ocean

“…Among them, the family Umagillidae (Dalytyphloplanida) is well known for parasitizing sea urchins, holothuroids and sipunculids. This family is a natural group (Van Steenkiste et al., 2013) represented by 68 species of parasitic worms. The genus Syndesmis presents twenty-four species parasitizing the intestine and coelomic cavity of sea urchins from different regions of the world (Brogger and Ivanov, 2010).…”

Molecular examination of kalyptorhynch diversity (Platyhelminthes: Rhabdocoela), including descriptions of five meiofaunal species from the north-eastern Pacific Ocean

“…There are about 530 described kalyptorhynch species (Van Steenkiste et al, 2013) but as is the case for other turbellarians (Artois & Schockaert, 2005b), this is likely a vast underestimate of the true diversity of the group. The Kalyptorhynchia contain two subclades: the Schizorhynchia (bifurcated proboscis) and the Eukalyptorhychia (unbranched proboscis) (Cannon, 1986;Willems et al, 2006a).…”