Isolated branches in the phylogeny of Platyhelminthes

Description

This dissertation examines the early phylogenetic divergences of the phylum Platyhelminthes using molecular sequence data, with an emphasis on the placement and evolutionary significance of several enigmatic and largely overlooked lineages. Firstly, I assess platyhelminth interrelationships using a representational sampling of all free-living orders for the “standard” 18S and 28S rRNA loci (plus two mtDNA markers). These analyses place numerous problematic taxa, most notably Gnosonesimida, which is recovered in a position consistent with the classical hypothesis in which this order retains a primitive form of ectolecithality. I also investigate the status of the crustacean-parasitic genus Genostoma, classified presently within Fecampiida, which has however been proposed on the basis of ultrastructural data to lie outside this order. These efforts robustly position Genostoma as the sister-taxon to the free-living order Prolecithophora, suggesting the recognition of a new higher taxon to accommodate this morphologically distinct, but poorly diverse lineage. To overcome the inherent limits of rRNA phylogenetics, I used massively parallel sequencing to survey transcriptomes from representatives of all “turbellarian” orders. From concatenation and consensus analyses of 512 orthologs, a robust signal of platyhelminth phylogeny emerges, congruent with previous results but also presenting several unanticipated relationships. Most notable among these, the monospecific order Bothrioplanida is recovered as the sister-group of Neodermata, the major vertebrate-parasitic clade within Platyhelminthes. These analyses prompt consideration of novel hypotheses on the origins and consequences of parasitism within Platyhelminthes, and motivate many previously unexplored comparisons among free-living taxa. The position of Platyhelminthes within Spiralia, and the related question of whether the phylum is “primitive” in morphology, remains controversial. I therefore also used transcriptomic data to resolve the phylogeny of Spiralia, with emphasis on the status of the “platyzoan” phyla, and on positioning several problematic interstitial lineages. These data robustly position Lobatocerebrum and Diurodrilus as members of Annelida. I also recover strong support for the non-monophyly of the platyzoan phyla, with Gnathifera as the earliest-splitting branch and a clade of Platyhelminthes and Gastrotricha as the nearest relative of Trochozoa. This phylogeny hence simultaneously highlights the importance of “reductive” processes in the evolution of interstitial organisms, as well as the possibility of the primitive nature of at least some of the “simple” features that have classically inspired zoological interest in Platyhelminthes. Finally, I present a focused inquiry on the internal phylogeny of one “isolated” flatworm clade, the continental order Prorhynchida. The recovered topology is broadly congruent with traditional classification, with most prorhynchid species falling into two genera. Remarkably, however, two rare taxa that share morphologically similar copulatory apparatus are recovered as unrelated basal branches, indicating the probable plesiomorphic nature of this morphology. Also, a little-known groundwater species, Geocentrophora boui, is supported as the sister taxon of the self-fertilizing genus Xenoprorhynchus, illuminating the functional specialization of the “copulatory” apparatus as a venom delivery system in this lineage. Field collections from this study uncovered over 31 new species, many of these unexpectedly terrestrial, more than doubling the known diversity of the order, and highlighting the need for continued systematic research on these remarkable but understudied animals.

Subjects