title: Spiralian phylogeny informs the evolution of microscopic lineages.
abstract: Despite rapid advances in the study of metazoan evolutionary history [1], phylogenomic analyses have so far neglected a number of microscopic lineages that possess a unique combination of characters and are thus informative for our understanding of morphological evolution. Chief among these lineages are the recently described animal groups Micrognathozoa and Loricifera, as well as the two interstitial "Problematica" Diurodrilus and Lobatocerebrum [2]. These genera show a certain resemblance to Annelida in their cuticle and gut [3, 4]; however, both lack primary annelid characters such as segmentation and chaetae [5]. Moreover, they show unique features such as an inverted body-wall musculature or a novel pharyngeal organ. This and their ciliated epidermis have led some to propose relationships with other microscopic spiralians, namely Platyhelminthes, Gastrotricha, and in the case of Diurodrilus, with Micrognathozoa [6, 7]-lineages that are grouped by some analyses into "Platyzoa," a clade whose status remains uncertain [1, 8-11]. Here, we assess the interrelationships among the meiofaunal and macrofaunal members of Spiralia using 402 orthologs mined from genome and transcriptome assemblies of 90 taxa. Lobatocerebrum and Diurodrilus are found to be deeply nested members of Annelida, and unequivocal support is found for Micrognathozoa as the sister group of Rotifera. Analyses using site-heterogeneous substitution models further recover a lophophorate clade and position Loricifera + Priapulida as sister group to the remaining Ecdysozoa. Finally, with several meiofaunal lineages branching off early in the diversification of Spiralia, the emerging concept of a microscopic, acoelomate, direct-developing ancestor of Spiralia is reviewed.
authors.affiliation: Department of Organismic and Evolutionary Biology, Museum of Comparative Zoology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA. Electronic address: claumer@ebi.ac.uk.
authors.forename: Christopher E
authors.initials: CE
authors.lastname: Laumer
authors.affiliation: Marine Biological Section, Department of Biology, University of Copenhagen, Universitetsparken 4, 2100 Copenhagen Ø, Denmark.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Bekkouche
authors.affiliation: Marine Biological Section, Department of Biology, University of Copenhagen, Universitetsparken 4, 2100 Copenhagen Ø, Denmark.
authors.forename: Alexandra
authors.initials: A
authors.lastname: Kerbl
authors.affiliation: Department of Ecology and Evolutionary Biology, Brown University, Providence, RI 02906, USA.
authors.forename: Freya
authors.initials: F
authors.lastname: Goetz
authors.affiliation: Biozentrum, University of Basel, Klingelbergstrasse 50, 4056 Basel, Switzerland.
authors.forename: Ricardo C
authors.initials: RC
authors.lastname: Neves
authors.affiliation: Natural History Museum of Denmark, Universitetsparken 15, 2100 Copenhagen Ø, Denmark.
authors.forename: Martin V
authors.initials: MV
authors.lastname: Sørensen
authors.affiliation: Natural History Museum of Denmark, Universitetsparken 15, 2100 Copenhagen Ø, Denmark.
authors.forename: Reinhardt M
authors.initials: RM
authors.lastname: Kristensen
authors.affiliation: Sars International Centre for Marine Molecular Biology, University of Bergen, Thormøhlensgate 55, 5008 Bergen, Norway.
authors.forename: Andreas
authors.initials: A
authors.lastname: Hejnol
authors.affiliation: Department of Ecology and Evolutionary Biology, Brown University, Providence, RI 02906, USA.
authors.forename: Casey W
authors.initials: CW
authors.lastname: Dunn
authors.affiliation: Department of Organismic and Evolutionary Biology, Museum of Comparative Zoology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA.
authors.forename: Gonzalo
authors.initials: G
authors.lastname: Giribet
authors.affiliation: Marine Biological Section, Department of Biology, University of Copenhagen, Universitetsparken 4, 2100 Copenhagen Ø, Denmark.
authors.forename: Katrine
authors.initials: K
authors.lastname: Worsaae
mesh_terms: D000818:Animals; D005075:Biological Evolution; D019143:Evolution, Molecular; D007448:Invertebrates / Q000033:anatomy & histology / Q000145:classification* / Q000235:genetics*; D010802:Phylogeny; D012403:Rotifera / Q000033:anatomy & histology / Q000145:classification / Q000235:genetics