Because of their simple organization the Acoela have been considered to be either primitive bilaterians or descendants of coelomates through secondary loss of derived features. Sequence data of 18S ribosomal DNA genes from non-fast evolving species of acoels and other metazoans reveal that this group does not belong to the Platyhelminthes but represents the extant members of the earliest divergent Bilateria, an interpretation that is supported by recent studies on the embryonic cleavage pattern and nervous system of acoels. This study has implications for understanding the evolution of major body plans, and for perceptions of the Cambrian evolutionary explosion.

https://science.sciencemag.org/content/sci/283/5409/1919.full.pdf

Acoel flatworms : Earliest extant bilaterian metazoans, not members of Platyhelminthes

Triploblastic relationships were examined in the light of molecular and morphological evidence. Representatives for all triploblastic "phyla" (except Loricifera) were represented by both sources of phylogenetic data. The 18S ribosomal (rDNA) sequence data for 145 terminal taxa and 276 morphological characters coded for 36 supraspecific taxa were combined in a total evidence regime to determine the most consistent picture of triploblastic relationships for these data. Only triploblastic taxa are used to avoid rooting with distant outgroups, which seems to happen be? cause of the extreme distance tl tat separates diploblastic from triploblastic taxa according to the 18S rDNA data. Multiple phylogenetic analyses performed with variable analysis parameters yield largely inconsistent results for certain groups such as Chaetognatha, Acoela, and Nemertodermatida. A normalized incongruence length metric is used to assay the relative merit of the multiple analy? ses. The combined analysis having the least character incongruence yields the following scheme of relationships of four main clades: (1) Deuterostomia (((Echinodermata + Enteropneusta) (Cephalo? chordata (Urochordata + Vertebrata)))); (2) Ecdysozoa ((((Priapulida + Kinorhyncha) (Nematoda + Nematomorpha)) ((Onychophora + Tardigrada) Arthropoda))); (3) Trochozoa (((Phoronida + Bra? chiopoda) (Entoprocta (Nemertea (Sipuncula (Mollusca (Pogonophora (Echiura + Annelida)))))))); and (4) Platyzoa (((Gnathostomulida (Cycliophora + Syndermata)) (Gastrotricha + Plathelminthes))). Chaetognatha, Nemertodermatida, and Bryozoa cannot be assigned to any one of these four groups. For the first time, a data analysis recognizes a clade of acoelomates, the Platyzoa (sensu Cavalier-Smith, Biol. Rev. 73:203-266, 1998). Other relationships that corroborate some morphological analyses are the existence of a clade that groups Gnathostomulida + Syndermata (= Gnathifera), which is ex? panded to include the enigmatic phylum Cycliophora, as sister group to Syndermata. (Ecdysozoa; Metazoa; morphology; phylogeny; Platyzoa; 18S rRNA; Triploblastica.)

/pdf/triploblastic-relationships-with-emphasis-on-the-acoelomates-15x91glt2a.pdf

Triploblastic relationships with emphasis on the acoelomates and the position of Gnathostomulida, Cycliophora, Plathelminthes, and Chaetognatha: a combined approach of 18S rDNA sequences and morphology.

The position of Xenacoelomorpha in the tree of life remains a major unresolved question in the study of deep animal relationships. Xenacoelomorpha, comprising Acoela, Nemertodermatida, and Xenoturbella, are bilaterally symmetrical marine worms that lack several features common to most other bilaterians, for example an anus, nephridia, and a circulatory system. Two conflicting hypotheses are under debate: Xenacoelomorpha is the sister group to all remaining Bilateria (= Nephrozoa, namely protostomes and deuterostomes) or is a clade inside Deuterostomia. Thus, determining the phylogenetic position of this clade is pivotal for understanding the early evolution of bilaterian features, or as a case of drastic secondary loss of complexity. Here we show robust phylogenomic support for Xenacoelomorpha as the sister taxon of Nephrozoa. Our phylogenetic analyses, based on 11 novel xenacoelomorph transcriptomes and using different models of evolution under maximum likelihood and Bayesian inference analyses, strongly corroborate this result. Rigorous testing of 25 experimental data sets designed to exclude data partitions and taxa potentially prone to reconstruction biases indicates that long-branch attraction, saturation, and missing data do not influence these results. The sister group relationship between Nephrozoa and Xenacoelomorpha supported by our phylogenomic analyses implies that the last common ancestor of bilaterians was probably a benthic, ciliated acoelomate worm with a single opening into an epithelial gut, and that excretory organs, coelomic cavities, and nerve cords evolved after xenacoelomorphs separated from the stem lineage of Nephrozoa.

/pdf/xenacoelomorpha-is-the-sister-group-to-nephrozoa-2clumn0vi0.pdf

Xenacoelomorpha is the sister group to Nephrozoa

?Annelids and arthropods have long been considered to be each other's closest relatives, as evidenced by similarities in their segmented body plans. In the first cladistic analysis of metazoan morphology accompanied by an explicit data matrix, Schram (Meglitsch and Schram, 1991, Invertebrate zoology, 3rd edition, Oxford Univ. Press, New York) suggested tentative support for this conventional "Articulata" hypothesis. Our reanalysis of the Schram data matrix yielded weak support for an alternative "Eutrochozoa" grouping of annelids, molluscs, and certain other spiralian phyla, exclusive of arthropods. Likewise, recent 18S ribosomal RNA se? quence comparisons have favored the Eutrochozoa hypothesis. This study presents a new analysis of 141 independently assembled characters, purported to represent the current state of knowledge of metazoan morphology and embryology. This maximum parsimony analysis resulted in robust support of Eutrochozoa. For this data compilation and method of analysis, the Articulata hy? pothesis could only be supported by adding multiple ad hoc proposals of evolutionary events. Instead, the more parsimonious Eutrochozoa hypothesis is favored as the best-supported current reconstruction of higher level animal genealogy. [Phylogeny; Metazoa; animal; Arthropoda; Annelida; Mollusca; morphology; embryology; RNA.] An enormous literature of descriptive and experimental work on the ontogeny and morphology of animals has accumu? lated over the last century and a half. Much is now known about the life history, anat? omy, and genetic organization of species in most of the more than 30 recognized animal phyla. Our modern interpretation of character homology rests on the results of these studies and on the understanding of animal evolution they make possible. The recent emphasis of cladistic methods in systematic biology, however, has fo? cused new attention on the evidential basis for asserting hypotheses of homology among characters (Wiley, 1981; Patterson, 1982). The criteria for homology formal? ized by Remane (1956) involve evaluation of similarities of ontogeny, composition, and anatomical position on a case-by-case basis. These similarities are now regarded by many systematists to be preliminary to the test of character congruence on a dadogram (Hennig, 1966; Patterson, 1982), and only shared derived characters (synapomorphies) are considered candidates for homology. Other similarities that result from independent or parallel evolution (homoplasies) or are retained from more ancient evolutionary transformation (plesiomorphies) are not considered relevant even when they are phenotypically iden? tical. Studies on the early evolution of major animal lineages and on the origins of im? portant features in their structural design have only recently begun to incorporate the character congruence approach of eval? uating phenotypic similarities. One ex? ample is the investigation of genealogical relationships among spiralian metazoans, including Arthropoda, Mollusca, Anneli? da, and several other less speciose phyla. Spiralians comprise >90% of all living metazoan (multicellular animal) species (Barnes, 1987; Brusca and Brusca, 1990), yet the genealogical relationships among many of them remain undocumented by phylo? genetic criteria. Members of the Spiralia have also been grouped as Protostomia by 1 Present address: Institute of Marine Sciences and Biology Department, University of California, Santa Cruz, California 95064, USA.

Annelida and Arthropoda are Not Sister Taxa: A Phylogenetic Analysis of Spiralian Metazoan Morphology

We used a data matrix of 65 morphological characters from 25 ingroup and 6 outgroup taxa, and an alignment comprising complete 18S rDNA sequences from 82 species of parasitic and free-living Platyhelminthes and from 19 species of lower invertebrates to analyse phylogenetic relationships of various platyhelminth taxa. Of the 1358 unambiguously alignable molecular positions, 995 were variable and 757 were phylogenetically informative (parsimony criterion); complete 18S rDNA sequences ranged in length from 1755 to 2873 bp. Main conclusions are: Neodermata are monophyletic, and the Trematoda, Monogenea and Cestoda within them are monophyletic as well. The sister group of the Neodermata is all the other Neoophora; the Kalyptorhynchia, Typhloplanida, Dalyelliida and Temnocephalida form one clade, and the last three another. Monophyly of the Seriata is rejected, but Polycladida/Macrostomida/Haplopharyngida are monophyletic, as are the last two taxa. As a consequence, validity of the taxon Trepaxonemata is rejected. Further studies must show the correct position of the Acoela and Nemertodermatida. It is stressed that morphological and molecular data in some respects lead to contradictory results, for instance concerning the position of the Fecampiidae/Urastoma/Ichthyophagaand the relative position of the Lecithoepitheliata. Denser sampling of taxa for molecular data, complementary sequences from independent genes, and inclusion of additional morphological data are necessary to resolve these contradictions.

The interrelationships of all major groups of Platyhelminthes: phylogenetic evidence from morphology and molecules

Within the last two decades, syntheses of both light-microscopic and ultrastructural characters have shown that there are three well-defined monophyletic groups within the Platyhelminthes: 1) the Catenulidale, 2) the Nemertodermatida-Acoela, and 3) the Haplopharyngida-Macrostomida-Polycladida-Neoophora (+ parasitic platyhelminth classes). However, the relationships among these three groups are problematic. The possible apomorphies that would unite them are either not true homologues (i.e. frontal organ), are mutually conflicting (i.e. 9+1 axoneme in spermatozoa vs. biflagellate spermatozoa, epidermal ciliary rootlet structure, and protonephridia), or are unrooted with any outgroup and hence untestable or uncertain as apomorphies (pro- tonephridia, mode of epidermal replacement, absence of accessory centrioles on cilia). The chief obstacle to deciphering the relationships of these groups is the lack of information on them; presently available information is insufficient to test potential synapomorphies and insufficient also to allow agreement upon a narrowly defined outgroup for the Turbellaria.

Is the Turbellaria polyphyletic

Using characters discernible through electron microscopy, we redefine the organ traditionally identified as the frontal organ in acoelomorph turbellarians as being a collection of two to several large mucus-secreting glands whose necks emerge together through a frontal pore at the exact apical pole of the body, i.e. at the point where the pattern of epidermal ciliary rootlets converges. Representatives that we have studied of each of the acoel families Paratomellidae, Diopisthoporidae, Solenofilomorphidae, Convolutidae, Otocelidae, and Mecynostomidae, as well as a representative of the Nemertodermatida, have such glands. Up to five additional types of glands that open anteriorly outside of the frontal pore, some of which are indistinguishable from glands of the general body wall, could be seen in the nemertodermatid, in Hesiolicium inops (Paratomellidae), and in representatives of the latter four acoel families. In Paratomella, three different types of glands open in diffuse fashion in a frontal glandular complex reminiscent of that in the Macrostomida.

Frontal organs in the Acoelomorpha (Turbellaria): Ultrastructure and phylogenetic significance

Although a variety of larval and adult invertebrates possess an anterior ‘frontal organ’, ‘frontal gland’ or ‘apical organ’, the degree of functional or phylogenetic relatedness of these various organs is largely unknown. The frontal organ in primitive turbellarians is important for phylogenetic analysis both within and without the Turbellaria, a group whose phylogenetic position in the Metazoa is of crucial importance to many current theories of bilaterian origins. In the present paper, the ultrastructure of the frontal organ in Diopisthoporus gymnopharyngeus sp.n. is studied, in connection with a description of this new acoel turbellarian species from North Carolina, U.S.A. The frontal organ of D. gymnopharyngeus comprises two identical gland cells that open at the exact apical pole of the body. There are no specific sensory associations with the opening of the frontal glands and, therefore, the frontal organ in this species is best regarded as purely glandular. This constitutes a major difference between the frontal organ in D. gymnopharyngeus and the glandulo-sensory apical organs of the Muller's and Gotte's larvae of polyclad turbellarians, as well as the purely sensory apical organs of the larvae of other invertebrates investigated to date. Similarities between the body wall glands and the frontal glands in D. gymnopharyngeus, coupled with the generally low level of differentiation of the frontal organ in this species, are in accord with von Graff's hypothesis that the frontal organ in acoels evolved from an anterior accumulation of body wall glands, and support the relatively primitive position of the Diopisthoporidae within the Acoela postulated from earlier studies of the parenchyma.

Fine‐Structure and Evolutionary Implications of the Frontal Organ in Turbellaria Acoela. 1 Diopisthoporus gymnopharyngeus sp.n.

Present models of turbellarian evolution depict the organism with a frontal organ — a complex of glands whose necks emerge at the anterior tip of the body — and therefore imply that this organ is homologous throughout the Turbellaria. However, comparisons of representatives of the Acoela and Macrostomida, two putatively primitive orders of the Turbellaria, show that frontal organs in these two are not similar in ultrastructure or histochemistry. The acoel Convoluta ‘pulchra’ had a prominent cluster of frontal mucous glands whose necks emerged together in a frontal pore at the exact apical pole of the organism, and an array of smaller glands of at least five other types opened at the anterior end, separately from and ventral to this pore. The ‘frontal organs’ (Stirndrusen) of two species of Macrostomum on the other hand, comprised an array of discretely emerging necks of at least two gland types including one with rhabdiform (rhammite) and one with globular mucous secretion granules neither of which emerge at the apical pole. In neither species did the organ appear to be sensory. Our findings indicate a low probability of homology between the frontal glands of the Acoela and Macrostomida.

Ultrastructure of the frontal organ in Convoluta and Macrostomum spp.: significance for models of the turbellarian archetype

Parahaploposthia thiophilus is described as a new species of the acoel turbellarian family Haploposthiidae. Although P. thiophilus exhibits strong anatomical similarities to other members of the genus Parahaploposthia, notably in the male copulatory organ, the male gonads and the frontal organ, this new species differs from all other members of its genus by the possession of a bursa. We have broadened the generic diagnosis of Parahaploposthia to include this feature, a measure that we prefer to the erection of a new genus based on a character (the bursa) of uncertain validity in the present system of acoel taxonomy. This species description is based primarily on information gleaned from the observation of living specimens by phase contrast microscopy, in order to illustrate the utility of an often neglected method that reveals important information not available from fixed and sectioned specimens, and which also may provide for more rapid and simple identification of acoel turbellarians by nonspecialists.

Julian P. S. Smith

Papers

Is the Turbellaria polyphyletic

Frontal organs in the Acoelomorpha (Turbellaria): Ultrastructure and phylogenetic significance

Fine‐Structure and Evolutionary Implications of the Frontal Organ in Turbellaria Acoela. 1 Diopisthoporus gymnopharyngeus sp.n.

Ultrastructure of the frontal organ in Convoluta and Macrostomum spp.: significance for models of the turbellarian archetype

Parahaploposthia thiophilus sp.n. and the Use of Living Specimens in Identification of Acoel Turbellarians