Comparative ultrastructure of the gastrotrich pharynx and the evolution of myoepithelial foreguts in aschelminthes

Summary Structural and ultrastructural data are presented for the myoepithelial pharynges of 20 species of Gastrotricha representing the marine Macrodasyida and marine and freshwater Chaetonotida. A comparative analysis reveals that pharynges with several plesiomorphic characters occur in Chordodasys among the Macrodasyida and Neodasys among the Chaetonotida. The Gastrotricha are systematized based on pharyngeal characters and the system is shown to be concordant with all recent classifications of the group. The plesiomorphic design of the Gastrotrich pharynx is given as: a cylindrical pharyngeal pump composed of monociliated myoglanduloepithelial cells surrounding a circular or oval lumen; radial myofibrils organized into several cross-striated sarcomeres with Z-discs composed of planar aggregations of dense bodies; excitation-contraction coupling is achieved by peripheral couplings of SR with the sarcolemma; apical cell surface with microvilli protruding through the two-layered cuticle; pharynx with at least 3 longitudinal tracts of monociliated sensory cells; nerves as at least 4 basal intraepithelial, longitudinal tracts of neurites. As a model for muscle cell evolution, the investigation postulates a monociliated, cross-striated myoglanduloepithelial cell as the original muscle cell design within the Gastrotricha. Triradiate myoepithelial foreguts occur only in Bryozoa (Ectoprocta), Gastrotricha, Nematoda and Tardigrada. The potential homology of pharyngeal organization of the latter three phyla is discussed. Based on pharyngeal structure, it is concluded that Gastrotricha (Chaetonotida-Paucitubulatina) and Nematoda share several apomorphic characters and share, therefore, a most recent common ancestor. Affinities of Tardigrada with Aschelminthes are considered feasible but currently inconclusive for lack of sufficient comparative ultrastructural data for the Tardigrada.     

Formation of malformed pharynx and neoplasia in the planarian Bdellocephala brunnea following treatment with a carcinogen

The effect of benzo(a)pyrene on regeneration of the pharynx in Bdellocephala brunnea Ijima et Kaburaki was studied by inserting a 5-µg crystal of this carcinogen into the pharyngeal cavity of worms from which the pharynx had been extirpated 2 d previously. Multiple pharynges formed in the regenerates, and neoplasia was induced in a few cases. The malformed pharynges appeared as protrusions overlapping the anterior part of the main pharynx. Electron microscopy of the pharynx in normal, intact worms shows eight layers comprising inner and outer ciliated epithelia, muscle, and glands.     

Structure and evolution of the pharynx simplex in acoel flatworms (Acoela)

The homology of pharynges within the mostly pharynx‐less Acoela has been a matter of discussion for decades and even the basic question of whether a pharynx is a primitive trait within the Acoela and homologous to the pharynx of platyhelminth turbellarians is open. By using fluorescence staining of musculature, as well as conventional histological techniques and transmission electron microscopy, the present study sets focus on the mouth and pharynx (where present) of seven species of Acoela within Paratomellidae, Solenofilomorphidae, Hofsteniidae, Proporidae, and Convolutidae, as well as one species of Nemertodermatida and Catenulida, respectively. It is shown that among the investigated families of acoels there is a great variability in muscle systems associated with the mouth and pharynx and that pharynx histology and ultrastructural characters are widely diverse. There are no close similarities between the acoel pharynges and the catenulid pharynx but there is a general resemblance of the musculature associated with the mouth in the representatives of Paratomellidae and Nemertodermatida. On the basis of the profound differences in pharynx morphology, three major conclusions are drawn: 1) the pharynges as present in Recent acoels are not homologous to the pharynx simplex characteristic for Catenulida and Macrostomida within the Platyhelminthes; 2) the different muscular pharynx types of acoels are not homologous between higher taxa and thus a single acoel‐type pharynx simplex cannot be defined; 3) the presence of a muscular pharynx most likely does not represent the ancestral state. J. Morphol, 2009. © 2008 Wiley‐Liss, Inc.     

Anatomy of the oral valve in nymphalid butterflies and a functional model for fluid uptake in Lepidoptera

The food canal of the proboscis of Lepidoptera serves for the uptake of nutrient fluids and the discharge of saliva. A valve was discovered at the entrance to the sucking pump in the head that separates these countercurrent flows in nymphalid butterflies. Three species of Nymphalidae were examined by dissections and light microscopic serial semithin sections. The sucking pump is a unit composed of three structures: (1) the oral valve, which is a projection of the epipharynx extending into the anterior cibarial lumen, (2) the expandable lumen, and (3) the posterior sphincter valve which controls influx into the oesophagus. Based on the microanatomical results, a functional model is presented to account for the uptake and swallowing of fluids and for the control of the salivary flow into the food canal of the proboscis. Dilator muscles of the sucking pump expand the lumen by pulling on the muscular dorso-anterior side. This opens the oral valve and fluid can be drawn into the lumen from the food canal of the proboscis. Circular compressor muscles which attach to both sides of the sclerotized ventro-posterior wall of the sucking pump reduce the size of the lumen; passively they close the oral valve and press fluid through the relaxed posterior sphincter opening into the oesophagus. According to this model saliva can be discharged into the food canal during the swallowing phase. The oral valve and pumping unit are similar in all studied species despite the fact that saliva presumably plays a special role in the derived pollen-feeding behaviour of one of them, viz. Heliconius melpomene.     

On the nature of pharyngeal muscle cells in the Tardigrada

As part of a transmission electron microscopic study of the embryological development in tardigrades, the ultrastructure of the pharynx was examined. The intent was to establish whether the pharyngeal muscle cells constitute an eclodermal myoepithelium (as in many aschelminth pharynges) or whether they are mesodermal (as in certain other aschelminth groups and among articulates). In the latter case the cuticle would be produced solely by specialized epithelial cells. The eutardigrade species Halobiotus crispae Kristensen, 1982, was investigated in four embryological stages, as a newly hatched juvenile, in the active adult stage, and in the hibernation stage pseudosimplex I. A comparison was made with the arthrolardigrade Actinarctus doryphorus Schulz, 1935, in the active adult stage and in the simplex stage. The results indicate that the tardigrade pharynx is an ectodermal myoepithelium. The muscles appear to be truly cross-striated and monosarcomerial. The phylogenetic implications of these findings are discussed briefly.     

The food pump of Aphelocheirus aestivalis F. as compared with that of typical Naucoridae (Heteroptera)

The heteropteran cibarium forms a sucking pump which conveys fluid foods into the pharynx. The food pumps of Hydrocorisae have the additional function of grinding or filtering particulate matter; they contain ridges, hairs, and sclerotized processes which have probably evolved at least twice among the hydrocorisine families. Aphelocheirus, like the Naucoridae, possesses a modified anteclypeus and a tripartite type of food pump. The main sucking action occurs in the pump's anterior and posterior regions, while the middle region is specialized for grinding and filtering. The anteclypeus has broadened and fused with other parts of the cranium, and is thus braced against the pull of the powerful cibarial dilator muscles. In the Naucoridae the three regions of the pump have the same functions as those of Aphelocheirus. The pumps of the five naucorid genera here studied are structurally very similar to each other but differ considerably from that of Aphelocheirus. Cibarial morphology, as well as respiratory differences, thus supports the contention that Aphelocheirus is not a member of the Naucoridae but should be placed in a separate family.     

Group I Introns from Zygomycota: Evolutionary Implications for the Fungal IC1 Intron Subgroup

The origins of fungal group I introns within nuclear small-subunit (nSSU) rDNA are enigmatic. This is partly because they have never been reported in basal fungal phyla (Zygomycota and Chytridiomycota), which are hypothesized to be ancestral to derived phyla (Ascomycota and Basidiomycota). Here we report group I introns from the nSSU rDNA of two zygomycete fungi, Zoophagus insidians (Zoopagales) and Coemansia mojavensis (Kickxellales). Secondary structure analyses predicted that both introns belong to the IC1 subgroup and that they are distantly related to each other, which is also suggested by different insertion sites. Molecular phylogenetic analyses indicated that the IC1 intron of Z. insidians is closely related to the IC1 intron inserted in the LSU rDNA of the basidiomycete fungus Clavicorona taxophila, which strongly suggests interphylum horizontal transfer. The IC1 intron of C. mojavensis has a low phylogenetic affinity to other fungal IC1 introns inserted into site 943 of nSSU rDNA (relative to E. coli 16S rDNA). It is noteworthy that this intron contains a putative ORF containing a His–Cys box motif in the antisense strand, a hallmark for nuclear-encoded homing endonucleases. Overall, molecular phylogenetic analyses do not support the placement of these two introns in basal fungal IC1 intron lineages. This result leads to the suggestion that fungal IC1 introns might have invaded or been transferred laterally after the divergence of the four major fungal phyla. Received: 8 February 2001 / Accepted: 1 November 2001     

Pharynx and intestine

The alimentary canal of polychaetes consists of a foregut, midgut, and hindgut. The alimentary canal shows different specializations even in homonomously segmented polychaetes. The foregut gives rise to the buccal cavity, pharnyx and oesophagus, the midgut may be divided into a stomach and the intestine proper. Since polychaetes use a wide spectrum of food sources, structures involved in feeding vary as well and show numerous specializations. In the foregut these specializations may be classified as one of the following types: dorsolateral folds, ventral pharynx, axial muscular pharynx, axial non-muscular proboscis and dorsal pharynx. The latter, typical of oligochaetous Clitellata, occurs rarely in polychaetes. The structure, evolution and phylogenetic importance of these different types are described and discussed. Axial muscular and ventral pharynges may be armed with jaws, sclerotized parts of the pharyngeal cuticle. Terminology, structure, occurrence and development of the jaws are briefly reviewed. Special attention has been paid to the jaws of Eunicida including extinct and extant forms. Conflicting theories about the evolution of the jaws in Eunicida are discussed. The epithelia of the intestine may form a pseudostratified epithelium composed of glandular cells, absorptive cells and ciliated cells or only one cell type having similar functions. A conspicuous feature in the intestine of certain polychaetes is the occurrence of unicellular tubular structures, called enteronephridia. So far these enteronephridia are only known in a few meiofauna species.     

Studies on the tardigrades. II. Fine structure of the pharynx of milnesium tardigradum doyère

The fine structure of the pharynx and associated valve of the tardigrade Milnesium tardigradum is presented. The pharynx consists of a triradiate, cuticle lined lumen surrounded by radially arranged muscle cells and special apical cells which cap each of the ventricles of the lumen. The valve is an unusual structure marking the anterior limit of the pharynx. It is a specialization of the cuticular lining of the foregut and the apical cells of the pharynx. The significance and phyletic affinities of these structures are discussed.     

Bacterial phylotypes associated with the digestive tract of the sea urchin Paracentrotus lividus and the ascidian <Emphasis Type="Italic">Microcosmus</Emphasis> sp.

We used sequencing and phylogenetic analysis of PCR-amplified 16S rRNA genes from bacteria that are associated with the esophagus/pharynx, stomach and intestine of two marine sympatric invertebrates but with different feeding mechanisms, namely the sea urchin Paracentrotus lividus (grazer) and the ascidian Microcomus sp. (suspension feeder). Amplifiable DNA was retrieved from all sections except the pharynx of the ascidian. Based on the inferred phylogeny of the retrieved sequences, the sea urchin’s esophagus is mainly characterized mostly by bacteria belonging to α-, γ-Proteobacteria and Bacteriodetes, most probably originating from the surrounding environment. The stomach revealed phylotypes that belonged to γ- and δ-Proteobacteria, Verrucomicrobia and Fusobacteria. Since the majority of their closest relatives are anaerobic species and they could be putative symbionts of the P. lividus stomach, in which anaerobic conditions also prevail. Seven out of eight phylotypes found in the sea urchin’s intestine belonged to sulfate reducing δ-Proteobacteria, and one to γ-Proteobacteria, with possible nutritional activities, i.e. degradation of complex organic compounds which is beneficial for the animal. The bacterial phylotypes of the ascidian digestive tract belonged only to the phyla of Actinobacteria and Proteobacteria. The stomach phylotypes of the ascidian were related to pathogenic bacteria possibly originating from the water column, while the intestine seemed to harbour putative symbiotic bacteria that are involved in the degradation of nitrogenous and other organic compounds, thus assisting ascidian nutrition. The text was submitted by the authors in English.     

Anatomy and ultrastructure of the ventral pharyngeal organs of Saccocirrus(Saccocirridae) and Protodriloides(Protodriloidae fam. n.) with remarks on the phylogenetic relationships within the Protodrilida (Annelida: Polychaeta)

General anatomy and ultrastructure of the ventral pharyngeal organs were investigated in Succocirrus krusudensis, Protodriloides chaetifer and P. symbioticus. Succocirrus papillocercus, a species without a ventral pharyngeal organ, was included for comparison. The two genera show homologous similarities in their pharynges: bulbus muscle composed of transverse muscle fibres and interstitial cells, those cells with small cell bodies and processes containing prominent tonofilaments which are orientated dorsoventrally and connect the bulbus epithelium with the investing muscle; bulbus muscle fibres circomyarian with nuclei and mitochondria located basally, investing muscle also with interstitial cells, which lack tonofilaments; oesophagus surrounded by gland cells opening into the pharynx. Most likely, a ventral pharynx with these characters was already present in the stem species of Saccocirridae, Protodriloidae fam. n. and Protodrilidae and evolutionary processes led to several changes: a tongue-like organ with a prominent tip and supporting elements is a synapomorphic character of Protodrilus and Succocirrus; the pharyngeal organ is reduced to stomodeal pouches and salivary glands in S. papillocercus and completely lost in Asromus raenioides. These results and data from previous studies are summarized in a tentative phylogenetic dendrogram and allow the introduction of a new family, Protodriloidae.     

EMG activity in hyoid muscles during pig suckling

Infant suckling is a complex behavior that includes cycles of rhythmic sucking as well as intermittent swallows. This behavior has three cycle types: 1) suck cycles, when milk is obtained from the teat and moved posteriorly into the valleculae in the oropharynx; 2) suck-swallow cycles, which include both a rhythmic suck and a pharyngeal swallow, where milk is moved out of the valleculae, past the larynx, and into the esophagus; and 3) postswallow suck cycles, immediately following the suck-swallow cycles. Because muscles controlling these behaviors are active in all three types of cycles, we tested the hypothesis that different patterns of electromyographic (EMG) activity in the mylohyoid, hyoglossus, stylohyoid, and thyrohyoid muscles of the pig characterized each cycle type. Anterior mylohyoid EMG activity occurred regularly in every cycle and was used as a cycle marker. Thyrohyoid activity, indicating the pharyngeal swallow, was immediately preceded by increased stylohyoid and hyoglossus activity; it divided the suck-swallow cycle into two phases. Timed from the onset of the suck-swallow cycle, the first phase had a relatively fixed duration while the duration of the second phase, timed from the thyrohyoid, varied directly with cycle duration. In short-duration cycles, the second phase could have a zero duration so that thyrohyoid activity extended into the postswallow cycle. In such cycles, all swallowing activity that occurred after the thyrohyoid EMG and was associated with bolus passage through the pharynx fell into the postswallow cycle. These data suggest that while the activity of some muscles, innervated by trigeminal and cervical plexus nerves, may be time locked to the cycle onset in swallowing, the cycle period itself is not. The postswallow cycle consequently contains variable amounts of pharyngeal swallowing EMG activity. The results exemplify the complexity of the relationship between rhythmic sucking and the swallow.     

Differential changes in human pharyngoesophageal motor excitability induced by swallowing,pharyngeal stimulation,and anesthesia

We investigated the effects of water swallowing, pharyngeal stimulation, and oropharyngeal anesthesia on corticobulbar and craniobulbar projections to human swallowing musculature. Changes in pathway excitability were measured via electromyography from swallowed intraluminal pharyngeal and esophageal electrodes to motor cerebral and trigeminal nerve magnetic stimulation. After both water swallowing and pharyngeal stimulation, pharyngoesophageal corticobulbar excitability increased (swallowing: pharynx = 59 +/- 12%, P < 0.001; esophagus = 45 +/- 20%, P < 0.05; pharyngeal stimulation: pharynx = 76 +/- 19%, P < 0.001; esophagus = 45 +/- 23%, P = 0.05), being early with swallowing but late with stimulation. By comparison, craniobulbar excitability increased early after swallowing but remained unaffected by pharyngeal stimulation. After anesthesia, both corticobulbar (pharynx =-24 +/- 10%, P < 0.05; esophagus = -28 +/- 7%, P < 0.01) and craniobulbar excitability showed a late decrease. Thus swallowing induces transient early facilitation of corticobulbar and craniobulbar projections, whereas electrical stimulation promotes delayed facilitation mainly in cortex. With removal of input, both corticobulbar and craniobulbar projections show delayed inhibition, implying a reduction in motoneuron and/or cortical activity.     

Key novelties in the evolution of the aquatic colonial phylum Bryozoa: evidence from soft body morphology

Molecular techniques are currently the leading tools for reconstructing phylogenetic relationships, but our understanding of ancestral, plesiomorphic and apomorphic characters requires the study of the morphology of extant forms for testing these phylogenies and for reconstructing character evolution. This review highlights the potential of soft body morphology for inferring the evolution and phylogeny of the lophotrochozoan phylum Bryozoa. This colonial taxon comprises aquatic coelomate filter‐feeders that dominate many benthic communities, both marine and freshwater. Despite having a similar bauplan, bryozoans are morphologically highly diverse and are represented by three major taxa: Phylactolaemata, Stenolaemata and Gymnolaemata. Recent molecular studies resulted in a comprehensive phylogenetic tree with the Phylactolaemata sister to the remaining two taxa, and Stenolaemata (Cyclostomata) sister to Gymnolaemata. We plotted data of soft tissue morphology onto this phylogeny in order to gain further insights into the origin of morphological novelties and character evolution in the phylum. All three larger clades have morphological apomorphies assignable to the latest molecular phylogeny. Stenolaemata (Cyclostomata) and Gymnolaemata were united as monophyletic Myolaemata because of the apomorphic myoepithelial and triradiate pharynx. One of the main evolutionary changes in bryozoans is a change from a body wall with two well‐developed muscular layers and numerous retractor muscles in Phylactolaemata to a body wall with few specialized muscles and few retractors in the remaining bryozoans. Such a shift probably pre‐dated a body wall calcification that evolved independently at least twice in Bryozoa and resulted in the evolution of various hydrostatic mechanisms for polypide protrusion. In Cyclostomata, body wall calcification was accompanied by a unique detachment of the peritoneum from the epidermis to form the hydrostatic membraneous sac. The digestive tract of the Myolaemata differs from the phylactolaemate condition by a distinct ciliated pylorus not present in phylactolaemates. All bryozoans have a mesodermal funiculus, which is duplicated in Gymnolaemata. A colonial system of integration (CSI) of additional, sometimes branching, funicular cords connecting neighbouring zooids via pores with pore‐cell complexes evolved at least twice in Gymnolaemata. The nervous system in all bryozoans is subepithelial and concentrated at the lophophoral base and the tentacles. Tentacular nerves emerge intertentacularly in Phylactolaemata whereas they partially emanate directly from the cerebral ganglion or the circum‐oral nerve ring in myolaemates. Overall, morphological evidence shows that ancestral forms were small, colonial coelomates with a muscular body wall and a U‐shaped gut with ciliary tentacle crown, and were capable of asexual budding. Coloniality resulted in many novelties including the origin of zooidal polymorphism, an apomorphic landmark trait of the Myolaemata.     

Adenylate cyclase in sea anemone implication for chemoreception

Adenylate cyclase of the sea anemoneAnthopleura elegantissima was found to be associated with the heavy particulate fraction of the cell and to be activated by NaF and 2-mercaptoethanol. Reduced glutathione, which elicits the ciliary swallowing response during feeding, also activated adenylate cyclase in particles from the oral disc and pharynx. The GSH effect was dependent on homogenization procedure, whereas the NaF and 2-mercaptoethanol activation was not. The activation of adenylate cyclase from the oral disc and pharynx by GSH was correlated with increased Ca²⁺ binding to the particulate fraction. When activation by GSH was abolished by mechanical homogenization, no increasea in Ca²⁺ binding was observed in the presence of GSH. It is suggested that chemoreception for the swallowing response of this organism is mediated by cyclic AMP control of Ca²⁺ distribution in the cell.     

Comparative ultrastructure of the pharynx simplex in turbellaria

Summary The simple pharynges in thirteen species of Turbellaria in the orders Macrostomida, Haplopharyngida, Catenulida, and Acoela have been studied by electron microscopy. After consideration of the functional aspects of the pharynx simplex, the relationship of the pharynx simplex ultrastructure to the phylogeny of the above mentioned groups is analyzed.The Haplopharyngida and Macrostomida are united as a group by the following characters: a pharynx transition zone of 1–5 circles of insunk cells with modified ciliary rootlets or no cilia, pharynx sensory cells without stereocilia collars and with a variable number of cilia, a prominent nerve ring with more than 30 axons circling the pharynx at the level of the beginning of the pharynx proper distal to the gland ring, 2 or more gland cell types in the pharynx, with at least two layers of muscle present and the longitudinal muscles derived from regular and special body wall circular muscles and a prominent post-oral nerve commissure. This specific arrangement can be distinguished from the other pharynx simplex types and is called the pharynx simplex coronatus.The catenulid pharynx simplex is characterized by the lack of a prominent nerve ring, no prominent post-oral commissure, a transition zone with epidermal type ciliary rootlets, recessed monociliated sensory cells, and one or no type of pharynx gland cell. The Acoela are specialized because of the epidermal type rootlets in the pharynx proper. They also lack a transition zone and a prominent nerve ring and have monociliated sensory cells different from the catenulid type.Ultrastructural characters of the pharynx simplex support the view that the Haplopharyngida-Macrostomida are monophyletic. The more primitive catenulid pharynx probably arose from a common ancestral pool with the Haplopharyngida and Macrostomida, although it does not appear possible presently to establish a clear monophyletic line for these forms. The various pharynx types within the Acoela appear to indicate independent origins with no clear link to the basic pharynx simplex type in the three other orders.Abbreviations Used in Figures a nerve axon - ar accessory rootlet - bb basal body - bn brain-nerve ring commissure - c caudal rootlet - ce centriole - ci cilium - cm circular muscle - cp ciliary pit - cu cuticle - cw cell web - d dictyosome - dp proximal pharynx proper cell - e epidermis - er rough endoplasmic reticulum - f fibrous rod - g gastrodermis - gc gastrodermal gland cell - he heterochromatin - i intercellular matrix - lc lateral nerve cord - lm longitudinal muscle - m mitochondria - mo mouth - mt microtubules - mv microvilli - n nucleus - nr nerve ring - ns neurosecretory granules - p pharynx proper - ph pharynx - po post-oral commissure - r rostral rootlet - rm radial muscle - s sphincter - sc sensory cell - sj septate junction - sr sensory rootlet - t transition zone - u ultrarhabdite - v vertical rootlet - va food vacuole - za zonula adhaerens - 1 type I gland cell - 2 type II gland cell - 3 type III gland cell - 4 type IV gland cell - 5 type V gland cell - 6 type VI gland cell - 7 type VII gland cell     

Morphology of the sweet potato whitefly,Bemisia tabaci (Homoptera,Aleyrodidae) relative to virus transmission

 The stylet bundle of the sweet potato whitefly, Bemesia tabaci, consists of paired mandibles and maxillae. The latter interlock to form the food and salivary canals. Its salivary system consists of paired primary and accessary glands in the thorax. Primary and accessory gland ducts on each side of the nerve cord fuse to form lateral ducts that course anteroventrally to the midline and continue in parallel down the hypopharynx to eventually fuse to form the single afferent duct of the salivary pump. Saliva exiting the pump via the efferent duct enters the salivary canal of the maxillae. Food from the maxillary food canal passes from the antecibarium to the postcibarium or sucking pump and, per os, to the pharynx and esophagus of the foregut. The esophagus extends from the head to the base of the abdomen where it and the anterior midgut intimately mingle with the anterior hindgut to form a filter chamber. The midgut then proceeds dorsocaudally before looping anteroventrally to join the hindgut. The latter gives off two fingerlike Malpighian tubules before entering the filter chamber, whence it proceeds dorsocaudally to the anus within the vasiform orifice. Where possible, the morphology of Bemisia is discussed in relation to plant virus transmission and the morphologies of more thoroughly studied homopteran vectors such as aphids and leafhoppers. Accepted: 9 July 1996     

Use of a deviant mitochondrial genetic code in yellow-green algae as a landmark for segregating members within the phylum

Several algae that were previously classified in the phylum Xanthophyta (yellow-green algae) were assigned in 1971 to a new phylum, Eustigmatophyta. It was anticipated that the number of algae reclassified to Eustigmatophyta would increase. However, due to the fact that the morphological characteristics that segregate eustigmatophytes from other closely related algae can be only obtained through laborious electron microscopic techniques, the number of members in this phylum have increased rather slowly. We attempted, therefore, to segregate two closely related groups of algae, eustigmatophytes and yellow-green algae, on the basis of a molecular phylogenetic tree as a means of providing an alternative method of distinguishing these phyla. We analyzed the mitochondrial cytochrome oxidase subunit I (COXI) gene sequences of eight algae classified as xanthophyceans and found that six manifested the expected deviant genetic code where AUA codes for methionine (AUA/Met), but not for isoleucine (AUA/Ile) as in the universal genetic code. The other two, Monodus sp. (CCMP 505) and Ophiocytium majus (CCAP 855/1), which were presumed to be yellow-green algae, and all the examined eustigmatophytes utilized AUA for Ile. In addition, the phylogenetic tree of COXI gene sequences showed that the six yellow-green algae bearing the AUA/Met deviant code composed a tight clade with a bootstrap value of 100%. The phylogenetic tree of the corresponding sequences from Monodus sp. and Ophiocytium majus and the eustigmatophytes also composed a tight cluster, but with a bootstrap value of 92%. These results strongly suggest that two previously classified members of yellow-green algae belong to the phylum Eustigmatophyta. Therefore, examination of the mitochondrial genetic code in algae appears to be a potentially very useful genetic marker for classifying these organisms, especially when it is considered with the results obtained through a molecular phylogenetic tree. Received: 14 December 1996 / Accepted: 3 April 1997     

Molecular clocks, molecular phylogenies and the origin of phyla

Erwin, Douglas H. 1989 07 15: Molecular clocks, molecular phylogenies and the origin of phyla. Lethaia , Vol. 22, pp. 251–257. Oslo. ISSN 0024–1164.
Protein, RNA and DNA sequences have been widely used to construct phylogenies and to calculate divergence times using a molecular clock. Reliance on molecular information is particularly attractive when fossil evidence is missing or equivocal, as in the Cambrian metazoan radiation. I consider the applicability of molecular clocks and phylogenetic analysis of molecular data to the origin of metazoan phyla, and conclude that molecular information is often ambiguous or misleading. Amino acid sequences are of limited use because the redundancy of the genetic code masks patterns of descent, while in a nucleotide sequence only four potential states exist at each site (the four nucleotide bases). In each case, homoplasy may often go undetected. The application of a molecular clock to resolve the timing of the metazoan radiation is unwarranted, while molecular phylogenetic reconstruction should be approached with care. A potentially more useful technique for phylogenetic reconstruction would be the use of patterns of genome structure and organization as characters. * Molecular clock, phylogenetics, metazoan radiation, origin of phyla .     

Nkx2.5 and Nkx2.6, homologs of Drosophila tinman, are required for development of the pharynx

Nkx2.5 and Nkx2.6 are murine homologs of Drosophila tinman. Their genes are expressed in the ventral region of the pharynx at early stages of embryogenesis. However, no abnormalities in the pharynges of embryos with mutations in either Nkx2.5 or Nkx2.6 have been reported. To examine the function of Nkx2.5 and Nkx2.6 in the formation of the pharynx, we generated and analyzed Nkx2.5 and Nkx2.6 double-mutant mice. Interestingly, in the double-mutant embryos, the pharynx did not form properly. Pharyngeal endodermal cells were largely missing, and the mutant pharynx was markedly dilated. Moreover, we observed enhanced apoptosis and reduced proliferation in pharyngeal endodermal cells of the double-mutant embryos. These results demonstrated a critical role of the NK-2 homeobox genes in the differentiation, proliferation, and survival of pharyngeal endodermal cells. Furthermore, the development of the atrium was less advanced in the double-mutant embryos, indicating that these two genes are essential for both pharyngeal and cardiac development.