Chaetognatha

The chaetognathes ( Chaetognatha ) are a phylum of predatory sailors. Their name comes from the mobile hooks which allow the capture of their preys (of the Greek khaitê , hair and gnathos jaw) but they are sometimes called towards Sagittarius because of their shape of arrow. The positioning of the chaetognathes within the tree of the animals was the object of a long controversy but of the serious arguments henceforth allow to classify them within the protostomiens, one of the two great lines of animals bilatériens with the deutérostomiens. The chaetognathes are a very old animal group as several recent fossils attest it dating from the lower Cambrien (even Paléontologie). They play an important role in the planktonique ecosystem like principal predatory direct of the the Copepoda and represent up to 10% of the Biomasse zooplancton.

The chaetognathes appear for the first time in drawings of the Dutch naturalist Martinus Slabber in 1775. Currently, the Phylum is composed of about thirty kinds and approximately 150 Espèce S whose size can vary from 2 mm to approximately 12 cm. One finds them in all the marine habitats, of the coastal areas to the oceanic benthos.

Morphology

The chaetognathes are segmented métazoaires Bilatériens Triploblastique S not metamerized nor. The general cavities of the trunk and the tail are entirely papered of mésoderme epithelial, and can thus be regarded as Cœlome S. Although the chaetognathe is basically dimeric, the adult body includes/understands three areas, separated by two Septum S: the cephalic septum separates the head and the trunk, the caudal septum separates the trunk and the tail. The chaetognathes are deprived of apparatuses respiratory and circulatory. However, a system known as hémal, localized in the extracellular matrices, consists of a perish-intestinal sine, a dorsal sine, and two side sines. No néphridie was observed. Their skin is monostratifié around the head and is pluristratifié on the remainder of the body. The chaetognathes have one or two pairs of side fins and a broad caudal fin, each one being supported by rays of unknown composition.

Digestive head and structures

The head of the chaetognathe has particular adaptations which one can think that they are related to the predation. On the level of the head, the oral slit is surrounded by hooks and teeth, used for the capture of preys, mainly of the crustacean planktonique such as the the Copepoda, or even of others chaetognathes. The former and latéro-ventral areas of the head are covered with a cuticularized Ectoderme monostratifié. This Cuticule constitutes a continuous structure with the teeth and the hooks, which contain a strong crystalline alpha-chitin rate. Two ectodermaux bodies characteristic of the phylum and unknown function are localized in the dorsal area of the head and the neck:

the body rétrocérébral, made up of two glandular areas with fine channels opening on a common former pore;
the ciliée crown, represented by two concentric bands of monociliées cells with flange.

The mouth opens on a Pharynx, then a Intestin (presenting at certain species of the former diverticula) followed by a short vertical rectum emerging in a ventral Anus located in front of the caudal septum. In its former half, the intestine contains in majority of the glandular cells, whereas the posterior part is made up of absorbing cells. The intestine is suspended in the general cavity via a dorsal Mésentère.

Muscular system

The musculature, very developed, is made of four longitudinal quadrants of Muscle S primary educations associated with four narrow bands with secondary muscles, two in dorsal position and two in side position. The primary education muscles are striated and include/understand in general two types of muscle fibers (has and B). Same manner, the secondary muscles are striated, but their ultrastructure is single, with two types of Sarcomère S. At the species benthoplanctonic or benthic, such as Spadella will cephaloptera , the primary education muscles contain a majority of muscle fibers of type has and one finds muscles oblique or transverse (phragmes).

Nervous system

The Nervous system is made up mainly of the dorsal ganglion cerebroid and a ventral ganglion located in the trunk. The ganglion cerebroid, bilobate, is placed at the top of the mouth and innerve primarily the sensors of the head (eyes, ciliée crown). A ring periœsophagial (or circumenteric) share of the dorsal ganglion and carries a pair of small vestibular ganglia which innervent the Pharynx and the muscles of the head. With semi-length of the trunk, or in the first third, is the ventral ganglion. It is connected to the cephalic nervous system by a pair the connective ones (the connective circumentériques ones). It has a dozen pairs of side nerves, and a pair of posterior nerves, which extends beyond the anus and is completed in contact with a plexus subepidermic. The eyes, having each one several ciliées sensory cells and a pigmented cell, constitute a very particular structure within the métazoaires. Several areas of the skin punctually present groups of ciliées cells sensitive to the vibrations, probably taking part in the detection of the preys.

The nervous system of the chaetognathes recently was the subject of a redescription based on the imagery confocale. This work made it possible to show that the ganglion ventral could not be compared with the ventral cords highly-strung person of the arthropods or annélides because it corresponds to the condensation of a nervous plexus intra-épidermal (i.e which does not cross the basal Lame). Indeed, no polarization is observed in the innervation of this ganglion which has nevertheless a structure interns very precise, suggesting thus that the chaetognathes could have set up a nervous system highly parallel to organized the others phyla.

Paleontology

The chaetognathes constitute one of the oldest phyla, of the fossil chaetognathes were found in famous layers of the cambrien such as the schist of Burgess or the Gisement of Chengjiang. Many cases were successively proposed like that of the oldest chaetognathe and of many specimens were recently described:

Amiskwia discovered by Walcott in famous the Faune of Burgess was generally regarded as a chaetognathe but this assumption was rejected thereafter.
Paucijaculum describes by Schram in 1973 is later (carboniferous) and constituted for one long period the oldest fossil.
Eognathacantha ercainella was described in 2002 by Chen and Wuang like a chaetognathe of the lower cambrien (520 My) in spite of a rather low quality of safeguarding.
the protoconodontes, microfossiles of the cambrien were proposed as of the hooks fossilized by Szaniawski on the basis of rather detailed comparative analysis.
Oesia disjuncta constitutes a return at the origin of the debate because this fossil of the schist of Burgess originally described by Walcott is interpreter in 2005 by Szaniawski like a chaetognathe.
Protosagitta spinosa quoted at the beginning by Chen and Wuang was the subject of a recent detailed analysis based on the analysis of certain fine structures preserved particularly well in new specimens. This fossil of the Gisement of Chengjiang (cambrien lower) thus constitutes the oldest fossil and best described to date.

Thus, not only the phylum of the chaetognathes is contemporary Radiation cambrienne but it had as of this time a morphology almost identical to that which it presents today. This case of morphological conservation is very surprising and relatively rare but also suggests that the lifestyle of the chaetognathe remained unchanged since the cambrien.

Reproduction and development

The chaetognathes are hermaphrodites and them reproduction is done by cross Fécondation. The development is direct, without Mue nor larval stage . The egg, transparency, undergoes a total and equal radiate segmentation. However, of the experiments of marking of the first blastomères showed that the tetrahedral provision of the blastomères at the stage four cells corresponded to the future body axes, a characteristic also present at the spiraliens.

A Blastula with a narrow Blastocœle develops. At the stage 64 cells, the germinal determinant of the lines, a round and small body formed after fecundation close to the vegetable pole, divides for the first time and is distributed in two blastomères, future the germinal cells paramount (CGP). At the following stage, a typical invagination forms the gastrula. Two antéro-side invaginations of endoderm form then two folds which migrate towards the blastopore directly inside the archentéron. These folds push with their point the CGP which divide again. It is a particular form of entérocœlie in which the Mésoderme is formed by folds which migrate inside the Archentéron, rather than by the formation of pockets which invade the blastocœle.

In the former area of the embryo are thus formed the Intestin, the bilateral mésodermaux bags (future Cœlome) and a ventral invagination stomodeale, whereas the Blastopore, located on the other hand, is closed. The differentiation of the areas ectodermales occupied by the adult nervous system is very early, the ventral ganglion appearing in the form of two bilateral cellular masses. The primary cœlomes of the head and the trunk separate then the embryo lengthen and are curved gradually inside egg. All the cavities are reduced then considerably. With the course this lengthening A/P, the whole of the nervous system continues its differentiation and one can then observe on the level of the ventral ganglion the installation of the neuropile (Axone S) and the future cerebroid ganglion in the head. Lastly, preceding the blossoming, the differentiation of the longitudinal muscles begins in the mésoderme from the trunk.

After the blossoming, the mésoderme of the newborn appears as an undifferentiated cellular mass in which it is impossible to locate the future tissue territories of the adult. The formation of the epithelium S lining the general cavity of the trunk proceeds during the first two days of development of the newborn. The progressive appearance of these fabrics leads to the final segregation of the germinal lines (formation of the caudal septum starting from specialized cells péritonéales separating the ovary S located before from the Testicule S located subsequently) and to the installation of the adult plan of organization: division of the body in three areas, reopening of the general cavities, opening of the digestive tract and differentiation of the cephalic structures. In a way surprising, after blossoming, in fact the posterior areas are different the first, and the head is the part of the body most tardily formed. To our knowledge, the chaetognathe is the only animal presenting this characteristic.

Systematic of the phylum

The key character of systematic of the phylum is the transverse presence or not of musculature (phragmes). This character makes it possible to subdivide the six S traditional: Sagitta , Pterosagitta , Spadella , Eukrohnia , Heterokrohnia and Krohnitta in two S: Phragmophora (presence of phragmes) and Aphragmophora (absence of phragme).

This subdivision was confirmed by analyzes of molecular Phylogénie carried out starting from ARN the gene of large under unit of ribosomal ARN 28S and thereafter of small under unit the ARNr 18S. This Phylogénie is in agreement with the principal morphological characters such as the number and the structure of the Nageoires, the type of muscle fibers or the head ratio/body. However, the molecular Phylogénie clarified the limit of the traditional characters: the planktonique species Pterosagitta draco deprived of transverse musculature (phragmes), classified before in the order of will aphragmophora, was included in the order of will phragmophora. This loss of the phragme at Pterosagitta draco can be explained the change of Milieu of life: this family member Benthique of the spadellidae would have adapted to the planktonique life .

Phylogenetic position of the phylum

A discussed morphology

Because of a division of natures traditionally allotted to the deutérostomiens and the protostomiens (table) the phylogenetic position of the chaetognathes was very largely discussed (Ghirardelli 1968,1994). In spite of a morphology pointing out that of the protostomiens, the traditional phylogenetic position of the chaetognathes was a long time that advanced by Hyman (1959) which regarded them as parents far away from the deutérostomiens because of their characters embryologic (see also Willmer, 1990). However, equitably considering the characters embryologic and morphological, Beklemishev (1969) concludes that the chaetognathes remained among the phyla most isolated from the animal world and placed them, with the brachiopods, out of the deutérostomiens and the protostomiens. A position found in the system of alive suggested by Rider-Smith (1998), where the chaetognathe is the single member of Chaetognathi, one of four major divisions of the protostomiens (with Lophozoa, Platyzoa and Ecdysozoa). Meglitsch and Schram (1991), followed by Eernisse and Al (1992) introduced, following their cladistic study of morphological matrices of characters, the chaetognathes in the aschelminthes. Nielsen (2001) considered the chaetognathes as the group of brother of the gnathostomulides and the rotifères, within Gnathifera, on the basis of of the chitinous hooks surrounding the mouth and innervation of the muscles of the vestibular ganglion. The obligatory hermaphrodism with the presence of the female gonades in former position compared to the male gonades was also a character suggested to bring closer the chaetognathes and the gnathostomulides (Zrzavy, 1993).

First analyzes of molecular phylogeny

It is into 1993 that, for the first time, the molecular tool was used to define the phylogenetic position of the chaetognathes. Starting from the analysis of the 18S of the species Sagitta elegans , Telford and Holland suggested that the chaetognathes would have diverged precociously within the bilatériens, at the same time as the platelminthes. This result was initially preserved during the year following with the inclusion at the same time of new species of chaetognathe and new phyla such as the nematodes, the gnatostomulides or the nématomorphes. However, the bringing together of the chaetognathes with the platyleminthe or the nematodes describes by the whole of this work could be explained by a phenomenon of long branches caused by the very fast rates of evolution observed for the ARN 18S in these various groups. Similar problems were encountered with the ARN 28S (LSU) what confirms the difficulty of the molecular analysis for the phylum of the chaetognathes.

A certain number of complementary arguments but never final accumulated thereafter like analyzes based on the intermediate filaments, the combined analysis of morphological matrices and even the search for a specific tissue marker (Haase and Al 2001). The whole of these studies propose various positions like a position isolated within the bilatériens, an inclusion in both clades known of protostomiens: the lophotrochozoaires and the ecdysozoaires, an exclusion of the ecdysozaires but none of them suggests the historical bringing together of the chaetognathes with the deutérostomiens.

To conclude on this outline from the controversy around the phylogenetic position from the chaetognathes, force is to note that the situation did not develop much since Darwin (1844), which regarded the chaetognathes as “remarkable for the darkness of their affinities”. In the works of Lecointre and Guyader (2001) and of Nielsen (2001), as in other work, the chaetognathes belong to the protostomiens, but always with a point of uncertainty, due in particular to the absence of reliable molecular data.

Mitochondriaux genomes

The sequencing of mitochondriaux genomes of chaetognathe made it possible to bring new molecular arguments based on the analysis of the alignment of the sequences of genes or the structure of the genome mitochondrial.

The genome mitochondrial of S. will cephaloptera proved to be very particular, at the same time by its size and its composition. With 11905 Pb, it is the smallest known genome mitochondrial of métazoaires and contains only 13 of 37 usual genes. More surprising was the incapacity to isolate the least ARN from transfer (ARNt) mitochondrial among 22 the usually present ones in the mitochondriaux genomes of animals. Among thirteen usual genes coding of proteins, the genes atp6 and atp8 are absent. In spite of this common organization far from, the various analyzes of the protein mitochondriales sequences clearly shows zones signature of protostomiens. There is no gene junction in common with the deutérostomiens in the genome mitochondrial of S. will cephaloptera . However, none of these data makes it possible to determine more precise affinities inside the protostomien group. Exactly two days before the treating article of the genome mitochondrial of S. cephaloptera is accepted by Molecular Biology and Evolution, the article of Helfenbein and Al (2004), on the genome mitochondrial of Paraspadella gotoi , was published in PNAS. Stressing the importance of this work for the decoding of animal phylogeny, Telford (2004b) published a comment covering the two articles in the part `News and Views' of the Nature newspaper. There are thus thus two complete genomes chaetognathes now available. The results of the two teams are equivalent: the genome mitochondrial of P. gotoi is very reduced (11424 Pb), and its study shows that the chaetognathes are protostomiens. Among the differences with the genome of S. will cephaloptera , one can quote the presence of single ARNt, that of methionine ( trnM ), or the fact that the genome includes/understands two whole of genes, each one transcribed in a different direction. The phylogenetic analysis of the sequences of the genome mitochondrial of P. gotoi watch that the chaetognathes are the brother group of the protostomiens, at least, specify the authors, of the protostomiens sampled in the study. The phylogenetic analyzes of the two genomes present several differences, likely to explain the light differences in result:

Helfenbein and Al (2004) used only eight protein mitochondriales sequences (cob, cox1, 2,3, nad1, 3,4 and 5) whereas the eleven proteins of the genome of S. cephaloptera (nad2, 4L and 6 in additional sequences) were used.
taxonomic sampling is appreciably different
the phylogenetic analysis from the primary sequences of P. gotoi was made only with the method of parsimony.

Phylogénomique

At Spadella will cephaloptera : 11526 ESTs were sequences by Génoscope (Evry, Paris). Starting from these sequences, a data file of 79 ribosomal proteins concaténées at 18 taxed, representing more: 11500 positions, was analyzed. New once, the chaetognathes is related to the protostomiens, confirming the results from the marker mitochondrial, and in basal position. A similar project, with Flaccisagitta enflata , has leads to a similar conclusion (affinity of the chaetognathes with the protostomiens) but with a different position inside the protostomiens: at the base of the lophotrochozoaires.

Implications of the phylogenetic position of the chaetognathes

The phylogenetic division of the bilatériens in protostomiens and deutérostomiens is traditionally based on the following characters: the origin of the cœlome, destiny of the blastopore or the segmentation of egg. The division of some of these characters between the chaetognathes and the deutérostomiens (entérocoelie, deuterostomy, trimerism, radiate cleavage; Count 1), as well as the absence of certain characteristics (1) molecular (genes hox , tissue marker; to see low for the genes hox of chaetognathes, Haase and Al 2001) or (2) morphological (mainly the moult, the presence of lophophores or larvae trochophores; Lecointre and Leguyader 2001) defining the ecdysozoaires and the lophotrochozoaires, support the position of the chaetognathes at the base of the protostomiens, with the image of our phylogenomic analysis (fig. 3). Thus, the characters of the deutérostomien type would have been preserved between the chaetognathes and the deutérostomiens and are lost in the line leading to the last common ancestor of the ecdysozoaires and the lophotrochozoaires.

However, the mode of formation of the cœlome, destiny of the blastopore, or the mode of cleavage of egg have been characters prone to controversy, and this for several decades (Lovtrup 1975, Bergström 1986, Nielsen 2001). For example, cases of deuterostomy can be observed at annélides or molluscs (Lovtrup 1975, Arendt and Nubler jung 1997), the brachiopods or the entéropneustes include/understand at the same time organizations schizocœliens and entérocœliens or the cœlome of the tardigrades is formed by entérocœlie (Bergström 1986, Nielsen 2001, Jenner 2004).

Lastly, the chaetognathes are not, actually, animals trimeric, but dimeric. The observation of the early stages of the embryology shows that the differentiation of the mésoderme leads to the formation of only two cœlomic pairs of bags: the cephalic primary coelome and the primary education cœlome of the trunk (Doncaster 1902).

This short recall shows that the ontological criteria which traditionally define the lines deutérostomiennes and protostomiennes can be misleading. And it is the taking into account of the animal diversity as a whole which will enable us not to over-estimate the conservation of characters actually very unstable and to better apprehend the evolution and these mechanisms. For this reason, the minor phyla still little studied, like the chaetognathes, have all their importance.

Evodevo

The Gène S hox code factors of transcription to homéodomaine. They are implied in the regionalization of the axis anteroposterior of the animals and were discovered at the Drosophile but then were characterized at all the bilatériens, like at the cnidaires.

At Spadella will cephaloptera , several genes hox were identified: a member of the group of paralogia median hox3 ( sceHox3 ), four genes ( sceMed1-4 ) and a mosaic gene which shares characteristics at the same time with the median and posterior gene classes ( sceMedPost ). None the specific genes hox of lophotrochozoaires, ecdysozoaires or deutérostomiens could be clearly identified. Several assumptions were planned to explain the presence of SceMedPost, but the phylogenetic position resulting from the analyzes of DNA mitochondrial and the ribosomal Protéine S would suggest rather than it is about a specific gene derived from the phylum.

The expression of one of these genes, sceMed4 , was studied with various stages of the development of S. will cephaloptera , where it is expressed in two side cellular solid masses on the level of the trunk. This area of expression is localized in the neuronal cellular bodies of the ventral ganglion in formation. No expression is detected in the remainder of the nervous system (ganglion cerebroid and neuropile ventral). These preliminary results represent the first data of expression of a gene hox at the chaetognathes, suggesting a role of SceMed4 in the regionalization of the central nervous system, a function which the genes hox at a great number of bilatériens provide.

References

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