Protostomy

The protostomes or protostomes (Protostomia, from the Greek «πρῶτος» -prōto- " primero" and «στόμα» -stoma- "mouth") are a grouping of phyla from the animal kingdom. Together with the deuterostomes, they form the two great lineages into which the animals with bilateral symmetry (Bilateria) are divided. The separation of these two fundamental evolutionary lines is ancestral; During the Cambrian explosion (about 540 million years ago) the differentiated protostomous and deuterostomous phyla already appeared.

The difference between the two clades, proposed by Grobben, lies in the embryonic origin of the mouth. In protostomes, the adult mouth derives from the embryonic blastopore (hence the name of the group), while in protostomes deuterostomes is of neoformation. However, in recent years it has been seen that the fate of the blastopore and the origin of the anus is much more complex and variable among groups of protostomes.

Definition and apomorphies

Karl Grobben, an Austrian biologist, coined the terms Protostomia (Urmundtiere) and Deuterostomia (Neumundtiere), which appear for the first time in Die systematische Einteilung des Tierreiches. In this book, the Austrian grouped all those animals in which the adult mouth originates from the embryonic blastopore into protostomes.. Although the classification concept proposed by Grobben (fate of the blastopore) was new, the group was not, since Hatschek had previously grouped the same taxa under the Zygoneura ensemble. The characteristic that he united with the Zygoneura is that in all of them the nerve cord was ventral and paired. Today, these two characters are considered as morphological apomorphies of the Protostomia clade.

Today, there is almost no doubt that protostomes form an evolutionary unit. The main apomorphies that support this group are:

• Simultaneous formation of the mouth and anus from the blastoporo, a condition known as amphistomy (against Grobben).
• Nervous system formed by a dorsal brain—circunfaringeo—and a couple of ventral nervous cords, sometimes fused (Hatschek's Zegoneury).
• Trocophorous primary larva, with a type ciliar system downstream (the primary larva is missing in the Ecdysozoa, who lack cilias).
• The first two embryo divisions originate a cell (D) that has a special destiny, contributing to the formation of much of the ectoderm, endomesoderm and germinal cells.

Fate of blastopore

The name protostomes derives from the fact that the mouth is formed from the blastopore, as opposed to deuterostomes, in which the blastopore originates only from the anus. However, the fate of the blastopore is not unique and for some groups it was taken for granted although not specifically studied.

When Martín-Durán et al. studied the embryonic development of Priapulus caudatus (Priapulida), a protostome, they verified that this species presented deuterostomy, that is, the blastopore gives rise to the anus, similar to what happens in deuterostomes. On the other hand, in some groups (not in all, but in the most numerous, such as Annelida, Arthropoda and Nematoda), the lateral lips of the blastopore fuse, leaving two openings that become the mouth and anus: they present < i>amphistomy. In the case of Polygordius, the posterior orifice closes completely and the anus appears later, due to an invagination of the body wall. Finally, in some groups such as the pycnogonids, the blastopore originates only from the mouth, in this case we speak of protostomy sensu stricto.

In summary, unlike the classical conception that protostomes present only protostomy, the reality is that the type of development varies according to the group in question, and may be due to protostomy, deuterostomy or commonly amphistomy (this last situation is considered apomorphic by Nielsen and other authors).

Seeing this scenario, scientists began to wonder what would then be the plesiomorphic character of Protostomia: protostomy or deuterostomy? Martín-Durán et al. concluded that:

• The expression of genes bra, cdx, gsc and foxA confirm the homology of the mouth and anus between the different phyla and that the ancestral condition of the Ecdysozoa is the deuterostomia.
• The hypothesis is argued that the common ancestor between Deuterostomia and Protostomia had deuterostomated-type development (as for the fate of their blastoporo). This is in opposition to the classic theories that protostomy It would be the ancestral condition.

Embryonic development

Spiral segment

As a general rule, Protostomia have determined development, starting from mosaic eggs. They form the mouth from the blastopore, by protostomy (mouth only), amphistomy (mouth and anus at the same time) or by deuterostomy (first the anus is formed from the blastopore and then the mouth at the opposite end of the animal, for example in Priapulida).

They can have T-shaped (as in Nematoda and Rotifera), or radial (as in Ectoprocta) or spiral (Platyhelminthes, Annelida, Nemertea, Mollusca) segmentation of the egg.

In the embryonic stage of 4 cells, one of them (cell D) differentiates from the rest and gives rise to the endomesoderm and germ cells. Nielsen names this pattern "quadrant segmentation", and proposes it as a synapomorphy of Protostomia.

The D «cell» (and its fate) has been identified in the Spiralia, in the mollusc Sphaerium, in the rotifer Asplanchna, in most of the Ecdysozoa (arthropods and nematodes) and in the Annelida. The Brachiopoda and Phoronida have segmentation patterns similar to the Deuterostomia.

Spiral segmentation

In this type of segmentation of the fertilized egg, the first two divisions are longitudinal and perpendicular to each other, while the subsequent divisions are oblique. As a result, daughter cells of different sizes are formed: macromeres and micromeres.

Newly formed micromeres move laterally in a clockwise (dextrotropic) or anticlockwise (levotropic) direction. The passage from 4 to 8 cells implies a dextrotropic shift. The following divisions alternate between both types of displacements.

Wilson developed an elegant coding system for spiral cleavage (Wilson's System), which allows one to trace the lineage of each embryonic cell:

• In the embryo of "4 cells", these are encoded as A, B, C and D, following the meaning of the clock needles (views from the animal pole).

• These 4 cells define a Macedonian quartet, and delimit 4 quadrants (then the name proposed by Nielsen of segmentation in quadrants). These cells are collectively encoded generically as Q.

1q (1q) derivatives_x)

2q and 2Q derivatives

1q and 1Q derivatives

Stadium of 64 cells

• The following division (4 → 8) is uneven and oblique, generating lower macromeras and upper micromeras, the latter move in a dextrotrotropic form. These form the first quartet of micromeras (1q): 1a, 1b, 1c and 1d. Now, the macromeras (which always remain in place, in this and the following divisions) are designated as 1Q: 1A, 1B, 1C and 1D. That is, each of the 4 cells (e.g. the A) is divided giving rise to a micromera (1a) and a macromera (1A).

• The Pass 8-16 cells involves the division of both macromerates (1Q) and micromeras (1q), and the new formed micromeras move in a levotropic sense:
  • Macromeras (1Q) are divided by producing a second quartet of micromeras (2q), and a second series of macromeras in situ (2Q).
  • Micromeras (1q) are also divided, producing new microphones that are identified with a subscript (1q)_x).

• The passage 16-32 cells again lead to a dextrotrotropic displacement of mycromerates daughters:
  • Macromeras (2Q) are again divided to form a third quartet of micromeras (3q), forming new macromeras (3Q) in situ.
  • The 12 microphones (2q_x) also be divided again, in this way, according to Wilson's system, the subscript is defined by 2 digits (2q)_xy). This way, from the 1st micromera₁ stem cells 1a₁₁ and 1a₁₂.

• The following division (32 → 64) occurs in a levotropic pattern, resulting thus far the following cells:

Generally, from the embryo of 64 cells the segmentation stops being spiral and acquires different patterns according to the groups. As can be seen, in each division the new cells rotate from right to left, distributing themselves in a pattern similar to a spiral, from which the name of this type of segmentation derives.

Late segmentation patterns

Later in development, distinct characteristic patterns can be observed as a consequence of the orientation of the first quartet of micromeres.

• The higher cells lie in the apex of the embryo (derivated from 1q₁₁₁♪ forming the rosette apical.
• In the anelids and other groups the micromeras derived from 1q₁₁₂ form the nestled cross (Mollusca: peripheral rosette), perpendicular to apical rosette.
• In the molluscs appears a mollusca cross formed from the derivatives of the 1q micromera₁₂.

spiral segmentation Trochus (Mollusca).

The Platyhelminthes, Nemertea, and Schizocoelia clade have a very uniform pattern of spiral cleavage. There are some differences in the arrangement of the cells that form a "cross" at the animal pole of the embryo (for example, between Platyhelminthes and Annelida), but the general pattern is the same.

Origin of Germ Layers

Invagination

Epibolia

During the third division of the embryo, the separation between the cells that will originate the ectoderm and the endoderm occurs. The quartet of micromeres (q) form the presumptive tissue that will originate the ectoderm: the "ectoblast". The macromeres (Q) give rise to the presumptive endoderm: the "endoblast."

The 3D macromere divides, giving rise to the 4d micromere, which will divide and form the presumptive mesoderm: the «mesoblast». This micromere (4d) is called the mesentoblast and gives rise to the mesoderm in all animals with spiral cleavage. When dividing, the mesentoblast gives rise to two teloblasts (4d₁ and 4d₂), which by mitosis give rise to two bands of mesodermal tissue in the area of contact between the ectoderm and endoderm, near the blastopore.

Gastrulation varies according to the group in question and the amount of yolk carried by the egg:

• Eggs with little vitela (microlecitos) give rise to a gallblastula, and gastrulation occurs mostly by invagination.
• Eggs with a lot of vitethelite (telecytes, macrolecites) give rise to a stereoblástula, and gastrulation occurs mostly by epibolia.

After the formation of the mesoderm, the formation of the coelom occurs, when it exists. In this case, and particularly in the Schizocoelia clade, the coelom is formed by hollowing out the mesoderm, a process known as schizocelia: a schizocoelom is formed.

Trochophore larva

During the final stage of embryonic development, the embryo can become a larva (indirect development: this larva can be a trochophore or non-trochophore larva) or develop directly into a juvenile (direct development). Since the studies carried out by Hatschek, the presence of trochophore larvae was suggested as a characteristic of all Protostomia. However, not all protostomia have this type of larva. Three broad generalities can be made about the trochophore larva:

• It is characteristic of all the members of the Schizocoelia or Trochozoa clado.
• It is present in others phylaconsidered as modified troophorasince his morphology differs from the basic model.
• The Ecdysozoa lack throcophore as a result of a secondary loss.

The following table shows the phyla with trochophore larva [+], modified trochophore [(+)], without trochophore larva [-] and no information available [?]:

Generalized trophorous larva in lateral vision A.- Episfera B.- Hiposfera 1.- Brain Ganglio2.- Syncipital Plate 3.- Prototroco 4.- Target 5.- Mesodermic outline 6.- Year 7. Protonefridio 8.- Digestive tube 9.- Boca 10.- Blastocel

Morphologically, the trochophore larva is characterized by being shaped like a spinning top or top (hence its name) and by being furrowed by ciliated bands. It can be lecithotrophic or planktotrophic, depending on embryonic development, and it moves thanks to the action of the ciliated bands, which in the case of planktotrophic larvae also participate in feeding. The body is divided into two portions, separated by the prototrochus (ciliary band): an upper hemisphere (episphere) and a lower hemisphere (hyposphere).

• At the level of the episphere the nerve system of the larva, formed by an apical organ, a couple of cerebral ganglia and a couple of more or less merged ventral nervous cords (the latter continue at the level of the hyposphere). From the apical organ (sincipital plate) there is a cilia (painting prick) characteristic of this larva.
• At the level of the hypoosphere the majority of the ciliated bands spread and it is here that some organs are shown: digestive tract (with central and ventral mouth, and terminal and lower anus) and protonefrids (2).

The internal cavity is represented by the embryonic blastocel. Distinctive of the trochophore larva is the presence of five ciliated bands: prototrochus (supraoral), metatrochus (suboral), adoral ciliated zone (circumoral), gastrotrochus (connects metatrochus with telotrochus), and telotrochus (supraanal).

• The "prototroco" and the "thetroco" (when present) form the locomotive system of the larva.
• The “prototroco”, the “adorable zone” and the “metatroco” form the larva feeding system, a collection system downstream.
• In the Lecitotrophic Troops the «metatroco» is missing and the «adorable zone» is undeveloped.
• In the larvae of some species, accessory ciliate bands (“acrotroco”, “meniscotroco”, “pretroco”) may appear.

Phylogenetic value of spiral cleavage

On the basis of spiral segmentation, the clade Spiralia was founded, which brought together -as main groups- the Platyhelminthes, Nemertea, Annelida and Mollusca, which follow the typical spiral model.

If all mosaic eggs, with determined development, were derived from an ancestor with spiral cleavage, it would be justified to include in a single clade groups with other types of cleavage that were traditionally interpreted as modified spiral (although sometimes they do not have much resemblance to the supposed "original" model).

The Spiralia (or some of them) also received the name of Trochozoa, within which different authors included more or less groups, depending on the evidence they had.

When the lophophorates (Brachiopoda, Phoronida) were proposed to be protostomes, they were joined to the earlier Trochozoa, under the name Lophotrochozoa. Thereafter the monophyly of the Lophotrochozoa was discussed, which is sometimes restricted to the Spiralia sensu stricto and sometimes incorporates other phyla. As there is not a minimum of agreement on the phylogeny of these groups, for now it is preferable not to use the concepts Lophotrochozoa and Spiralia, although probably the latter may have more foundation.

Coelom

The mesoderm of protostomes arises as a solid tissue that grows as two masses lateral to the blastopore and usually proliferates lining the ectoderm inside. As it develops it can:

• Generate different tissues and organs in the blastocelic cavity (seudocellomad animals).
• Produce a parenchyma that fills the blastocel (aloused or parenchimios).
• Forming two masses that hollow, originating a cavity upholstered by peritoneum (schizocellomad animals).

When a body cavity forms, the surrounding mesoderm can differentiate:

• Dermis (a mesodérmic connective tissue that is integrated into the epithelium): can be thick, thin or missing (most invertebrates have no dermis).
• The muscles of the body wall (typically a layer of circular muscle and, below, a longitudinal muscle).
• Peritoneum (an inner epithelium, with basal membrane, which can sometimes be totally or partly lost during ontgeny or -in several phyla of animals with body cavity- never forming).

Traditionally, parenchymal animals (without a body cavity) are called acelomates, pseudocoelomates those that have a body cavity that is not lined by peritoneum and coelomates to those with peritoneum. They are merely descriptive concepts, without any evolutionary connotation.

Acoelomate Animals

In an acoelomate the mesoderm forms a parenchyma that occupies most of the internal space of the body. However, they are not totally solid, because lacunae open within the parenchyma in which interstitial fluid accumulates, leaving small cavities that surround the gonads or other structures. A clear example of this are the species belonging to the Platyhelminthes phylum.

Pseudocelomate Animals

Pseudocelomate is any animal whose general body cavity is not covered by peritoneum (regardless of why it is not): not a concept with phylogenetic value. Formerly they formed the edge Aschelminthes (today obsolete), but since the pseudocel is not homologous in all cases, the pseudocoelomates cannot be considered as a single edge because they share that character. The pseudocoelom can arise by two different mechanisms, depending on the group in question:

• Animals with stereo. In these animals the pseudocelloma is formed by boner of a syncicial parenchyma (Rotifera, Gnathostomulida).
• Animals with jealousy. In these animals the pseudoceloma is the blastocel, which persists in the adult (Nematoda, Nematomorpha, Kinorhyncha, Priapulida, Loricifera).

Cross-section of a jealous animal.

A particular case is gastrotric, which have no body cavity. However, many scientists consider them pseudocoelomates where the pseudocoelom disappeared as a consequence of an invasion of mesodermal parenchyma (they are functional acoelomates).

In many species such as nematodes, the pseudocoelom is not an empty cavity: it contains fluid that functions as a hydrostatic skeleton and is always partially occupied by large cells (pseudocelomocytes) of poorly understood function. Mechanically, pseudocoelomocytes contribute to organs occupying relatively stable positions.

Coelomate Animals

In a coelomate, the general body cavity is surrounded by peritoneum, which integrates with the body wall as a somatopleura, surrounds the intestine as splanchnopleura, and forms the mesenteries that unite both pleurae. Examples of coelomate animals are the Annelida segmented worms.

Distribution of chin in animals according to Willmer.

Chitin

Protonefridia with famiger cell

Chitin is a carbohydrate, polymer of N-acetylglucosamine (a sugar derivative in which a hydroxyl group [-OH] is replaced by an amino group [-NH₂ ].Its monomers are linked to each other with β-1,4 bonds, just like the glucose molecules that make up cellulose.It is the most abundant natural polymer after cellulose.It is also present in fungi.

The presence of chitin has been detected in at least 21 animal phyla (Willmer). Most (19) belong to the Protostomia clade, and two of these are more basal taxa (sponges and ctenophores).

Since chitin exists in most of the animal phyla and in the other groups (fungi), and that it is absent only in Deuterostomia, the most parsimonious hypothesis is to think that chitin is a synapomorphy of animals. Therefore, it follows that in Protostomia chitin is plesiomorphic, while in Deuterostomia its absence is apomorphic.

Excretory system

Excretion is the process of eliminating nitrogenous wastes that come from cellular metabolism. It involves three stages: ultrafiltration, reabsorption and secretion of urine. It is carried out by different structures, depending on the phylum: protonephridia, metanephridia, glandular nephridia, sacciform nephridia (arthropods), glomeruli or other organs.

The type of nephridia is related to the type and width of the general body cavity of the animal. Thus, smaller animals (less than 1 mm) and gelatinous ones usually lack an excretory system (but not excretion).

Protonephridia

A protonephridium is a structure derived from the ectoderm, which by itself fulfills the three excretion processes: ultrafiltration, reabsorption and secretion.

It is a tube closed at its end by an terminal cell, which has one or more special undulipodia (cilia/flagella), which move the liquid (urine in formation) along a tube It flows out through a nephropore.

Smaller animals and larvae usually have only two protonephridia, which usually end together in a pore spore. In larger animals, there may be many protonephridia (sometimes thousands) joining their ducts into two large nephridioducts, which may lead to a urinary bladder.

When there are only one or two undulipodia, it is said to be a protonephridian with solenocytes; when the terminal cell has a tuft of cilia, it is said to be a protonephridian with a flaming cell. The difference is not absolute and, in general, an organism is only said to have protonephridia, unless it is a typical example of one of those two models.

The terminal cell is surrounded by a porous basement membrane, through which ultrafiltration of the fluid from the cavity (or parenchyma interstices if there is no cavity) occurs. The movement of the undulipodia or undulipodia is what produces a negative pressure (suction) that is enough for small molecules to pass into the protonephridium. Along the nephridioduct, metabolically usable substances are reabsorbed, which are returned to the coelom (and waste substances are secreted into the tube, helping to form the final urine).

Metanephridia

Metanephage.

A metanephridian differs from a protonephridian because:

• They are of mesodérmic origin.
• They do not perform the ultrafiltration (this is done in small cells associated with the circulatory system).
• They are open to jealousy by a ciliated mouth, usually in the form of funnel (conciliated nephrostoma).

Podocytes are cells that have finger-like extensions (pedicels), associated with blood vessels. Blood pressure causes small, positive molecules to pass through the coelomic fluid, crossing the basement membrane.

The metanephridia take up the coelomic fluid and, by reabsorption and secretion along the nephridioduct, produce urine, which is transported to the nephropore.

Protonephridia or metanephridia?

The existence of metanephridia assumes a circulatory system with podocytes: there are no metanephridia in acoelomate or pseudocoelomate animals, where there is no peritoneum.

Coelomate larvae develop protonephridia, of ectodermal origin, before they complete the formation of the general body cavity and circulatory system. In coelomates, the larval protonephridia disappear during metamorphosis and metanephridia are formed. Only by exception do larval protonephridia remain in the adult.

Animals that have a greatly reduced coelom modify the metanephridia, originating structures that function differently: for example, molluscs (Bojanus's organ) and arthropods (Malpighian tubes).

Adults of Platyhelminthes, Nemertea, Rotifera, Priapulida, Kinorhyncha, Loricifera, Gnathostomulida, Entoprocta, and larvae of schizocoelomates (mollusks and annelids), Phoronida, Ectoprocta, and Gastrotricha have protonephridia.

Relationship between excretor system and player (a) Detached protonefridio pipeline; each opens in an independent pore (gonoporo and nefroporo); (b) fusion of the gonoduct funnel with the protonefride duct; a single pore allows the emission of gametas and the exit of the urine; (c) Separate gonoduct from metanefridio; two pores; (d) Fusion of the gonoduct funnel in front of the metanefridial funnel.

Adults of Nematomorpha, Ectoprocta, Cycliophora do not have an excretory system.

Nephromyxia

The use of the ducts of the excretory system for the emission of gametes that mature free in the coelom is called nephromyxia. It can occur in both animals with protonephridia (protonephromyxia) and in animals with metanephridia (metanephromyxia). Fertilization in both cases is generally external (ovuliparous), but there are also cases of internal fertilization (oviparous), without copulation and without long incubations.

Considering this and the relationship between excretory and reproductive systems, the following can be given:

• Separate gonoduct from protonefridio; each opens in an independent pore (gonoporo and nefroporo).
• Fusion of the gonoduct funnel with the protonefride duct; a single pore allows the emission of gametas and the exit of the urine.
• Gonoduct separated from the metanefridio; two pores.
• Fusion of the gonoduct funnel in front of the metanefridial funnel.

Systematics and phylogeny

Traditionally, parenchymal animals (without a body cavity) are called acelomates, pseudocelomates those that have a body cavity that is not lined by peritoneum and coelomates to those with peritoneum. In the past, the traditional systematics of the 20th century gave these concepts an evolutionary meaning, considering them natural groups:

• Acelomados: Platyhelminthes and Nemertea.
• Pseudocellomados: Nematoda, Rotifera, Priapulida, Gastrotricha, etc.
• Celomados: Annelida, Arthropoda, Mollusca, Bryozoa, etc.

A phylogenetic sequence was assumed: acelomate → pseudocelomate → coelomate. This evolutionary concept of coelomy is completely obsolete, because today it is known that:

• In some jealousysecondaryly, the body cavity is filled with parenchyma: functional acelomados (e.g., snails, leeches).
• The pseudocelloma is homoplastic: phyla with different phylogenetic origin lack peritoneum; others are jealous that have lost it and in others cavity is a neoformation.
• The peritoneum it is homoplastic: it appeared at least twice in evolution, by two different ontogenetic mechanisms (in deuterostomates the peritoneum derived from endoderm).

Consequently, thanks to cladistic analyzes based on morphological and molecular data, this classical and erroneous view of protostomes began to change. The relationships between annelids and arthropods (Articulata) have been invalidated and the Ecdysozoa hypothesis has emerged, according to which arthropods are related to pseudocoelomate groups that shed their cuticles. The new phylogenetic systematics divides protostomes into the Ecdysozoa and Spiralia clades:

• Ecdysozoa: includes animals that have a cuticle of organic material (quitin; collagen) of three or more layers, which is regularly renewed by mudas (ecdisis) for which a specific enzyme (ecdisone) is synthesized. In turn they lack locomotive cilias, which translates into the formation of spermatozoid ameboids.

• Spiralia: Includes animals that have spiral segmentation of the egg, however not all its members possess this characteristic, but it has been suggested that the latter could evolve from ancestors that had this embryonic development pattern. It includes very diverse morphologically supported filos by all molecular analysis. Its members may have throcophorous evolutionary sympathies.

Phylogenetic tree

The phylogeny of prototosmos is very difficult to solve, molecular analyzes give very different results with the placement of the groups. Although it all depends on the method used in the analysis. Currently, phyla such as Platyhelminthes, Tardigrada, and Rotifera have rapid rates of evolution, which prevents the reconstruction of an adequate phylogenetic tree and leads to the attraction of long branches. Recent phylogenetic studies (2019) using slower evolving species that are less prone to long branch attraction and other studies attempting to avoid systematic error have given the following phylogeny.

An alternative phylogenetic tree published in the book "The Invertebrate Tree Of Life" (2020) is the one proposed by Gonzalo Giribet. The marked nodes are defended by the author: