Evolution of Otx paralogue usages in early patterning of the vertebrate head

To assess evolutional changes in the expression pattern of Otx paralogues, expression analyses were undertaken in fugu, bichir, skate and lamprey. Together with those in model vertebrates, the comparison suggested that a gnathostome ancestor would have utilized all of Otx1, Otx2 and Otx5 paralogues in organizer and anterior mesendoderm for head development. In this animal, Otx1 and Otx2 would have also functioned in specification of the anterior neuroectoderm at presomite stage and subsequent development of forebrain/midbrain at somite stage, while Otx5 expression would have already been specialized in epiphysis and eyes. Otx1 and Otx2 functions in anterior neuroectoderm and brain of the gnathostome ancestor would have been differentially maintained by Otx1 in a basal actinopterygian and by Otx2 in a basal sarcopterygian. Otx5 expression in head organizer and anterior mesendoderm seems to have been lost in the teleost lineage after divergence of bichir, and also from the amniotes after divergence of amphibians as independent events. Otx1 expression was lost from the organizer in the tetrapod lineage. In contrast, in a teleost ancestor prior to whole genome duplication, Otx1 and Otx2 would have both been expressed in the dorsal margin of blastoderm, embryonic shield, anterior mesendoderm, anterior neuroectoderm and forebrain/midbrain, at respective stages of head development. Subsequent whole genome duplication and the following genome changes would have caused different Otx paralogue usages in each teleost lineage. Lampreys also have three Otx paralogues; their sequences are highly diverged from gnathostome cognates, but their expression pattern is well related to those of skate Otx cognates.     

Ablation of a Single Cell From Eight-cell Embryos of the Amphipod Crustacean Parhyale hawaiensis

The amphipod Parhyale hawaiensis is a small crustacean found in intertidal marine habitats worldwide. Over the past decade, Parhyale has emerged as a promising model organism for laboratory studies of development, providing a useful outgroup comparison to the well studied arthropod model organism Drosophila melanogaster. In contrast to the syncytial cleavages of Drosophila, the early cleavages of Parhyale are holoblastic. Fate mapping using tracer dyes injected into early blastomeres have shown that all three germ layers and the germ line are established by the eight-cell stage. At this stage, three blastomeres are fated to give rise to the ectoderm, three are fated to give rise to the mesoderm, and the remaining two blastomeres are the precursors of the endoderm and germ line respectively. However, blastomere ablation experiments have shown that Parhyale embryos also possess significant regulatory capabilities, such that the fates of blastomeres ablated at the eight-cell stage can be taken over by the descendants of some of the remaining blastomeres. Blastomere ablation has previously been described by one of two methods: injection and subsequent activation of phototoxic dyes or manual ablation. However, photoablation kills blastomeres but does not remove the dead cell body from the embryo. Complete physical removal of specific blastomeres may therefore be a preferred method of ablation for some applications. Here we present a protocol for manual removal of single blastomeres from the eight-cell stage of Parhyale embryos, illustrating the instruments and manual procedures necessary for complete removal of the cell body while keeping the remaining blastomeres alive and intact. This protocol can be applied to any Parhyale cell at the eight-cell stage, or to blastomeres of other early cleavage stages. In addition, in principle this protocol could be applicable to early cleavage stage embryos of other holoblastically cleaving marine invertebrates.     

The fine structure of Comanthus japonica (Echinodermata: Crinoidea) from zygote through early gastrula

The fine structure of the early embryo of Comanthus has been described by scanning and transmission electron microscopy at approximately 20-min intervals from zygote (20 min) through early gastrula (260 min). In normally developing (and presumably monospermic) embryos, some non-fertilizing sperm were invariably trapped in the perivitelline space; this suggests that there is an effective block to polyspermy at the level of the plasma membrane. No trace of a hyaline layer is encountered in the pervitelline space. At first cleavage, which begins unilaterally at the animal pole, the contractile ring filaments are rather thick (50–150 Å) in comparison to those known for other marine invertebrates. From first cleavage through early gastrula, the lateral surfaces of the blastomeres are broadly adherent, and there is an intercellular material, presumably an adhesive, in the intercellular space. The blastocoel first appears during the four-cell stage. From the eight-cell stage through the start of gastrulation, only one opening, the vegetal pore, connects the blastocoel with the perivitelline space. Gastrulation begins at the 50–100-cell stage, while the vegetal pore is still open, and a clearly defined blastula stage is bypassed. Gastrulation is by a novel process, which I have called holoblastic involution. At gastrulation the eight most vegetal blastomeres, which encircle the vegetal pore, shoot out erect, unbranched filopodia for many microns through the blastocoel. The filopodia adhere to the blastocoelic surfaces of the animal blastomeres and contract, pulling the vegetal blastomeres into the blastocoel. The migrated vegetal blastomeres adhere to one another, forming the entoderm in the vegetal region of the embryo; the remaining blastomeres become the ectoderm. Soon after the completion of cell migration, the entodermal blastomeres appear to cast off their contractile microappendages and adhesive membranes into the blastocoel.     

Hox cluster organization in the jawless vertebrate Petromyzon marinus

Large-scale gene amplifications may have facilitated the evolution of morphological innovations that accompanied the origin of vertebrates. This hypothesis predicts that the genomes of extant jawless fish, scions of deeply branching vertebrate lineages, should bear a record of these events. Previous work suggests that nonvertebrate chordates have a single Hox cluster, but that gnathostome vertebrates have four or more Hox clusters. Did the duplication events that produced multiple vertebrate Hox clusters occur before or after the divergence of agnathan and gnathostome lineages? Can investigation of lamprey Hox clusters illuminate the origins of the four gnathostome Hox clusters? To approach these questions, we cloned and sequenced 13 Hox cluster genes from cDNA and genomic libraries in the lamprey, Petromyzon marinus. The results suggest that the lamprey has at least four Hox clusters and support the model that gnathostome Hox clusters arose by a two-round-no-cluster-loss mechanism, with tree topology [(AB)(CD)]. A three-round model, however, is not rigorously excluded by the data and, for this model, the tree topologies [(D(C(AB))] and [(C(D(AB))] are most parsimonious. Gene phylogenies suggest that at least one Hox cluster duplication occurred in the lamprey lineage after it diverged from the gnathostome lineage. The results argue against two or more rounds of duplication before the divergence of agnathan and gnathostome vertebrates. If Hox clusters were duplicated in whole-genome duplication events, then these data suggest that, at most, one whole genome duplication occurred before the evolution of vertebrate developmental innovations.     

Characterization of Brachyury genes in the dogfish S. canicula and the lamprey L. fluviatilis. Insights into gastrulation in a chondrichthyan

In order to gain insights into the evolution of gastrulation mechanisms among vertebrates, we have characterized a Brachyury-related gene in a lamprey, Lampetra fluviatilis, and in a chondrichthyan, Scyliorhinus canicula. These two genes, respectively termed LfT and ScT, share with their osteichthyan counterparts prominent expression sites in the developing notochord, the tailbud, but also a transient expression in the prechordal plate, which is likely to be ancestral among vertebrates. In addition, the lamprey LfT gene is transcribed in the endoderm of the pharyngeal arches and the epiphysis, two expression sites that have not been reported thus far in gnathostomes, and, as in the chick, in the differentiating nephrotomes. Since Brachyury expression in nascent mesoderm and endoderm is highly conserved among vertebrates as well as cephalochordates, we have used this marker to identify these cell populations during gastrulation in the dogfish. The results suggest that these cells are initially present over the whole margin of the blastoderm and are displaced during gastrulation to its posterior part, which may correspond to the site of mesoderm and endoderm internalization. These data provide the first molecular characterization of gastrulation in a chondrichthyan. They indicate that gastrulation in the dogfish and in some amniotes shares striking similarities despite the phylogenetic distance between these species. This supports the hypothesis that the extensively divergent morphologies of gastrulae among vertebrates largely result from adaptations to the presence of yolk.     

Lamins of the sea lamprey (Petromyzon marinus) and the evolution of the vertebrate lamin protein family

Lamin proteins are found in all metazoans. Most non-vertebrate genomes including those of the closest relatives of vertebrates, the cephalochordates and tunicates, encode only a single lamin. In teleosts and tetrapods the number of lamin genes has quadrupled. They can be divided into four sub-types, lmnb1, lmnb2, LIII, and lmna, each characterized by particular features and functional differentiations. Little is known when during vertebrate evolution these features have emerged. Lampreys belong to the Agnatha, the sister group of the Gnathostomata. They split off first within the vertebrate lineage. Analysis of the sea lamprey (Petromyzon marinus) lamin complement presented here, identified three functional lamin genes, one encoding a lamin LIII, indicating that the characteristic gene structure of this subtype had been established prior to the agnathan/gnathostome split. Two other genes encode lamins for which orthology to gnathostome lamins cannot be designated. Search for lamin gene sequences in all vertebrate taxa for which sufficient sequence data are available reveals the evolutionary time frame in which specific features of the vertebrate lamins were established. Structural features characteristic for A-type lamins are not found in the lamprey genome. In contrast, lmna genes are present in all gnathostome lineages suggesting that this gene evolved with the emergence of the gnathostomes. The analysis of lamin gene neighborhoods reveals noticeable similarities between the different vertebrate lamin genes supporting the hypothesis that they emerged due to two rounds of whole genome duplication and makes clear that an orthologous relationship between a particular vertebrate paralog and lamins outside the vertebrate lineage cannot be established.     

Cell lineage analysis in ascidian embryos by intracellular injection of a tracer enzyme: I. Up to the eight-cell stage

Cell lineages during development of ascidian embryos were analyzed by injection of horseradish peroxidase as a tracer enzyme into identified cells at the one-, two-, four-, and eight-cell stages of the ascidians, Halocynthia roretzi, Ciona intestinalis, and Ascidia ahodori. Identical results were obtained with eggs of the three different species examined. The first cleavage furrow coincided with the bilateral symmetry plane of the embryo. The second furrow did not always divide the embryo into anterior and posterior halves as each of the anterior and posterior cell pairs gave rise to different tissues according to their destinies, which became more definitive in the cell pairs at the eight-cell stage. Of the blastomeres constituting the eight-cell stage embryo, the a4.2 pair (the anterior animal blastomeres) differentiated into epidermis, brain, and presumably sense organ and palps. Every descendant cell of the b4.2 pair (the posterior animal blastomeres) has been thought to become epidermis; however, the horseradish peroxidase injection probe revealed that the b4.2 pair gave rise to not only epidermis but also muscle cells at the caudal tip region of the developing tailbud-stage embryos. The A4.1 pair (the anterior vegetal blastomeres) developed into endoderm, notochord, brain stem, spinal cord, and also muscle cells next the caudal tip muscle cells. From the B4.1 pair (the posterior vegetal blastomeres) originated muscle cells of the anterior and middle parts of the tail, mesenchyme, endoderm, endodermal strand, and also notochord at the caudal tip region. These results clearly demonstrate that muscle cells are derived not only from the B4.1 pair, as has hitherto been believed, but also from both the b4.2 and A4.1 pairs.     

Maternal information and localized maternal mRNAs in eggs and early embryos of the ascidian Halocynthia roretzi

Summary

The mosaic behavior of blastomeres isolated from ascidian embryos has been taken as evidence that localized ooplasmic factors (cytoplasmic determinants) specify tissue precursor cells during embryogenesis. Experiments involving the transfer of egg cytoplasm have revealed the presence and localization of various kinds of cytoplasmic determinants in eggs of Halocynthia roretzi. Three cell fates, epidermis, muscle and endoderm, are fixed by cytoplasmic determinants. The three kinds of tissue determinants move in different directions during ooplasmic segregation. Prior to the onset of the first cleavage the three kinds of determinants reside in egg regions that correspond to the future fate map of the embryo and then they are differentially partitioned into specific blastomeres. In addition to tissue-specific determinants, there is evidence suggesting that ascidian eggs contain localized cytoplasmic factors that are responsible for controlling the cleavage pattern and morphogenetic movements. Transplantation of posterior-vegetal egg cytoplasm to an anterior-vegetal position causes a reversal of the anterior-posterior polarity of the cleavage pattern. Localized cytoplasmic factors in the posterior-vegetal region are involved in the generation of a unique cleavage pattern. When vegetal pole cytoplasm is transplanted to the animal pole or equatorial position of the egg, ectopic gastrulation occurs at the site of transplantation. This finding supports the idea that vegetal pole cytoplasm specifies the site of gastrulation. Recently, we started a cDNA project to analyze maternal mRNAs. An arrayed cDNA library of fertilized eggs of H. roretzi was constructed, and more than 2000 clones have been partially sequenced so far. To estimate the proportion of the maternal mRNAs that are localized in the egg and embryo, 150 randomly selected clones were examined by in situ hybridization. We found eight mRNAs that are localized in the eight-cell embryo, of which three were localized to the myoplasm (a specific region of the egg cytoplasm that is partitioned into muscle-lineage blastomeres) of the egg, and then to the postplasm of cleavage-stage embryos. These results indicate that the proportion of localized messages is much higher than we expected. These localized maternal messages may be involved in the regulation of various developmental processes.     

Hox gene expression patterns in Lethenteron japonicum embryos--insights into the evolution of the vertebrate Hox code

The Hox code of jawed vertebrates is characterized by the colinear and rostrocaudally nested expression of Hox genes in pharyngeal arches, hindbrain, somites, and limb/fin buds. To gain insights into the evolutionary path leading to the gnathostome Hox code, we have systematically analyzed the expression pattern of the Hox gene complement in an agnathan species, Lethenteron japonicum (Lj). We have isolated 15 LjHox genes and assigned them to paralogue groups (PG) 1-11, based on their deduced amino acid sequences. LjHox expression during development displayed gnathostome-like spatial patterns with respect to the PG numbers. Specifically, lamprey PG1-3 showed homologous expression patterns in the rostral hindbrain and pharyngeal arches to their gnathostome counterparts. Moreover, PG9-11 genes were expressed specifically in the tailbud, implying its posteriorizing activity as those in gnathostomes. We conclude that these gnathostome-like colinear spatial patterns of LjHox gene expression can be regarded as one of the features already established in the common ancestor of living vertebrates. In contrast, we did not find evidence for temporal colinearity in the onset of LjHox expression. The genomic and developmental characteristics of Hox genes from different chordate species are also compared, focusing on evolution of the complex body plan of vertebrates.     

Early development and acquisition of Zooxanthellae in the temperate symbiotic sea anemone Anthopleura ballii (Cocks)

The ova of Anthopleura ballii become infected with zooxanthellae (endosymbiotic dinoflagellates) of maternal origin just prior to spawning. After fertilization, the zygotes undergo radial, holoblastic cleavage, and then gastrulate by invagination to form ciliated planulae. Because the zooxanthellae are localized on one side of the ovum-and later, within the blastomeres at one end of the embryo-invagination leads to the zooxanthellae being restricted to the planular endoderm and hence to the gastrodermal cells of the adult anemone. We propose that maternal inheritance of zooxanthellae plays an important part in the success of these temperate sea anemones, which live in regions where potential sources of zooxanthellae are scarce.     

Mechanisms of heart development in the Japanese lamprey, Lethenteron japonicum

SUMMARY Vertebrate hearts have evolved from undivided tubular hearts of chordate ancestors. One of the most intriguing issues in heart evolution is the abrupt appearance of multichambered hearts in the agnathan vertebrates. To explore the developmental mechanisms behind the drastic morphological changes that led to complex vertebrate hearts, we examined the developmental patterning of the agnathan lamprey Lethenteron japonicum . We isolated lamprey orthologs of genes thought to be essential for heart development in chicken and mouse embryos, including genes responsible for differentiation and proliferation of the myocardium ( LjTbx20, LjTbx4/5 , and LjIsl1/2A ), establishment of left–right heart asymmetry ( LjPitxA ), and partitioning of the heart tube ( LjTbx2/3A ), and studied their expression patterns during lamprey cardiogenesis. We confirmed the presence of the cardiac progenitors expressing LjIsl1/2A in the pharyngeal and splanchnic mesoderm and the heart tube of the lamprey. The presence of LjIsl1/2A -positive cardiac progenitor cells in cardiogenesis may have permitted an increase of myocardial size in vertebrates. We also observed LjPitxA expression in the left side of lamprey cardiac mesoderm, suggesting that asymmetric expression of Pitx in the heart has been acquired in the vertebrate lineage. Additionally, we observed LjTbx2/3A expression in the nonchambered myocardium, supporting the view that acquisition of Tbx2/3 expression may have allowed primitive tubular hearts to partition, giving rise to multichambered hearts.     

The fate of isolated blastomeres with respect to germ cell formation in the amphipod crustacean Parhyale hawaiensis

Germ cells may be specified through the localization of germ line determinants to specific cells in early embryogenesis, or by inductive signals from neighboring cells to germ cell precursors in later embryogenesis. Such determinants can be produced and localized during or after oogenesis, either autonomously by oocytes or by associated nutritive cells. In Drosophila, each oocyte is connected to nurse cells by cytoplasmic bridges, and determinants synthesized in nurse cells are transported through these bridges to the oocyte. However, the Drosophila model may not be applicable to all arthropods, since in many species of all four extant arthropod classes, gametogenesis functions without nurse cells. In this paper, I use immunodetection of Vasa protein to study germ cell development in the amphipod crustacean Parhyale hawaiensis, a species whose ovaries lack nurse cells and whose eggs lack obvious polarity. Previous cell lineage analyses have shown that all three germ layers and the germ line are exclusively specified by third cleavage. In the present study, I use a molecular marker to follow germ cell development during P. hawaiensis embryogenesis. I determine the capacity of individual blastomeres to form germ cells by isolating blastomeres at early cleavage stages and provide experimental evidence for localized germ cell determinants at the two-cell stage in P. hawaiensis. These experiments indicate that many aspects of early amphipod development, including timing and symmetry of cell division, the transition from holoblastic to superficial cleavage, and possibly some gastrulation movements, are cell autonomous following first cleavage.     

Early Evolution of Conserved Regulatory Sequences Associated with Development in Vertebrates

Comparisons between diverse vertebrate genomes have uncovered thousands of highly conserved non-coding sequences, an increasing number of which have been shown to function as enhancers during early development. Despite their extreme conservation over 500 million years from humans to cartilaginous fish, these elements appear to be largely absent in invertebrates, and, to date, there has been little understanding of their mode of action or the evolutionary processes that have modelled them. We have now exploited emerging genomic sequence data for the sea lamprey, Petromyzon marinus, to explore the depth of conservation of this type of element in the earliest diverging extant vertebrate lineage, the jawless fish (agnathans). We searched for conserved non-coding elements (CNEs) at 13 human gene loci and identified lamprey elements associated with all but two of these gene regions. Although markedly shorter and less well conserved than within jawed vertebrates, identified lamprey CNEs are able to drive specific patterns of expression in zebrafish embryos, which are almost identical to those driven by the equivalent human elements. These CNEs are therefore a unique and defining characteristic of all vertebrates. Furthermore, alignment of lamprey and other vertebrate CNEs should permit the identification of persistent sequence signatures that are responsible for common patterns of expression and contribute to the elucidation of the regulatory language in CNEs. Identifying the core regulatory code for development, common to all vertebrates, provides a foundation upon which regulatory networks can be constructed and might also illuminate how large conserved regulatory sequence blocks evolve and become fixed in genomic DNA.     

Molecular phylogenetics of gnathostomous (jawed) fishes: old bones, new cartilage

Cartilaginous fishes (chondrichthyans) have traditionally been taken as an early offshoot among jawed vertebrates. To examine some crucial chondrichthyan relationships, we have sequenced the mitochondrial genomes of the holocephalan Chimaera monstrosa (ratfish) and the basal galeomorph species Heterodontus francisci (horn shark) and analysed them together with the corresponding data set of several other chondrichthyans, teleosts, the coelacanth, the African lungfish and the bichir. The rooting point of the tree was established using unequivocal outgroups, the sea lamprey , the sea lancelet or echinoderms. The phylogenetic analyses identified monophyletic Chondrichthyes in a terminal position in the piscine tree, lending no support to the traditionally accepted basal position of cartilaginous fishes among extant gnathostomes. The findings suggest that the cartilage characterizing extant chondrichthyans is a retention of an embryonic condition, thus representing a derived rather than a primitive phylogenetic and developmental stage. Similarly, the analyses suggest that the open gill slits of neoselachians (sharks and rays) constitute a derived state compared to the operculum (gill cover) characterizing bony fishes and holocephalans. The analyses did not support the so-called Squalea/Galea hypothesis which posits that batomorphs (sharks, rays) have arisen from recent selachians (sharks). Inconsistent with the common understanding of piscine and gnathostome evolution, the two taxa having lungs, the African lungfish and the bichir, had a basal position in the piscine tree. The findings put into question the phylogenetic validity of the taxonomic nomenclature attributed to various vertebrate, notably piscine, clades.     

Mesoderm and ectoderm lineages in the crustacean Parhyale hawaiensis display intra-germ layer compensation

In Parhyale hawaiensis, the first three divisions are holoblastic and asymmetric, resulting in an embryo comprised of eight cells—four macromeres and four micromeres. Lineage studies performed at this stage demonstrate that the progeny of each cell contribute to specific portions of different germ layers. However, it is not known if this lineage pattern means a given blastomere is committed to its specific fate, indicative of mosaic development, or if regulation can occur between blastomere progeny so that the loss of a blastomere could be compensated for during development. Furthermore, if compensation occurs, what would be the source of such replacement? To investigate these possibilities, we performed ablation experiments at the eight-cell stage. We find that loss of blastomeres results in compensation. To determine the compensation pattern, we combined ablation and cell lineage tracing to reveal that progeny of mesoderm and ectoderm producing blastomeres display intra-germ layer compensation. Furthermore, by ablating lineages later in development, we identify a key interval between gastrulation and germband elongation after which compensation no longer occurs. Our results suggest that Parhyale possesses a mechanism to assess the status of mesoderm and ectoderm formation and alter development to replace the missing portions of these lineages.     

Functional diversification of sea lamprey globins in evolution and development

Agnathans have a globin repertoire that markedly differs from that of jawed (gnathostome) vertebrates. The sea lamprey (Petromyzon marinus) harbors at least 18 hemoglobin, two myoglobin, two globin X, and one cytoglobin genes. However, agnathan hemoglobins and myoglobins are not orthologous to their cognates in jawed vertebrates. Thus, blood-based O₂ transport and muscle-based O₂ storage proteins emerged twice in vertebrates from a tissue-globin ancestor. Notably, the sea lamprey displays three switches in hemoglobin expression in its life cycle, analogous to hemoglobin switching in vertebrates. To study the functional changes associated with the evolution and ontogenesis of distinct globin types, we determined O₂ binding equilibria, type of quaternary assembly, and nitrite reductase enzymatic activities of one adult (aHb5a) and one embryonic/larval hemoglobin (aHb6), myoglobin (aMb1) and cytoglobin (Cygb) of the sea lamprey. We found clear functional differentiation among globin types expressed at different developmental stages and in different tissues. Cygb and aMb1 have high O₂ affinity and nitrite reductase activity, while the two hemoglobins display low O₂ affinity and nitrite reductase activity. Cygb and aHb6 but not aHb5a show cooperative O₂ binding, correlating with increased stability of dimers, as shown by gel filtration and molecular modeling. The high O₂-affinity and the lack of cooperativity confirm the identity of the sea lamprey aMb1 as O₂ storage protein of the muscle. The dimeric structure and O₂-binding properties of sea lamprey and mammalian Cygb were very similar, suggesting a conservation of function since their divergence around 500 million years ago.     

The competence of Xenopus blastomeres to produce neural and retinal progeny is repressed by two endo-mesoderm promoting pathways

Only a subset of cleavage stage blastomeres in the Xenopus embryo is competent to contribute cells to the retina; ventral vegetal blastomeres do not form retina even when provided with neuralizing factors or transplanted to the most retinogenic position of the embryo. These results suggest that endogenous maternal factors in the vegetal region repress the ability of blastomeres to form retina. Herein we provide three lines of evidence that two vegetal-enriched maternal factors (VegT, Vg1), which are known to promote endo-mesodermal fates, negatively regulate which cells are competent to express anterior neural and retinal fates. First, both molecules can repress the ability of dorsal-animal retinogenic blastomeres to form retina, converting the lineage from neural/retinal to non-neural ectodermal and endo-mesodermal fates. Second, reducing the endogenous levels of either factor in dorsal-animal retinogenic blastomeres expands expression of neural/retinal genes and enlarges the retina. The dorsal-animal repression of neural/retinal fates by VegT and Vg1 is likely mediated by Sox17alpha and Derriere but not by XNr1. VegT and Vg1 likely exert their effects on neural/retinal fates through at least partially independent pathways because Notch1 can reverse the effects of VegT and Derriere but not those of Vg1 or XNr1. Third, reduction of endogenous VegT and/or Vg1 in ventral vegetal blastomeres can induce a neural fate, but only allows expression of a retinal fate when both BMP and Wnt signaling pathways are concomitantly repressed.     

First cleavages,preblastula and blastula in the parthenogenetic mite Archegozetes longisetosus (Acari,Oribatida) indicate holoblastic rather than superficial cleavage

The mode of cleavage in the Acari is generalized as superficial or intralecithal, with a preceding phase of total (holoblastic) cleavage, but the knowledge is fragmentary and conclusions have been inconsistent, even when relating to the same species. Since no data about early embryology is available for the speciose group Oribatida, we studied Archegozetes longisetosus using transmission electron microscopy. We focused on early cleavages and the formation of the blastula, as these are the important and controversial points in early embryology of the Acari. We expected, as postulated for other acarine eggs, the early cleavages to be holoblastic and followed by a superficial preblastoderm stage. The early cleavages of A. longisetosus are holoblastic and blastomeres give rise to yolk-free micromeres and macromeres containing all the yolk. In contrast to expectations, the micromeres do not form a superficial preblastoderm layer. They are scattered along the embryonic surface and form an external, monocellular layer that covers the whole surface of the embryo. Since each of the existing TEM studies of mites shows this same pattern, and since this specialized form of total cleavage seems to be unique in Chelicerata, it may be the general mode of cleavage in Acari. However, the question will require much more investigation, especially since most data relate to the Actinotrichida and very few are currently available for species in the other major group, the Anactinotrichida.