Early segmentation in the mite Archegozetes longisetosus reveals conserved and derived aspects of chelicerate development.

The arthropod body plan is comprised of several repeating segments along the anteroposterior body axis. This high degree of conservation, however, obfuscates the wide degree of underlying developmental variation present across and within arthropod groups. In chelicerates, the arthropod clade containing mites, spiders, scorpions, and horseshoe crabs, development is the most similar at the stages following early germ band segmentation. Comparative studies of chelicerate segmentation prior to these events, however, remain scarce. In order to elucidate and identify possible shared and derived aspects of chelicerate segmentation, we followed the early prosomal (anterior) segmentation in the model mite Archegozetes longisetosus using the expression of the conserved segmental marker hedgehog (hh). Our data indicate that the ancestral chelicerate likely utilized the gene hedgehog in a group of cells surrounding the germ disc. We also provide evidence that chelicerate segmentation, albeit via the conserved "short/intermediate germ" mode, progresses differently in the prosoma between Archegozetes and spiders and thus early, anterior segmentation in chelicerates is heterochronic.

A study of embryonic development in eriophyoid mites (Acariformes, Eriophyoidea) with the use of the fluorochrome DAPI and confocal microscopy.

The embryonic development of four eriophyoid mite species, Cecidophyopsis ribis, Phytoptus avellanae, Oziella liroi and Loboquintus subsquamatus, has been studied with the use of fluorochrome DAPI and confocal microscopy. The first three nuclear divisions occur on the egg periphery (the groups of 2, 4, and 6 nuclei have been recorded), while the biggest part of yolk remains undivided. After four or five nuclear divisions all nuclei are situated only in one sector of the embryo, while other sectors contain only yolk suggesting possible meroblastic cleavage. Later, the formation of superficial blastoderm takes place. A few large yolk cells are situated inside the embryo. Germ band formation initiates as funnel-like cell invagination and leads to formation of a typical stage with four paired prosomal buds (chelicerae, palps, legs I and II). Each palp contains two lobes (anterior and posterior), the adult subcapitulum is presumably a fusion product of the anterior pair of the lobes. Neither rudiments of legs III and IV, traces of opisthosomal segments nor remnants of the prelarval exuvium under the egg shell were detected. Overall, the pattern of embryonic development in eriophyoids re-emphasizes the peculiarity of this ancient group of miniaturized phytoparasitic animals, and invites researches to pursue a deeper investigation of various fundamental aspects of this aberrant group of Acari. Further studies using various fluorescent dyes and transmission electron microscopy are needed to visualize plasma membranes and clarify the pattern of early cleavage of eriophyoids.

The expression of limb gap genes in the mite Archegozetes longisetosus reveals differential patterning mechanisms in chelicerates.

The modular organization of arthropod limbs has lead to the evolution of a diversity of appendages within this phylum. A conserved trait within the arthropods is the utilization of a conserved set of regulatory genes that specify the appendage podomeres along the proximo-distal axis, termed the limb gap genes. These include extradenticle, homothorax, dachshund, and Distal-less. The deployment of these genes in the most basally branching arthropod group, the chelicerates, has only been studied in detail in two chelicerate groups, the harvestmen and spiders. Given the broad range of appendage diversity within the chelicerates, comparative studies of gap gene deployment in other chelicerates groups is needed. We therefore followed limb gap gene expression in a member of the largest chelicerate group, Acari, the oribatid mite Archegozetes longisetosus. We show that in contrast to many arthropod species, A. longisetosus expresses homothorax and extradenticle exclusively in the proximal portion of the appendages, which refutes the hypothesis of a sister-group relationship between chelicerates and myriapods. We also provide evidence that mites posses the ancestral chelicerate condition of possessing three-segmented chelicerae, which also express the gene dachshund. This adds support to the hypothesis that a cheliceral dachshund domain is ancestral to arachnids. Lastly, we provide evidence that the suppression of the fourth pair of walking legs, a putative synapomorphy for Acari, is accomplished by repressing the development of the medial and distal regions of the limb.

Delayed mite hatching in response to mechanical stimuli simulating egg predation attempts.

Delayed or induced hatching in response to predation risk has been reported mainly in aquatic systems, where waterborne cues from predators and injured neighbouring eggs are available. Newly emerged larvae of the terrestrial predatory mite Neoseiulus womersleyi are vulnerable to predation by con- and heterospecific predatory mites, whereas their eggs are not. We examined whether N. womersleyi embryos delay hatching in response to artificial mechanical stimuli that simulates egg predation attempts. When embryos near the hatching stage were artificially stimulated every 5 min for 60 min, most stopped hatching for the duration of the 60-min period, whereas unstimulated embryos did not. Stimulated embryos resumed hatching when the treatment was stopped, and the proportion of hatched stimulated embryos caught up with that of unstimulated embryos within 120 min after stimuli stopped. Since hatching did not stop in response to changes in gravity direction, the effect of direct mechanical stimuli on the eggs was considered a proximate factor in delayed hatching. These results suggest that N. womersleyi embryos recognise immediate predation risk via mechanical stimuli, and delay hatching until the predation risk is reduced.

Microscopical anatomy of larva of cheladonta costulata (Acarina: Trombiculidae) II. Body tegument and digestive system.

The cuticle of gnathosoma of Cheladonta costulata larva consists of a smooth and firm epicuticle and sclerotized procuticle. The cuticle of the idiosoma is elastic. The wall of the digestive system is formed by cells lying on the basal membrane. During the process of sucking, however, the intestinal cells, which were originally spherical, get an appearance of a club reaching up to the lumen of intestine. In a fully engorged larva the intestine appears to be united and fill the major part of body. The lumen of the intestine is filled with eosinophilic granules of various size, among which are minute granules with basophilic tinge. The excretory bladder is located between posterior lobes of the intestine. In an unengorged larva, the excretory bladder is conical and possesses a conspicuous wall of a cellular structure. Its lumen is empty. In a fully engorged larva, the excretory bladder is oval, covered with a thin membrane, and its lumen is filled with guanine crystals.

Microscopical anatomy of larva of Cheladonta costulata (Acarina: Trombiculidae). I. Glands.

The larva of Ch. costulata possesses seven paired glands which can be differentiated according to their position and histological structure. In five of them, the dynamics of their secretory or excretory activity may be studied histologically during the feeding process, when not only the deposition of secretion in the gland cells, but also the size of the whole gland is changed. The morphology of the remaining glands is not affected by feeding.

A structuralist theory of evolution reconsidered.

The structuralist theory of evolution is reconsidered in the light of new discoveries. According to this theory, the evolutionary potentialities are in the genotype (a hierarchically ordered set of interacting elements) and manifest themselves in the course of morphogenesis in association with changes in the environment. It is demonstrated that this theory is in fact the development of a long philosophical tradition, in which Darwin and Neo-Darwinism did not participate. New discoveries in the field of molecular cytogenetics confirm the ideas of evolutionary potentiality and hierarchical genotypic ordering. It is demonstrated that gene regulation can manifest itself in association with instabilities of the morphogenetic field and the attainment of a new equilibrium; this change could be connected with changes in the environment, but has nothing to do with natural selection.

Peculiarities in ontogenesis and reproductive cycle of the mite Varroa jacobsoni (Parasitiformes, Varroidae) and its relation to the host Apis mellifera.

On the basis on functional morphology and ecology of Varroa jacobsoni the reproductive cycle of female mites was divided into 5 stages: copulation, puberty, preoviposition, oviposition, rehabilitation. The whole ontogeny from egg to formed protonymph in the chorion proceeds very intensively (26 to 30 hours). Besides the reproductive potential is realized during an extremely short period (6 to 7 days) which duration is determined by the total duration of the mite's ontogeny, so that the development of adult mites in all of the viable eggs laid has to be completed by the time the bees leave the brood.

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.

Lead-induced hsp70 and hsp60 pattern transformation and leg malformation during postembryonic development in the oribatid mite, Archegozetes longisetosus Aoki.

The study aimed at analysing the impact of high lead concentrations on the morphological integrity and the stress protein hsp70 and hsp60 levels during postembryonic development of the oribatid mite, Archegozetes longisetosus. Independent of the treatment, the recorded hsp70 levels were far higher than the hsp60 levels in all investigated stages. There was a tendency towards lower hsp70 and hsp60 levels with proceeding development (deutonymph>tritonymph>adult) in untreated animals. Both the hsp70 and hsp60 levels in all investigated quiescent stages prior to moult were higher than in the corresponding active stages independent from lead exposure. Continuous lead treatment from the larval stage onwards caused malformation of the 4th pair of legs and, in parallel, a shift to elevated hsp70 (but not hsp60) levels in all subsequent stages, compared to controls. Neither effects occurred when continuous lead treatment started later in development. In this case, elevated hsp60 levels could particularly be found in those stages respectively following the initially exposed stage. The hsp70 response became obvious even quicker in tritonymphs and adults, where hsp70 level peaks could be observed right in those stages the lead exposure had started in.

Expression of homeobox genes shows chelicerate arthropods retain their deutocerebral segment.

Expression patterns of six homeobox containing genes in a model chelicerate, the oribatid mite Archegozetes longisetosus, were examined to establish homology of chelicerate and insect head segments and to investigate claims that the chelicerate deutocerebral segment has been reduced or lost. engrailed (en) expression, which has been used to demonstrate the presence of segments in insects, fails to demonstrate a reduced deutocerebral segment. Expression patterns of the chelicerate homologs of the Drosophila genes Antennapedia (Antp), Sex combs reduced (Scr), Deformed (Dfd), proboscipedia (pb), and orthodenticle (otd) confirm direct correspondence of head segments. The chelicerate deutocerebral segment has not been reduced or lost. We make further inferences concerning the evolution of heads and Hox genes in arthropods.

Of mites and zen: expression studies in a chelicerate arthropod confirm zen is a divergent Hox gene.

We have cloned, from an oribatid mite, a gene homologous to the zerknült (zen) genes of insects and the Hox 3 genes of vertebrates. Hox genes specify cell fates in specific regions of the body in all metazoans studied and are expressed in antero-posteriorly restricted regions of the embryo. This is true of the vertebrate Hox 3 but not of the zen genes, the insect homologs, and it has been proposed that the zen genes have lost their Hox-like function in the ancestor of the insects. We studied expression of a mite Hox 3/zen homolog and found that it is expressed in a discrete antero-posterior region of the body with an anterior boundary coinciding with that of the chelicerate homolog of the Drosophila Hox gene, proboscipedia, and propose that its loss of Hox function in insects is due to functional redundancy due to this overlap with another Hox gene.

Vasa expression and germ-cell specification in the spider mite Tetranychus urticae.

The specification of germ cells is an important process during the development of all animals. Expression of an evolutionarily conserved gene such as vasa can be used as a marker for germ cell fate. We have isolated a vasa-related gene from the two-spotted spider mite (Tetranychus urticae) and used it to examine the segregation of germ cells in this animal. In spider mites, vasa expression first appears in a group of cells that do not join the initial blastoderm surface. Instead, these cells remain in the interior of the blastoderm and then migrate to posterior regions of the embryo, where they form a cluster that appears in regions of the embryo consistent with the gonads. The expression pattern of this spider mite vasa homologue implies a novel process acts to specify germ cells in this species and that the specification of germ cells is an evolutionarily labile process.