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

Insights into gene manipulation techniques for Acari functional genomics

Functional genomics is an essential tool for elucidating the structure and function of genes in any living organism. Here, we review the use of different gene manipulation techniques in functional genomics of Acari (mites and ticks). Some of these Acari species inflict severe economic losses to managed crops and health problems to humans, wild and domestic animals, but many also provide important ecosystem services worldwide. Currently, RNA interference (RNAi) is the leading gene expression manipulation tool followed by gene editing via the bacterial type II Clustered Regularly Interspaced Short Palindromic Repeats and associated protein 9 system (CRISPR-Cas9). Whilst RNAi, via siRNA, does not always lead to expected outcomes, the exploitations of the CRISPR systems in Acari are still in their infancy and are limited only to CRISP/Cas9 to date. In this review, we discuss the advantages and disadvantages of RNAi and CRISPR-Cas9 and the technical challenges associated with their exploitations. We also compare the biochemical machinery of RNAi and CRISPR-Cas9 technologies. We highlight some potential solutions for experimental optimization of each mechanism in gene function studies. The potential benefits of adopting various CRISPR-Cas9 systems for expanding on functional genomics experiments in Acari are also discussed.

Blastocoel morphogenesis: A biophysics perspective

The blastocoel is a fluid-filled cavity characteristic of animal embryos at the blastula stage. Its emergence is commonly described as the result of cleavage patterning, but this historical view conceals a large diversity of mechanisms and overlooks many unsolved questions from a biophysics perspective. In this review, we describe generic mechanisms for blastocoel morphogenesis, rooted in biological literature and simple physical principles. We propose novel directions of study and emphasize the importance to study blastocoel morphogenesis as an evolutionary and physical continuum.

De novo biosynthesis of simple aromatic compounds by an arthropod (Archegozetes longisetosus)

The ability to synthesize simple aromatic compounds is well known from bacteria, fungi and plants, which all share an exclusive biosynthetic route—the shikimic acid pathway. Some of these organisms further evolved the polyketide pathway to form core benzenoids via a head-to-tail condensation of polyketide precursors. Arthropods supposedly lack the ability to synthesize aromatics and instead rely on aromatic amino acids acquired from food, or from symbiotic microorganisms. The few studies purportedly showing de novo biosynthesis via the polyketide synthase (PKS) pathway failed to exclude endosymbiotic bacteria, so their results are inconclusive. We investigated the biosynthesis of aromatic compounds in defence secretions of the oribatid mite Archegozetes longisetosus. Exposing the mites to a diet containing high concentrations of antibiotics removed potential microbial partners but did not affect the production of defensive benzenoids. To gain insights into benzenoid biosynthesis, we fed mites with stable-isotope labelled precursors and monitored incorporation with mass spectrometry. Glucose, malonic acid and acetate, but not phenylalanine, were incorporated into the benzenoids, further evidencing autogenous biosynthesis. Whole-transcriptome sequencing with hidden Markov model profile search of protein domain families and subsequent phylogenetic analysis revealed a putative PKS domain similar to an actinobacterial PKS, possibly indicating a horizontal gene transfer.

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.

Triggering chemical defense in an oribatid mite using artificial stimuli

Most oribatid mites are well known for their exocrine oil gland secretions, from which more than a hundred different chemical components (hydrocarbons, terpenes, aromatics and alkaloids) have been described. The biological functions of these secretions have remained enigmatic for most species, but alarm-pheromonal and allomonal functions have been hypothesized, and demonstrated in some cases. Here, we tested different experimental stimuli to induce the release of defensive secretions in the model oribatid mite Archegozetes longisetosus Aoki. Whereas various mechanical stimuli did not result in a reproducible and complete expulsion of oil gland secretions, repeated treatments with hexane led to complete discharge. Life history parameters such as survival, development and reproduction were not influenced by the hexane treatment. Repeated hexane treatments also resulted in a complete depletion of oil glands in Euphthiracarus cribrarius Berlese.