A partial genome assembly of the miniature parasitoid wasp, Megaphragma amalphitanum

The first complete 3D reconstruction and morphofunctional mapping of an insect eye

The structure of compound eyes in arthropods has been the subject of many studies, revealing important biological principles. Until recently, these studies were constrained by the two-dimensional nature of available ultrastructural data. By taking advantage of the novel three-dimensional ultrastructural dataset obtained using volume electron microscopy, we present the first cellular-level reconstruction of the whole compound eye of an insect, the miniaturized parasitoid wasp Megaphragma viggianii. The compound eye of the female M. viggianii consists of 29 ommatidia and contains 478 cells. Despite the almost anucleate brain, all cells of the compound eye contain nuclei. As in larger insects, the dorsal rim area of the eye in M. viggianii contains ommatidia that are believed to be specialized in polarized light detection as reflected in their corneal and retinal morphology. We report the presence of three 'ectopic' photoreceptors. Our results offer new insights into the miniaturization of compound eyes and scaling of sensory organs in general.

Chromosomal-level genome assembly of Trichogramma dendrolimi (Trichogrammatidae: Hymenoptera)

Trichogramma dendrolimi is an essential egg parasitic wasp with a broad host range and has been widely used for controlling agricultural and forestry pests. Despite the availability of fragmented genomes of Trichogramma, a high-quality, chromosome-level genome reference is not yet available for this diverse genus. In this study, we assembled a high-quality chromosome-level genome of T. dendrolimi using PacBio CLR long sequencing and further scaffolded it with Hi-C technologies. The genome size was 216.24 Mb, featuring a N50 of 39.07 Mb, and 98.79% of scaffolds were anchored to five chromosomes. In addition, we annotated 12,902 protein-coding genes and 60.74 Mb repeat sequences for this genome assembly. In conclusion, our chromosome-level genome assembly provided important genomic resources that benefit the utilization of Trichogramma parasitoids in pest control.

A complete reconstruction of the early visual system of an adult insect

For most model organisms in neuroscience, research into visual processing in the brain is difficult because of a lack of high-resolution maps that capture complex neuronal circuitry. The microinsect Megaphragma viggianii, because of its small size and non-trivial behavior, provides a unique opportunity for tractable whole-organism connectomics. We image its whole head using serial electron microscopy. We reconstruct its compound eye and analyze the optical properties of the ommatidia as well as the connectome of the first visual neuropil-the lamina. Compared with the fruit fly and the honeybee, Megaphragma visual system is highly simplified: it has 29 ommatidia per eye and 6 lamina neuron types. We report features that are both stereotypical among most ommatidia and specialized to some. By identifying the "barebones" circuits critical for flying insects, our results will facilitate constructing computational models of visual processing in insects.

Multiscale head anatomy of Megaphragma (Hymenoptera: Trichogrammatidae)

Methods of three-dimensional electron microscopy have been actively developed recently and open up great opportunities for morphological work. This approach is especially useful for studying microinsects, since it is possible to obtain complete series of high-resolution sections of a whole insect. Studies on the genus Megaphragma are especially important, since the unique phenomenon of lysis of most of the neuron nuclei was discovered in species of this genus. In this study we reveal the anatomical structure of the head of Megaphragma viggianii at all levels from organs to subcellular structures. Despite the miniature size of the body, most of the organ systems of M. viggianii retain the structural plan and complexity of organization at all levels. The set of muscles and the well-developed stomatogastric nervous system of this species correspond to those of larger insects, and there is also a well-developed tracheal system in the head of this species. Reconstructions of the head of M. viggianii at the cellular and subcellular levels were obtained, and of volumetric data were analyzed. A total of 689 nucleated cells of the head were reconstructed. The ultrastructure of M. viggianii is surprisingly complex, and the evolutionary benefits of such complexity are probably among the factors limiting the further miniaturization of parasitoid wasps.

The 3D ultrastructure of the chordotonal organs in the antenna of a microwasp remains complex although simplified

Insect antennae are astonishingly versatile and have multiple sensory modalities. Audition, detection of airflow, and graviception are combined in the antennal chordotonal organs. The miniaturization of these complex multisensory organs has never been investigated. Here we present a comprehensive study of the structure and scaling of the antennal chordotonal organs of the extremely miniaturized parasitoid wasp Megaphragma viggianii based on 3D electron microscopy. Johnston's organ of M. viggianii consists of 19 amphinematic scolopidia (95 cells); the central organ consists of five scolopidia (20 cells). Plesiomorphic composition includes one accessory cell per scolopidium, but in M. viggianii this ratio is only 0.3. Scolopale rods in Johnston's organ have a unique structure. Allometric analyses demonstrate the effects of scaling on the antennal chordotonal organs in insects. Our results not only shed light on the universal principles of miniaturization of sense organs, but also provide context for future interpretation of the M. viggianii connectome.

Metamorphosis and denucleation of the brain in the miniature wasp Megaphragma viggianii (Hymenoptera: Trichogrammatidae)

Holometabolan brains undergo structural and allometric changes and complex reorganizations during metamorphosis. In minute egg parasitoids, brain formation is shifted to the late larva and young pupa, due to extreme de-embryonization. The brains of Megaphragma wasps undergo denucleation, the details of which remained unknown. We describe the morphological and volumetric changes in the brain of Megaphragma viggianii (Trichogrammatidae) during pupal development with emphasis on the lysis of nuclei and show that the absolute and relative volume of the brain decrease by a factor of 5 from prepupa to adult at the expense of the cell body rind. The first foci of lysis appear during early pupal development, but most nuclei (up to 97%) are lost between pharate adult and adult. The first signs of lysis (destruction of the nuclear envelopes) occur in pupae with red eyes. The number of lysis foci (organelle destruction and increasing number of lysosomes and degree of chromatin compaction) strongly increases in pupae with black eyes. The cell body rind volume strongly decreases during pupal development (in larger insects it increases slightly or remains unchanged). Elucidation of the lysis of nuclei in neurons and of the functioning of an anucleate brain is an important objective for neuroscience.

Intergeneric hybridization of two stickleback species leads to introgression of membrane-associated genes and invasive TE expansion

Interspecific hybridization has occurred relatively frequently during the evolution of vertebrates. This process usually abolishes reproductive isolation between the parental species. Moreover, it results in the exchange of genetic material and can lead to hybridogenic speciation. Hybridization between species has predominately been observed at the interspecific level, whereas intergeneric hybridization is rarer. Here, using whole-genome sequencing analysis, we describe clear and reliable signals of intergeneric introgression between the three-spined stickleback (Gasterosteus aculeatus) and its distant mostly freshwater relative the nine-spined stickleback (Pungitius pungitius) that inhabit northwestern Russia. Through comparative analysis, we demonstrate that such introgression phenomena apparently take place in the moderate-salinity White Sea basin, although it is not detected in Japanese sea stickleback populations. Bioinformatical analysis of the sites influenced by introgression showed that they are located near transposable elements, whereas those in protein-coding sequences are mostly found in membrane-associated and alternative splicing-related genes.

Revision of the World Species of Megaphragma Timberlake (Hymenoptera: Trichogrammatidae)

Megaphragma species are important models for basic organismal research, and many are potential biological control agents. We present the first extensive revision of species of the genus Megaphragma based on morphological and molecular data. Our revision includes all previously described species, 6 of which are synonymized, and 22 of which are described here as new. We also provide the first key to all species of the genus and reconstruct their phylogeny based on 28S and CO1 molecular markers. The following species are synonymized with M. longiciliatum Subba Rao: M. aligarhensis Yousuf and Shafee syn. nov.; M. amalphitanum Viggiani syn. nov.; M. decochaetum Lin syn. nov.; M. magniclava Yousuf and Shafee syn. nov.; M. shimalianum Hayat syn. nov.M. anomalifuniculi Yuan and Lou syn. nov. is synonymized with M. polychaetum Lin. The following species are described as new: M. antecessor Polaszek and Fusu sp. nov.; M. breviclavum Polaszek and Fusu sp. nov.; M. chienleei Polaszek and Fusu sp. nov.; M. cockerilli Polaszek and Fusu sp. nov.; M. digitatum Polaszek and Fusu sp. nov.; M. fanenitrakely Polaszek and Fusu sp. nov.; M. funiculatum Fusu, Polaszek, and Viggiani sp. nov.; M. giraulti Viggiani, Fusu, and Polaszek sp. nov.; M. hansoni Polaszek, Fusu, and Viggiani sp. nov.; M. kinuthiae Polaszek, Fusu, and Viggiani sp. nov.; M. liui Polaszek and Fusu sp. nov.; M. momookherjeeae Polaszek and Fusu sp. nov.; M. nowickii Polaszek, Fusu, and Viggiani sp. nov.; M. noyesi Polaszek and Fusu sp. nov.; M. pintoi Viggiani sp. nov.; M. polilovi Polaszek, Fusu, and Viggiani sp. nov.; M. rivelloi Viggiani sp. nov.; M. tamoi Polaszek, Fusu, and Viggiani sp. nov.; M. tridens Fusu, and Polaszek sp. nov.; M. uniclavum Polaszek and Fusu sp. nov.; M. vanlentereni Polaszek and Fusu sp. nov.; M. viggianii Fusu, Polaszek, and Polilov sp. nov.

An integrated transcriptomic and proteomic approach to identify the main Torymus sinensis venom components

During oviposition, ectoparasitoid wasps not only inject their eggs but also a complex mixture of proteins and peptides (venom) in order to regulate the host physiology to benefit their progeny. Although several endoparasitoid venom proteins have been identified, little is known about the components of ectoparasitoid venom. To characterize the protein composition of Torymus sinensis Kamijo (Hymenoptera: Torymidae) venom, we used an integrated transcriptomic and proteomic approach and identified 143 venom proteins. Moreover, focusing on venom gland transcriptome, we selected additional 52 transcripts encoding putative venom proteins. As in other parasitoid venoms, hydrolases, including proteases, phosphatases, esterases, and nucleases, constitute the most abundant families in T. sinensis venom, followed by protease inhibitors. These proteins are potentially involved in the complex parasitic syndrome, with different effects on the immune system, physiological processes and development of the host, and contribute to provide nutrients to the parasitoid progeny. Although additional in vivo studies are needed, initial findings offer important information about venom factors and their putative host effects, which are essential to ensure the success of parasitism.

Conservative route to genome compaction in a miniature annelid

The causes and consequences of genome reduction in animals are unclear because our understanding of this process mostly relies on lineages with often exceptionally high rates of evolution. Here, we decode the compact 73.8-megabase genome of Dimorphilus gyrociliatus, a meiobenthic segmented worm. The D. gyrociliatus genome retains traits classically associated with larger and slower-evolving genomes, such as an ordered, intact Hox cluster, a generally conserved developmental toolkit and traces of ancestral bilaterian linkage. Unlike some other animals with small genomes, the analysis of the D. gyrociliatus epigenome revealed canonical features of genome regulation, excluding the presence of operons and trans-splicing. Instead, the gene-dense D. gyrociliatus genome presents a divergent Myc pathway, a key physiological regulator of growth, proliferation and genome stability in animals. Altogether, our results uncover a conservative route to genome compaction in annelids, reminiscent of that observed in the vertebrate Takifugu rubripes.

This study reports the genome of the miniature segmented annelid Dimorphilus gyrociliatus and reveals no drastic changes in genome architecture and regulation, unlike other cases of genome miniaturization.