Dermestes maculatus: an intermediate-germ beetle model system for evo-devo

Regulatory gene function handoff allows essential gene loss in mosquitoes

Regulatory genes are often multifunctional and constrained, which results in evolutionary conservation. It is difficult to understand how a regulatory gene could be lost from one species’ genome when it is essential for viability in closely related species. The gene paired is a classic Drosophila pair-rule gene, required for formation of alternate body segments in diverse insect species. Surprisingly, paired was lost in mosquitoes without disrupting body patterning. Here, we demonstrate that a paired family member, gooseberry, has acquired paired-like expression in the malaria mosquito Anopheles stephensi. Anopheles-gooseberry CRISPR-Cas9 knock-out mutants display pair-rule phenotypes and alteration of target gene expression similar to what is seen in Drosophila and beetle paired mutants. Thus, paired was functionally replaced by the related gene, gooseberry, in mosquitoes. Our findings document a rare example of a functional replacement of an essential regulatory gene and provide a mechanistic explanation of how such loss can occur.

Cheatle Jarvela et al. demonstrate in the mosquito Anopheles stephensi that the paired gene was functionally replaced by the gene gooseberry, even though paired is essential in other insects such as fruit flies and beetles. This study contributes to the understanding of how essential genes are lost despite their importance during development.

Stagewise resolution of temperature-dependent embryonic and postembryonic development in the cowpea seed beetle Callosobruchus maculatus (F.)

BACKGROUND

The thermal plasticity of life-history traits receives wide attention in the recent biological literature. Of all the temperature-dependent traits studied, developmental rates of ectotherms are especially often addressed, and yet surprisingly little is known about embryonic responses to temperature, including changes in the thermal thresholds and thermal sensitivity during early development. Even postembryonic development of many cryptically living species is understood superficially at best.

RESULTS

This study is the first to estimate the exact durations of developmental stages in the cowpea seed beetle C. maculatus from oviposition to adult emergence at five permissive constant temperatures from 20 to 32 °C. Early embryonic development was tracked and documented by means of destructive sampling and subsequent confocal imaging of fluorescently stained specimens. Late embryonic and early larval development was studied with the use of destructive sampling and light microscopy. Well-resolved temporal series based on thousands of embryos allowed precise timing of the following developmental events: formation of the blastoderm; formation, elongation, and retraction of the germ band; dorsal closure; the onset and completion of sclerotization of the cuticle; hatching, and penetration of the first-instar larva into the cowpea seed. Pupation and adult eclosion were observed directly through an incision in the seed coat. The thermal phenotype of C. maculatus was found to vary in the course of ontogeny and different stages scaled disproportionately with temperature, but pitfalls and caveats associated with analyses of relative durations of individual stages are also briefly discussed.

CONCLUSION

Disproportionate changes in developmental durations with temperature may have important implications when study design requires a high degree of synchronization among experimental embryos or when the occurrence of particular stages in the field is of interest, as well as in any other cases when development times need to be estimated with precision. This work provides one of the first examples of integration of embryological techniques with ecophysiological concepts and will hopefully motivate similar projects in the future. While experiments with Drosophila continue to be the main source of information on animal development, knowledge on other model species is instrumental to building a broader picture of developmental phenomena.

An Overview of Embryogenesis: External Morphology and Transcriptome Profiling in the Hemipteran Insect Nilaparvata lugens

During embryogenesis of insects, the morphological and transcriptional changes are important signatures to obtain a better understanding of insect patterning and evolution. The brown planthopper Nilaparvata lugens is a serious insect pest of rice plants, but its embryogenesis has not uncovered. Here, we described embryonic development process of the pest and found it belongs to an intermediate-germ mode. The RNA-seq data from different times (6, 30, 96, and 150 h, after egg laying) of embryogenesis were then analyzed, and a total of 10,895 genes were determined as differentially expressed genes (DEGs) based on pairwise comparisons. Afterward, 1,898 genes, differentially expressed in at least two comparisons of adjacent embryonic stages were divided into 10 clusters using K means cluster analysis (KMCA). Eight-gene modules were established using a weighted gene co-expression network analysis (WGCNA). Gene expression patterns in the different embryonic stages were identified by combining the functional enrichments of the stage-specific clusters and modules, which displayed the expression level and reprogramming of multiple developmental genes during embryogenesis. The "hub" genes at each embryonic stage with possible crucial roles were identified. Notably, we found a "center" set of genes that were related to overall membrane functions and might play important roles in the embryogenesis process. After parental RNAi of the MSTRG.3372, the hub gene, the embryo was observed as abnormal. Furthermore, some homologous genes in classic embryonic development processes and signaling pathways were also involved in embryogenesis of this insect. An improved comprehensive finding of embryogenesis within the N. lugens reveals better information on genetic and genomic studies of embryonic development and might be a potential target for RNAi-based control of this insect pest.

Shifting roles of Drosophila pair-rule gene orthologs: segmental expression and function in the milkweed bug Oncopeltus fasciatus

The discovery of pair-rule genes (PRGs) in Drosophila revealed the existence of an underlying two-segment-wide prepattern directing embryogenesis. The milkweed bug Oncopeltus fasciatus, a hemimetabolous insect, is a more representative arthropod: most of its segments form sequentially after gastrulation. Here, we report the expression and function of orthologs of the complete set of nine Drosophila PRGs in Oncopeltus Seven Of-PRG-orthologs are expressed in stripes in the primordia of every segment, rather than every other segment; Of-runt is PR-like and several orthologs are also expressed in the segment addition zone. RNAi-mediated knockdown of Of-odd-skipped, paired and sloppy-paired impacted all segments, with no indication of PR-like register. We confirm that Of-E75A is expressed in PR-like stripes, although it is not expressed in this way in Drosophila, demonstrating the existence of an underlying PR-like prepattern in Oncopeltus These findings reveal that a switch occurred in regulatory circuits, leading to segment formation: while several holometabolous insects are 'Drosophila-like', using PRG orthologs for PR patterning, most Of-PRGs are expressed segmentally in Oncopeltus, a more basally branching insect. Thus, an evolutionarily stable phenotype - segment formation - is directed by alternate regulatory pathways in diverse species.

Speeding up anterior-posterior patterning of insects by differential initialization of the gap gene cascade

Recently, it was shown that anterior-posterior patterning genes in the red flour beetle Tribolium castaneum are expressed sequentially in waves. However, in the fruit fly Drosophila melanogaster, an insect with a derived mode of embryogenesis compared to Tribolium, anterior-posterior patterning genes quickly and simultaneously arise as mature gene expression domains that, afterwards, undergo slight posterior-to-anterior shifts. This raises the question of how a fast and simultaneous mode of patterning, like that of Drosophila, could have evolved from a rather slow sequential mode of patterning, like that of Tribolium. In this paper, we propose a mechanism for this evolutionary transition based on a switch from a uniform to a gradient-mediated initialization of the gap gene cascade by maternal Hb. The model is supported by computational analyses and experiments.

Conservation and variation in pair-rule gene expression and function in the intermediate-germ beetle Dermestes maculatus

A set of pair-rule (PR) segmentation genes (PRGs) promotes the formation of alternate body segments in Drosophila melanogaster Whereas Drosophila embryos are long-germ, with segments specified more or less simultaneously, most insects add segments sequentially as the germband elongates. The hide beetle Dermestes maculatus represents an intermediate between short- and long-germ development, ideal for comparative study of PRGs. We show that eight of nine Drosophila PRG orthologs are expressed in stripes in Dermestes Functional results parse these genes into three groups: Dmac-eve, -odd and -run play roles in both germband elongation and PR patterning; Dmac-slp and -prd function exclusively as complementary, classic PRGs, supporting functional decoupling of elongation and segment formation; and orthologs of ftz, ftz-f1, h and opa show more variable function in Dermestes and other species. While extensive cell death generally prefigured Dermestes PRG RNAi-mediated cuticle defects, an organized region with high mitotic activity near the margin of the segment addition zone is likely to have contributed to truncation of eve^RNAi embryos. Our results suggest general conservation of clock-like regulation of PR stripe addition in sequentially segmenting species while highlighting regulatory rewiring involving a subset of PRG orthologs.

Establishment of molecular genetic approaches to study gene expression and function in an invasive hemipteran, Halyomorpha halys

Hemiptera is a large clade of insects understudied in terms of developmental biology. Halyomorpha halys, the Brown Marmorated Stink Bug (BMSB, referred to throughout as H. halys), is an invasive hemipteran pest of the mid-Atlantic region of the USA that has rapidly spread to other regions in recent years, devastating a wide range of crops using a piercing and sucking mechanism. Its phylogenetic position, polyphagous habits, and rapid spread in the USA suggested that H. halys would be an ideal system to broaden our knowledge of developmental mechanisms in insects. We and others previously generated transcriptome sequences from different life stages of this insect. Here, we describe tools to examine gene expression patterns in whole-mount H. halys embryos and to test the response of H. halys to RNA interference (RNAi). We show that spatial and temporal patterns of gene expression in H. halys can be effectively monitored by both immunostaining and in situ hybridization. We also show that delivery of dsRNA to adult females knocks down gene function in offspring, using the homeotic gene Sex combs reduced (Scr). Knockdown of Hh-Scr resulted in dramatic malformations of the mouthparts, demonstrating for the first time that RNAi is effective in this species. Our results suggest that, despite difficulties with long-term laboratory culture of H. halys, this species shows promise as a developmental system.

Speed regulation of genetic cascades allows for evolvability in the body plan specification of insects

During the anterior-posterior fate specification of insects, anterior fates arise in a nonelongating tissue (called the "blastoderm"), and posterior fates arise in an elongating tissue (called the "germband"). However, insects differ widely in the extent to which anterior-posterior fates are specified in the blastoderm versus the germband. Here we present a model in which patterning in both the blastoderm and germband of the beetle Tribolium castaneum is based on the same flexible mechanism: a gradient that modulates the speed of a genetic cascade of gap genes, resulting in the induction of sequential kinematic waves of gap gene expression. The mechanism is flexible and capable of patterning both elongating and nonelongating tissues, and hence converting blastodermal to germband fates and vice versa. Using RNAi perturbations, we found that blastodermal fates could be shifted to the germband, and germband fates could be generated in a blastoderm-like morphology. We also suggest a molecular mechanism underlying our model, in which gradient levels regulate the switch between two enhancers: One enhancer is responsible for sequential gene activation, and the other is responsible for freezing temporal rhythms into spatial patterns. This model is consistent with findings in Drosophila melanogaster, where gap genes were found to be regulated by two nonredundant "shadow" enhancers.

Rearing and Double-stranded RNA-mediated Gene Knockdown in the Hide Beetle, Dermestes maculatus

Advances in genomics have raised the possibility of probing biodiversity at an unprecedented scale. However, sequence alone will not be informative without tools to study gene function. The development and sharing of detailed protocols for the establishment of new model systems in laboratories, and for tools to carry out functional studies, is thus crucial for leveraging the power of genomics. Coleoptera (beetles) are the largest clade of insects and occupy virtually all types of habitats on the planet. In addition to providing ideal models for fundamental research, studies of beetles can have impacts on pest control as they are often pests of households, agriculture, and food industries. Detailed protocols for rearing and maintenance of D. maculatus laboratory colonies and for carrying out dsRNA-mediated interference in D. maculatus are presented. Both embryonic and parental RNAi procedures-including apparatus set up, preparation, injection, and post-injection recovery-are described. Methods are also presented for analyzing embryonic phenotypes, including viability, patterning defects in hatched larvae, and cuticle preparations for unhatched larvae. These assays, together with in situ hybridization and immunostaining for molecular markers, make D. maculatus an accessible model system for basic and applied research. They further provide useful information for establishing procedures in other emerging insect model systems.

Emerging developmental genetic model systems in holometabolous insects

The number of insect species that are amenable to functional genetic studies is growing rapidly and provides many new research opportunities in developmental and evolutionary biology. The holometabolous insects represent a disproportionate percentage of animal diversity and are thus well positioned to provide model species for a wide variety of developmental processes. Here we discuss emerging holometabolous models, and review some recent breakthroughs. For example, flies and midges were found to use structurally unrelated long-range pattern organizers, butterflies and moths revealed extensive pattern formation during oogenesis, new imaging possibilities in the flour beetle Tribolium castaneum showed how embryos break free of their extraembryonic membranes, and the complex genetics governing interspecies difference in head shape were revealed in Nasonia wasps.