Influential Insider: Wolbachia, an Intracellular Symbiont, Manipulates Bacterial Diversity in Its Insect Host

Facultative intracellular symbionts like the α-proteobacteria Wolbachia influence their insect host phenotype but little is known about how much they affect their host microbiota. Here, we quantified the impact of Wolbachia infection on the bacterial community of the cabbage root fly Delia radicum by comparing the microbiota of Wolbachia-free and infected adult flies of both sexes. We used high-throughput DNA sequencing (Illumina MiSeq, 16S rRNA, V5-V7 region) and performed a community and a network analysis. In both sexes, Wolbachia infection significantly decreased the diversity of D. radicum bacterial communities and modified their structure and composition by reducing abundance in some taxa but increasing it in others. Infection by Wolbachia was negatively correlated to 8 bacteria genera (Erwinia was the most impacted), and positively correlated to Providencia and Serratia. We suggest that Wolbachia might antagonize Erwinia for being entomopathogenic (and potentially intracellular), but would favor Providencia and Serratia because they might protect the host against chemical plant defenses. Although they might seem prisoners in a cell, endocellular symbionts can impact the whole microbiota of their host, hence its extended phenotype, which provides them with a way to interact with the outside world.

A high-quality functional genome assembly of Delia radicum L. (Diptera: Anthomiidae) annotated from egg to adult

Belowground herbivores are overseen and underestimated, even though they can cause significant economic losses in agriculture. The cabbage root fly Delia radicum (Anthomyiidae) is a common pest in Brassica species, including agriculturally important crops, such as oil seed rape. The damage is caused by the larvae, which feed specifically on the taproots of Brassica plants until they pupate. The adults are aboveground-living generalists feeding on pollen and nectar. Female flies are attracted by chemical cues in Brassica plants for oviposition. An assembled and annotated genome can elucidate which genetic mechanisms underlie the adaptation of D. radicum to its host plants and their specific chemical defenses, in particular isothiocyanates. Therefore, we assembled, annotated and analyzed the D. radicum genome using a combination of different Next Generation Sequencing and bioinformatic approaches. We assembled a chromosome-level D. radicum genome using PacBio and Hi-C Illumina sequence data. Combining Canu and 3D-DNA genome assembler, we constructed a 1.3 Gbp genome with an N50 of 242 Mbp and 6 pseudo-chromosomes. To annotate the assembled D. radicum genome, we combined homology-, transcriptome- and ab initio-prediction approaches. In total, we annotated 13,618 genes that were predicted by at least two approaches. We analyzed egg, larval, pupal and adult transcriptomes in relation to life-stage specific molecular functions. This high-quality annotated genome of D. radicum is a first step to understanding the genetic mechanisms underlying host plant adaptation. As such, it will be an important resource to find novel and sustainable approaches to reduce crop losses to these pests.

Identification of morphologically challenging Delia (Diptera: Anthomyiidae) species from field vegetable crops using polymerase chain reaction–restriction fragment length polymorphism (PCR-RFLP)

Root feeding by the larvae of multiple Delia species can lead to economic loss in many agricultural crops. Field vegetables are subject to infestations by a species complex composed of Delia radicum (Linnaeus) (Diptera: Anthomyiidae), a pest in brassica crops (Brassicaceae), Delia antiqua (Meigen), believed to cause the majority of crop damage in onions, and the generalists Delia florilega (Zetterstedt), Delia platura (Meigen), and Botanophila fugax (Meigen) (Diptera: Anthomyiidae). Correct species identification is necessary to implement field management strategies, but these species are challenging to identify morphologically. We propose a polymerase chain reaction–restriction fragment length polymorphism method as a molecular tool to distinguish between five species of Delia and between two genetic lines of D. platura. The mitochondrial DNA cytochrome c oxidase subunit 1 barcode fragment is targeted, then the polymerase chain reaction product digested with four different restriction enzymes (AccI, BsrI, MlyI, and StyI). The BsrI enzyme distinguishes the two genetic lines of D. platura and D. florilega. The MlyI enzyme identifies B. fugax from the Delia species. Combining BsrI, StyI, AccI, and MlyI into double digestion reactions allows for rapid diagnostics among the species tested. Our method was validated using DNA from specimens collected in eastern Canada. This method provides tools in ecological and environmental studies where these species are of interest.

Connection between Nutrient Content and Resistance to Selected Pests Analyzed in Brassicaceae Hybrids

Rapeseed (Brassica napus L. ssp. oleifera Metzg) is attacked by a wide range of pests. Breeding of resistant cultivars is one of the best methods of protecting crops against insects, and non-cultivated species of Brassicaceae can be used as resistance donors. In this study, we investigated the relationship between nutrient content and resistance to two commonly occurring pests (Delia radicum and Brevicoryne brassicae) in 12 Brassicaceae hybrid combinations. The experiment was conducted at Poznań University of Life Sciences Experimental Station located in Dłoń. The assessment was carried out for three years (2017, 2018, and 2019) on randomly chosen individuals in two growth stages: vegetative stage and flowering stage. The resistance to B. brassicae and D. radicum was observed in all three years, however, macronutrient and micronutrient contents as well as total nitrogen, total sulfur, and sulfur sulfate were observed only in 2019. Statistically significant negative association between Ca and resistance to B. brassicae and D. radicum was discovered. Additionally, positive association between Cu and resistance to B. brassicae was observed. The importance of selected mineral elements in plant resistance is widely discussed.

Root-associated entomopathogenic fungi manipulate host plants to attract herbivorous insects

Root-associated entomopathogenic fungi (R-AEF) indirectly influence herbivorous insect performance. However, host plant-R-AEF interactions and R-AEF as biological control agents have been studied independently and without much attention to the potential synergy between these functional traits. In this study, we evaluated behavioral responses of cabbage root flies [Delia radicum L. (Diptera: Anthomyiidae)] to a host plant (white cabbage cabbage Brassica oleracea var. capitata f. alba cv. Castello L.) with and without the R-AEF Metarhizium brunneum (Petch). We performed experiments on leaf reflectance, phytohormonal composition and host plant location behavior (behavioral processes that contribute to locating and selecting an adequate host plant in the environment). Compared to control host plants, R-AEF inoculation caused, on one hand, a decrease in reflectance of host plant leaves in the near-infrared portion of the radiometric spectrum and, on the other, an increase in the production of jasmonic, (+)-7-iso-jasmonoyl-l-isoleucine and salicylic acid in certain parts of the host plant. Under both greenhouse and field settings, landing and oviposition by cabbage root fly females were positively affected by R-AEF inoculation of host plants. The fungal-induced change in leaf reflectance may have altered visual cues used by the cabbage root flies in their host plant selection. This is the first study providing evidence for the hypothesis that R-AEF manipulate the suitability of their host plant to attract herbivorous insects.

Relationships among root maggot (Delia spp., Diptera: Anthomyiidae) infestation, root injury, and seed yields of canola Brassica napus L. and Brassica rapa L.

This study investigated the relationships between numbers of Delia species (Delia spp. Rob.-Desv., Diptera: Anthomyiidae) flies or eggs and root injury levels of Brassica napus L. and Brassica rapa L. (Brassicaceae) canola (oilseed rape), and between root injury levels and canola seed yields. Wild-collected Delia species adult flies or laboratory-obtained Delia radicum (L.) eggs were introduced into laboratory or field cages containing B. napus or B. rapa plants. The degree of root injury was strongly associated with infestation level using both flies and eggs. Linear response curves between numbers of Delia flies and root injury levels and quadrilinear response curves between numbers of D. radicum eggs and root injury levels showed that root damage increased with increasing levels of infestation; at high egg infestation levels, root damage levelled off in both Brassica species. Seed yield was not clearly related to fly or egg infestation rate, nor to ensuing root damage. No consistent yield relationship in B. napus ‘AC Excel’ was found with levels of root damage seen in the investigation. Conversely, B. rapa cultivar ‘AC Boreal’ seed yields tended to increase above uninfested controls when feeding damage was light but decreased with increased root injury. Despite procedures used to prevent infestation in Delia-free control cages, some root maggot damage was found in control cages in all 4 yr of the field study. Changes in climate, land use patterns, and cropping systems could influence the economic impact of this pest complex on canola in the future.

In-field screening for host plant resistance to Delia radicum and Brevicoryne brassicae within selected rapeseed cultivars and new interspecific hybrids

Rapeseed (Brassica napus) can be attacked by a wide range of pests, for example, cabbage root fly (Delia radicum) and cabbage aphid (Brevicoryne brassicae). One of the best methods of pest management is breeding for insect resistance in rapeseed. Wild genotypes of Brassicaceae and rapeseed cultivars can be used as a source of resistance. In 2017, 2018, and 2019, field trials were performed to assess the level of resistance to D. radicum and B. brassicae within 53 registered rapeseed cultivars and 31 interspecific hybrid combinations originating from the resources of the Department of Genetics and Plant Breeding of Poznań University of Life Sciences (PULS). The level of resistance varied among genotypes and years. Only one hybrid combination and two B. napus cultivars maintained high level of resistance in all tested years, i.e., B. napus cv. Jet Neuf × B. carinata – PI 649096, Galileus, and Markolo. The results of this research indicate that resistance to insects is present in Brassicaceae family and can be transferred to rapeseed cultivars. The importance of continuous improvement of rapeseed pest resistance and the search for new sources of resistance is discussed; furthermore, plans for future investigations are presented.