Recent Advancements in Studies on Chemosensory Mechanisms Underlying Detection of Semiochemicals in Dacini Fruit Flies of Economic Importance (Diptera: Tephritidae)

Dacini fruit flies mainly contain two genera, Bactrocera and Zeugodacus, and include many important pests of fruits and vegetables. Their life cycle is affected by various environmental cues. Among them, multiple characteristic semiochemicals have remarkable effects on their reproductive and host-finding behaviors. Notably, floral fragrances released from so-called fruit fly orchids strongly attract males of several Dacini fruit fly species. Focusing on the strong attraction of male flies to particular chemicals, natural and synthetic lures have been used for pest management. Thus, the perception of semiochemicals is important to understand environmental adaptation in Dacini fruit flies. Since next-generation sequencers are available, a large number of chemosensory-related genes have been identified in Dacini fruit flies, as well as other insects. Furthermore, recent studies have succeeded in the functional analyses of olfactory receptors in response to semiochemicals. Thus, characterization of molecular components required for chemoreception is under way. However, the mechanisms underlying chemoreception remain largely unknown. This paper reviews recent findings on peripheral mechanisms in the perception of odors in Dacini fruit flies, describing related studies in other dipteran species, mainly the model insect Drosophilamelanogaster. Based on the review, important themes for future research have also been discussed.

The Dacini fruit fly fauna of Sulawesi fits Lydekker's line but also supports Wallacea as a biogeographic region (Diptera, Tephritidae)

Although there is scientific consensus on most of the major biogeographic regions in the world, the demarcation of the area connecting Southeast Asia with Australia and Oceania remains debated. Two candidate boundaries potentially explain faunistic diversity patterns in the regions: Lydekker's and Wallace's lines. The islands in between both 'lines' are jointly termed Wallacea, with Sulawesi as the largest landmass. We surveyed Dacini fruit flies (Tephritidae: Dacinae) in Sulawesi between 2016 and 2019 using traps baited with male lures, resulting in 4,517 collected flies. We identified all specimens to species level, which adds 15 new species records to the island, bringing the total number of Dacini species in Sulawesi to 83. The biogeographic affinity of species in the updated checklist reveals a strong connection with former 'Sunda' (41% of species); validating Lydekker's line, but also a high level of endemism (47% of species), confirming the uniqueness of Wallacea as a biogeographic region. We further describe a new species, Bactrocera (Bactrocera) niogreta Doorenweerd, sp. nov. and discuss the taxonomy of several interesting species.

Comparative Microbiomics of Tephritid Frugivorous Pests (Diptera: Tephritidae) From the Field: A Tale of High Variability Across and Within Species

The family Tephritidae includes some of the most notorious insect pests of agricultural and horticultural crops in tropical and sub-tropical regions. Despite the interest in the study of their gut microbiome, our present knowledge is largely based on the analysis of laboratory strains. In this study, we present a first comparative analysis of the gut microbiome profiles of field populations of ten African and Mediterranean tephritid pests. For each species, third instar larvae were sampled from different locations and host fruits and compared using 16S rRNA amplicon sequencing and a multi-factorial sampling design. We observed considerable variation in gut microbiome diversity and composition both between and within fruit fly species. A “core” microbiome, shared across all targeted species, could only be identified at most at family level (Enterobacteriaceae). At genus level only a few bacterial genera (Klebsiella, Enterobacter, and Bacillus) were present in most, but not all, samples, with high variability in their relative abundance. Higher relative abundances were found for seven bacterial genera in five of the fruit fly species considered. These were Erwinia in Bactrocera oleae, Lactococcus in B. zonata, Providencia in Ceratitis flexuosa, Klebsiella, and Rahnella in C. podocarpi and Acetobacter and Serratia in C. rosa. With the possible exception of C. capitata and B. dorsalis (the two most polyphagous species considered) we could not detect obvious relationships between fruit fly dietary breadth and microbiome diversity or abundance patterns. Similarly, our results did not suggest straightforward differences between the microbiome profiles of species belonging to Ceratitis and the closely related Bactrocera/Zeugodacus. These results provide a first comparative analysis of the gut microbiomes of field populations of multiple economically relevant tephritids and provide base line information for future studies that will further investigate the possible functional role of the observed associations.

Development of internal COI primers to improve and extend barcoding of fruit flies (Diptera: Tephritidae: Dacini)

Accurate species-level identifications underpin many aspects of basic and applied biology; however, identifications can be hampered by a lack of discriminating morphological characters, taxonomic expertise or time. Molecular approaches, such as DNA "barcoding" of the cytochrome c oxidase (COI) gene, are argued to overcome these issues. However, nuclear encoding of mitochondrial genes (numts) and poor amplification success of suboptimally preserved specimens can lead to erroneous identifications. One insect group for which these molecular and morphological problems are significant are the dacine fruit flies (Diptera: Tephritidae: Dacini). We addressed these issues associated with COI barcoding in the dacines by first assessing several "universal" COI primers against public mitochondrial genome and numt sequences for dacine taxa. We then modified a set of four primers that more closely matched true dacine COI sequence and amplified two overlapping portions of the COI barcode region. Our new primers were tested alongside universal primers on a selection of dacine species, including both fresh preserved and decades-old dry specimens. Additionally, Bactrocera tryoni mitochondrial and nuclear genomes were compared to identify putative numts. Four numt clades were identified, three of which were amplified using existing universal primers. In contrast, our new primers preferentially amplified the "true" mitochondrial COI barcode in all dacine species tested. The new primers also successfully amplified partial barcodes from dry specimens for which full length barcodes were unobtainable. Thus we recommend these new primers be incorporated into the suites of primers used by diagnosticians and quarantine labs for the accurate identification of dacine species.

Detection and characterization of bacterial endosymbionts in Southeast Asian tephritid fruit fly populations

BACKGROUND

Various endosymbiotic bacteria, including Wolbachia of the Alphaproteobacteria, infect a wide range of insects and are capable of inducing reproductive abnormalities to their hosts such as cytoplasmic incompatibility (CI), parthenogenesis, feminization and male-killing. These extended phenotypes can be potentially exploited in enhancing environmentally friendly methods, such as the sterile insect technique (SIT), for controlling natural populations of agricultural pests. The goal of the present study is to investigate the presence of Wolbachia, Spiroplasma, Arsenophonus and Cardinium among Bactrocera, Dacus and Zeugodacus flies of Southeast Asian populations, and to genotype any detected Wolbachia strains.

RESULTS

A specific 16S rRNA PCR assay was used to investigate the presence of reproductive parasites in natural populations of nine different tephritid species originating from three Asian countries, Bangladesh, China and India. Wolbachia infections were identified in Bactrocera dorsalis, B. correcta, B. scutellaris and B. zonata, with 12.2-42.9% occurrence, Entomoplasmatales in B. dorsalis, B. correcta, B. scutellaris, B. zonata, Zeugodacus cucurbitae and Z. tau (0.8-14.3%) and Cardinium in B. dorsalis and Z. tau (0.9-5.8%), while none of the species tested, harbored infections with Arsenophonus. Infected populations showed a medium (between 10 and 90%) or low (< 10%) prevalence, ranging from 3 to 80% for Wolbachia, 2 to 33% for Entomoplasmatales and 5 to 45% for Cardinium. Wolbachia and Entomoplasmatales infections were found both in tropical and subtropical populations, the former mostly in India and the latter in various regions of India and Bangladesh. Cardinium infections were identified in both countries but only in subtropical populations. Phylogenetic analysis revealed the presence of Wolbachia with some strains belonging either to supergroup B or supergroup A. Sequence analysis revealed deletions of variable length and nucleotide variation in three Wolbachia genes. Spiroplasma strains were characterized as citri-chrysopicola-mirum and ixodetis strains while the remaining Entomoplasmatales to the Mycoides-Entomoplasmataceae clade. Cardinium strains were characterized as group A, similar to strains infecting Encarsia pergandiella.

CONCLUSIONS

Our results indicated that in the Southeast natural populations examined, supergroup A Wolbachia strain infections were the most common, followed by Entomoplasmatales and Cardinium. In terms of diversity, most strains of each bacterial genus detected clustered in a common group. Interestingly, the deletions detected in three Wolbachia genes were either new or similar to those of previously identified pseudogenes that were integrated in the host genome indicating putative horizontal gene transfer events in B. dorsalis, B. correcta and B. zonata.

Genetic structure and range expansion of Zeugodacus Cucurbitae (Diptera: Tephritidae) in Africa

Hypotheses about the worldwide colonization routes of the melon fly, Zeugodacus cucurbitae (Diptera: Tephritidae), are mainly based on sparse historical records. Here we aim at reconstructing the colonization history of the African continent based on an improved description of the population structure of Z. cucurbitae and approximate Bayesian analyses. Individuals of Z. cucurbitae were sampled in 17 localities from East, West and Central Africa and genotyped at 19 microsatellite markers. Bayesian analyses showed intracontinental population structuring with populations from Uganda diverging from those of Tanzania and populations from Burundi and Kenya showing traces of admixture with West African samples. Approximate Bayesian Computation provided support to the hypothesis of a single introduction Z. cucurbitae into East Africa and subsequent expansion to West Africa, each colonization event was followed by a bottleneck that promoted population divergence within Africa. Parameter estimates suggested that these events are roughly compatible with the historical records of Z. cucurbitae presence in sub-Saharan Africa (viz. 1936 in East Africa and 1999 in West Africa) and allow excluding alternative hypotheses on older or multiple introductions of Z. cucurbitae.

Descriptions of four new species of Bactrocera and new country records highlight the high biodiversity of fruit flies in Vietnam (Diptera, Tephritidae, Dacinae)

Recent snap-shot surveys for fruit flies in Vietnam in 2015 and 2017 using traps baited with the male Dacinae fruit fly lures methyl eugenol, cue-lure and zingerone, collected 56 species, including 11 new country records and another 11 undescribed species, four of which are described in this paper. This increases the number of described species known to occur in Vietnam from 78 to 93. Species accumulation curves, based on the Chao 2 mean estimate, suggest that we collected 60-85 % of the local fauna at the sites sampled, and that species diversity decreases with increasing latitude. The four new species are named: Bactrocera (Tetradacus) ernesti Leblanc & Doorenweerd sp. n., B. (Asiadacus) connecta Leblanc & Doorenweerd sp. n., B. (Parazeugodacus) clarifemur Leblanc & Doorenweerd sp. n., and B. (Bactrocera) adamantea Leblanc & Doorenweerd sp. n. In addition to morphological data COI DNA sequence data of both the COI-5P and COI-3P mitochondrial DNA gene regions is provided. Three of the four newly described species are morphologically and genetically easily distinguished from all other members of Dacini. Bactroceraclarifemur sp. n. is superficially similar to B.pendleburyi (Perkins) based on morphology, but there are several apomorphic characters to distinguish the two. Both COI and a segment of the nuclear gene Elongation Factor 1 alpha separate the two species as well.

Description of a new species of Dacus from Sri Lanka, and new country distribution records (Diptera, Tephritidae, Dacinae)

A fruit fly survey in the Sinharaja and Knuckles National Parks in Sri Lanka (2016), using traps baited with the male lures methyl eugenol, cue-lure, and zingerone, yielded 21 species of Dacini fruit flies. Of these, three species, viz. Bactroceraamarambalensis Drew, B.dongnaiae Drew & Romig, and B.rubigina (Wang & Zhao), are new country occurrence records, and Dacus (Mellesis) ancoralis Leblanc & Doorenweerd, sp. n. is described as a new species. The Sri Lankan Dacini fruit fly fauna is now comprised of 39 species.

A global checklist of the 932 fruit fly species in the tribe Dacini (Diptera, Tephritidae)

The correct application of the scientific names of species is neither easy nor trivial. Mistakes can lead to the wrong interpretation of research results or, when pest species are involved, inappropriate regulations and limits on trade, and possibly quarantine failures that permit the invasion of new pest species. Names are particularly challenging to manage when groups of organisms encompass a large number of species, when different workers employ different philosophical views, or when species are in a state of taxonomic flux. The fruit fly tribe Dacini is a species-rich taxon within Tephritidae and contains around a fifth of all known species in the family. About 10% of the 932 currently recognized species are pests of commercial fruits and vegetables, precipitating quarantines and trade embargos. Authoritative species lists consist largely of scattered regional treatments and outdated online resources. The checklist presented here is the first global overview of valid species names for the Dacini in almost two decades, and includes new lure records. By publishing this list both in paper and digitally, we aim to provide a resource for those studying fruit flies as well as researchers studying components of their impact on agriculture. The list is largely a consolidation of previous works, but following the results from recent phylogenetic work, we transfer one subgenus and eight species to different genera: members of the Bactrocera subgenus Javadacus Hardy, considered to belong to the Zeugodacus group of subgenera, are transferred to genus Zeugodacus; Bactrocera pseudocucurbitae White, 1999, stat. rev., is transferred back to Bactrocera from Zeugodacus; Zeugodacus arisanicus Shiraki, 1933, stat. rev., is transferred back to Zeugodacus from Bactrocera; and Z. brevipunctatus (David & Hancock, 2017), comb. n.; Z. javanensis (Perkins, 1938), comb. n.; Z. montanus (Hardy, 1983), comb. n.; Z. papuaensis (Malloch, 1939), comb. n.; Z. scutellarius (Bezzi, 1916), comb. n.; Z. semisurstyli (Drew & Romig, 2013), comb. n.; and Z. trilineatus (Hardy, 1955), comb. n. are transferred from Bactrocera to Zeugodacus.