Six years of fruit fly surveys in Bangladesh: a new species, 33 new country records and discovery of the highly invasive Bactrocera carambolae (Diptera, Tephritidae)

Genomes of the cosmopolitan fruit pest Bactrocera dorsalis (Diptera: Tephritidae) reveal its global invasion history and thermal adaptation

INTRODUCTION

The oriental fruit fly Bactrocera dorsalis is one of the most destructive agricultural pests worldwide, with highly debated species delimitation, origin, and global spread routes.

OBJECTIVES

Our study intended to (i) resolve the taxonomic uncertainties between B. dorsalis and B. carambolae, (ii) reveal the population structure and global invasion routes of B. dorsalis across Asia, Africa, and Oceania, and (iii) identify genomic regions that are responsible for the thermal adaptation of B. dorsalis.

METHODS

Based on a high-quality chromosome-level reference genome assembly, we explored the population relationship using a genome-scale single nucleotide polymorphism dataset generated from the resequencing data of 487 B. dorsalis genomes and 25 B. carambolae genomes. Genome-wide association studies and silencing using RNA interference were used to identify and verify the candidate genes associated with extreme thermal stress.

RESULTS

We showed that B. dorsalis originates from the Southern India region with three independent invasion and spread routes worldwide: (i) from Northern India to Northern Southeast Asia, then to Southern Southeast Asia; (ii) from Northern India to Northern Southeast Asian, then to China and Hawaii; and (iii) from Southern India toward the African mainland, then to Madagascar, which is mainly facilitated by human activities including trade and immigration. Twenty-seven genes were identified by a genome-wide association study to be associated with 11 temperature bioclimatic variables. The Cyp6a9 gene may enhance the thermal adaptation of B. dorsalis and thus boost its invasion, which tended to be upregulated at a hardening temperature of 38 °C. Functional verification using RNA interference silencing against Cyp6a9, led to the specific decrease in Cyp6a9 expression, reducing the survival rate of dsRNA-feeding larvae exposed to extreme thermal stress of 45 °C after heat hardening treatments in B. dorsalis.

CONCLUSION

This study provides insights into the evolutionary history and genetic basis of temperature adaptation in B. dorsalis.

Effect of various depths of pupation on adult emergence of interspecific hybrid of Bactrocera carambolae and Bactrocera dorsalis

The depth of the pupation is one of the important factors in the success of fruit flies to become imago. The objective of this study was to evaluate the effect of soil depth on survival, normality dan development time of adult interspecific hybrids of Bactrocera carambolae (Drew & Hancock) and B. dorsalis (Hendel). The experiments were carried out in a laboratory consisting of seven depths of pupation treatments (4 cm, 10 cm, 20 cm, 30 cm, 40 cm, 50 cm, and 60 cm) with four replications. Soil depth had a significant, negative effect on the survival of the emergence and development time of B. dorsalis and B. carambolae hybrids. The emergence rate was found to decrease with the increase in pupation depth. The higher survival of the emergence of the hybrid occurred at a depth of 4 cm (95% ± 1.91) and 10 cm (86% ± 2.58), while the lower survival occurred at a depth of 50 cm (12% ± 1.63) and 60 cm (5% ± 3.79). Normal imagoes were found in all soil depths except in 60 cm depth, where all imagoes had abnormal morphology. Means of development time ranged from 8.88 to 10.63 days. The depth of pupation influences the duration of pupae development. The means of development time at a depth of 4–40 cm was similar, but at a depth of 50 cm and 60 cm, a significantly longer time of development were observed. for more effective fruit fly control, this study suggests burying rotten fruit in the soil at a depth of 50 cm or more as a preventive measure for the development of fruit flies.

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.