Researchers have successfully mapped the nearly complete genomes of a susceptible bed bug strain and a superstrain that exhibits approximately 20,000 times more resistance to insecticides, providing an extensive insight into the mutations contributing to their resilience.
The results of their research were published in the journal Insects.
While bed bugs are not known to spread diseases to humans, their bites can lead to itching and secondary infections of the skin. The extensive application of insecticides, including DDT which is now prohibited, almost eradicated these blood-sucking pests by the 1960s, rendering infestations uncommon. However, in the last two decades, there has been a notable comeback of bed bugs, largely due to the resistance mutations they have acquired against various insecticides.
Resistance can emerge through several mechanisms, including the production of enzymes that break down insecticides (metabolic resistance) or the development of thicker outer coverings that prevent chemical penetration (penetration resistance). Previous research has identified some mutations and gene expressions associated with insecticide resistance, but a complete understanding of all the mutations involved has been lacking due to a lack of whole-genome sequencing of resistant strains.
A team of researchers led by Hidemasa Bono, a professor at Hiroshima University’s Graduate School of Integrated Sciences for Life, aimed to fill this knowledge gap by mapping the genomes of both susceptible and resistant bed bug strains from Japan. They used strains derived from wild bed bugs (Cimex lectularius) collected 68 years ago in Isahaya City, Nagasaki, for the susceptible samples, while the resistant strains were bred from specimens found in a Hiroshima City hotel in 2010. The researchers found that the resistant samples exhibited a remarkable 19,859-fold stronger resistance to pyrethroids—the most widely used insecticides for managing bed bugs—surpassing levels seen in many previously identified superstrains. The bed bug specimens were supplied by Fumakilla Limited, a chemical manufacturing company based in Japan.
Assembling the Genome Puzzle
Sequencing a genome is comparable to putting together a gigantic jigsaw puzzle, which can be anywhere from about 160,000 to 160 billion pieces in size. To achieve the most comprehensive bed bug genomes so far, the researchers employed long-read sequencing techniques, which capture extended segments of DNA—similar to having larger sections of the puzzle assembled. In contrast, traditional short-read sequencing covers only small fragments, often resulting in frustrating gaps.
Utilizing their advanced methods, the researchers were able to create a nearly complete representation of both genomes, with nearly every piece accurately positioned. They achieved 97.8% completeness and a quality value (QV) of 57.0 for the susceptible strain, and 94.9% completeness with a QV of 56.9 for the resistant strain. A QV exceeding 30 indicates high-quality sequences with an error rate of less than 0.1%. Both strains also exceeded the N50 value of the earlier reference genome for C. lectularius, Clec2.1, meaning they had fewer gaps and more intact regions of the genomic puzzle.
Discovering Known and New Resistance Mutations
Once the genomes were fully sequenced, the research team identified protein-coding genes, evaluated their functions, and examined their activity levels through transcriptional analysis. They discovered 3,938 transcripts containing amino acid mismatches, with 729 of those mutated transcripts being specifically associated with insecticide resistance.
“We determined the genome sequence of insecticide-resistant bed bugs, which showed a 20,000-fold higher resistance compared to susceptible ones. By comparing the amino acid sequences, we pinpointed 729 transcripts with mutations that confer resistance,” explained Kouhei Toga, the first author of the study and a postdoctoral researcher in the Laboratory of Genome Informatics at HU’s Graduate School of Integrated Sciences for Life.
“These transcripts included genes involved in DNA damage response, cell cycle regulation, insulin metabolism, and lysosome functions, suggesting that these molecular pathways may contribute to the development of pyrethroid resistance in bed bugs.”
By leveraging insights from previous insect studies, the researchers confirmed established resistance mutations and identified new ones, which could aid in developing more targeted and effective pest management strategies.
“We pinpointed numerous genes likely implicated in insecticide resistance, many of which have not been previously associated with resistance in bed bugs. Modifying these genes could yield significant insights into the evolution and mechanisms of insecticide resistance,” Toga added.
“Moreover, this research broadens the array of target genes for tracking changes in allele distribution and frequency, potentially playing a crucial role in assessing resistance levels in wild populations. This study underscores the potential of genome-wide approaches to enhance our understanding of insecticide resistance in bed bugs.”
The research team also included Fumiko Kimoto and Hiroki Fujii, both affiliated with Fumakilla Limited.
