Milan, 18 December. (Adnkronos Salute) – They are the terror of the ‘globetrotters’. Bed bugs strike in the still of the night, and while there is no evidence that they transmit disease to humans, their bites often cause itchy skin rashes and secondary infections. Widespread use of insecticides, including the now banned DDT, nearly wiped out populations of these blood-sucking insects in the 1960s, making infestations rare.
But over the past 20 years, the world has witnessed their resurgence, partly due to the resistance mutations they have developed against these insecticides. In a new study, Japanese scientists have mapped with unprecedented detail the genomes of both a susceptible strain and a super bed bug about 20 thousand times more resistant to insecticides, offering – explain the authors – the “broadest overview so far of the entire extent of their resistance mutations”.
The discoveries are published in the journal ‘Insects’. Resistance can occur through several mechanisms, such as the production of enzymes that detoxify insecticides (metabolic resistance) or the development of thicker outer layers to block chemicals (penetration resistance). Previous studies have identified some of the mutations and gene expressions linked to insecticide resistance. However, the full extent of mutations driving resistance remains unknown as no research has sequenced the entire genome of insecticide-resistant strains.
The team led by Hidemasa Bono, a professor at Hiroshima University has mapped the genomes of susceptible and resistant bed bug strains from Japan to fill this gap. Sensitive strains descending from wild bed bugs (Cimex lectularius) collected 68 years ago in the fields of Isahaya City, Nagasaki, have come under the microscope. Meanwhile, resistant strains were bred from samples collected from a hotel in Hiroshima City in 2010. Tests revealed that the resistant samples had 19,859 times stronger resistance to pyrethroids, the most commonly used insecticide for bedbug control of beds, exceeding levels observed in many previously identified super strains. All samples were provided by Fumakilla Limited, a chemical company based in Japan.
Sequencing a genome, experts explain, is like assembling a huge puzzle, which extends from approximately 160 thousand to 160 billion pieces. To map the most complete bed bug genomes to date, researchers have used an innovative method, which simply puts together entire sections of puzzle pieces. Traditional sequencing, in contrast, covers only small fragments, often causing frustrating gaps, the authors reason.
The researchers obtained 97.8% completeness and a quality value (Qv) of 57.0 for the susceptible strain and 94.9% completeness and a Qv of 56.9 for the resistant strain. In short, a step forward compared to expected attempts. After completely sequencing the genomes, the team identified the protein-coding genes, determined their functions and assessed their activity via transcriptional analysis. Thus, 3,938 transcripts with amino acid mismatches were discovered. Of these, 729 mutated transcripts were linked to insecticide resistance. “We determined the genome sequence of insecticide-resistant bed bugs, which showed 20,000 times greater resistance than susceptible bed bugs. By comparing the amino acid sequences between susceptible and resistant bed bugs, we identified 729 transcripts with mutations specifics of resistance,” summarizes first author Kouhei Toga, postdoctoral researcher (Hiroshima University).
“These transcripts – he continues – included genes related to the response to DNA damage, cell cycle regulation, insulin metabolism and lysosome functions. This suggests that these molecular pathways could play a role in the development of pyrethroid resistance in bed bugs. We have identified a large number of genes likely involved in insecticide resistance, many of which have not previously been reported to be associated with resistance in bed bugs. Genome editing of these genes could provide valuable evolutionary insights and on the mechanisms of resistance to insecticides”.
