The nicotinic acetylcholine receptor gene family of the silkworm, Bombyx mori.

BACKGROUND

Nicotinic acetylcholine receptors (nAChRs) mediate fast synaptic cholinergic transmission in the insect central nervous system. The insect nAChR is the molecular target of a class of insecticides, neonicotinoids. Like mammalian nAChRs, insect nAChRs are considered to be made up of five subunits, coded by homologous genes belonging to the same family. The nAChR subunit genes of Drosophila melanogaster, Apis mellifera and Anopheles gambiae have been cloned previously based on their genome sequences. The silkworm Bombyx mori is a model insect of Lepidoptera, among which are many agricultural pests. Identification and characterization of B. mori nAChR genes could provide valuable basic information for this important family of receptor genes and for the study of the molecular mechanisms of neonicotinoid action and resistance.

RESULTS

We searched the genome sequence database of B. mori with the fruit fly and honeybee nAChRs by tBlastn and cloned all putative silkworm nAChR cDNAs by reverse transcriptase-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE) methods. B. mori appears to have the largest known insect nAChR gene family to date, including nine alpha-type subunits and three beta-type subunits. The silkworm possesses three genes having low identity with others, including one alpha and two beta subunits, alpha 9, beta2 and beta 3. Like the fruit fly and honeybee counterparts, silkworm nAChR gene alpha 6 has RNA-editing sites, and alpha 4, alpha 6 and alpha 8 undergo alternative splicing. In particular, alternative exon 7 of Bm alpha 8 may have arisen from a recent duplication event. Truncated transcripts were found for Bm alpha 4 and Bm alpha 5.

CONCLUSION

B. mori possesses a largest known insect nAChR gene family characterized to date, including nine alpha-type subunits and three beta-type subunits. RNA-editing, alternative splicing and truncated transcripts were found in several subunit genes, which might enhance the diversity of the gene family.