Tracking the colonisation history of the invasive species Varroa destructor
Résumé
The development of molecular tools and the theory of invasion genetics have stimulated interest in the study of the invasion of new environments or new hosts by alien species. This approach has recently been applied to better understand the colonisation history of Varroa destructor, an invasive parasitic mite that has dramatically expanded its geographical distribution area after it shifted from its original host, the Asian cavity-nesting Apis cerana, to the honey bee A. mellifera during the last century. Varroa is currently considered as the major pest of honey bees in most parts of the world. Initial work found little evidence of genetic variability in V. destructor. More recent work, taking advantage of the sequencing of the mite’s complete mitochondrial (mt) genome, made it possible to define specific PCR primers for amplifying long DNA fragments covering different mitochondrial genes. Mitochondrial sequences (a total of 2,700 nucleotides) display substantial polymorphism. In total 20 haplotypes were identified, representing nucleotide diversity of 0.75%. Interestingly, six of them were detected in mites infesting A. mellifera and represented a nucleotide diversity of 0.4%. This contrasts with the remarkable homogeneity of the two haplotypes (known as the Korean and Japan haplotypes) that have invaded A. mellifera worldwide. It thus appears that although only two Varroa genotypes have managed to colonise A. mellifera worldwide (presumably carried by humans), multiple shifts from its natural host A. cerana to this species have occurred in Asia. This and other reports of variation in V. destructor in relation with the invasion process are reviewed and discussed, including the threatening possibility that additional genotypes might spread into A. mellifera in the future.