Hi both Peters and All

> However, does this mean one clonal line or many?

The best paper I could find on this is by Solignac and colleagues in 2005.  There are two types of V. destructor on A. mellifera which seem to trace back to two original strains jumping from A. cerana to A. mellifera.  On top of that there has been some mixing between the two strains, and some new generation of mutants giving resistance to pyrethroids (interestingly called 'parathyroids' on one web page I saw this morning!).

Calling them 'clones' doesn't seem quite right, even though I used the term before and the paper cited below uses the word in its title.  'Quasi-clones' brought about by repeated (but not total) inbreeding, perhaps.

The paper below notes that the variant DNA in the cytoplasm always matches the variant DNA in the nucleus.  The DNA in the nucleus was checked by looking at the fast-evolving microsatellites, and the small amount of variation noted was compatible with new variants occuring during the 50 years since the jump to A. mellifera.

There has been some mixing between the Japanese and Korean strains where they meet, throwing up various combinations of the genes of the two.

On the topic of the rapidity of the evolution of Varroa where enormous selection pressure is applied (ie miticides) it is not too surprising that new mutants with resistance have emerged.  The sodium channel protein essential to nerve function has been mentioned already.  In other arthropods, detoxifying enzymes such as monooxygenases also have an effect.  Perhaps in Varroa, as in some insects, resistance arose in a step-wise, incomplete manner - possibly with individual components coming together in those rare matings which are not brother-sister, but distant cousin-distant cousin.

But is there enough variation to allow the weak selection pressure found in a few isolated generations in a forest somewhere bring about non-virulence in Varroa?  I doubt that very much, and would reckon that other factors are permitting the co-existence of forest bees and forest Varroa.

all the best

Gavin  
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Solignac, M.; Cornuet, J. M.; Vautrin, D.; Conte, Y. le; Anderson, D.; Evans, J.; Cros-Arteil, S.; Navajas, M. (2005) The invasive Korea and Japan types of Varroa destructor, ectoparasitic mites of the Western honeybee (Apis mellifera), are two partly isolated clones. Proceedings of the Royal Society of London. Series B, Biological Sciences 272 (1561) : 411-419 
 
Abstract:  Varroa destructor, now a major pest of the Western honey bee, Apis mellifera, switched from its original host, the Eastern honey bee, A. cerana, approximately 50 years ago. So far, only two out of several known mitochondrial haplotypes of V. destructor have been found to be capable of reproducing on A. mellifera (Korea and Japan). These haplotypes are associated in almost complete cytonuclear disequilibrium to diagnostic alleles at 11 microsatellite loci. By contrast, microsatellite polymorphism within each type is virtually absent, because of a severe bottleneck at the time of host change. Accordingly, 12 mitochondrial sequences of 5185 nucleotides displayed 0.40% of nucleotide divergence between haplotypes and no intra haplotype variation. Hence, each type has a quasi-clonal structure. The nascent intratype variability is subsequent to the clone formation 50 years ago: in both types, the variant alleles differ from the most common by one (in
 10 cases), two (five cases) or three (one case) repeated motifs. In addition to individuals of the two 'pure' types, five F1 hybrids and 19 recombinant individuals (Japan alleles introgressed into the Korea genetic background) were detected. The existence of F1 and recombinant individuals in admixed populations requires that double infestations of honey bee cells occur in a high proportion but the persistence of pure types suggests a post-zygotic isolation between the two clones.

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