Host Specificity in the Honeybee Parasitic Mite, <i>Varroa spp</i>. in <i>Apis mellifera</i> and <i>Apis cerana</i>

<div><p>The ectoparasitic mite <i>Varroa destructor</i> is a major global threat to the Western honeybee <i>Apis mellifera</i>. This mite was originally a parasite of <i>A</i>. <i>cerana</i> in Asia but managed to spill over into colonies of <i>A</i>. <i>mellifera</i> which had been introduced to this continent for honey production. To date, only two almost clonal types of <i>V</i>. <i>destructor</i> from Korea and Japan have been detected in <i>A</i>. <i>mellifera</i> colonies. However, since both <i>A</i>. <i>mellifera</i> and <i>A</i>. <i>cerana</i> colonies are kept in close proximity throughout Asia, not only new spill overs but also spill backs of highly virulent types may be possible, with unpredictable consequences for both honeybee species. We studied the dispersal and hybridisation potential of <i>Varroa</i> from sympatric colonies of the two hosts in Northern Vietnam and the Philippines using mitochondrial and microsatellite DNA markers. We found a very distinct mtDNA haplotype equally invading both <i>A</i>. <i>mellifera</i> and <i>A</i>. <i>cerana</i> in the Philippines. In contrast, we observed a complete reproductive isolation of various Vietnamese <i>Varroa</i> populations in <i>A</i>. <i>mellifera</i> and <i>A</i>. <i>cerana</i> colonies even if kept in the same apiaries. In light of this variance in host specificity, the adaptation of the mite to its hosts seems to have generated much more genetic diversity than previously recognised and the <i>Varroa</i> species complex may include substantial cryptic speciation.</p></div

Principal Component Analysis.

<p>Genetic clustering of the <i>Varroa</i> mites based on their genotype at six microsatellite markers. The <i>Varroa</i> mites were sampled in <i>A</i>. <i>cerana</i> (<b>Vc</b>) and <i>A</i>. <i>mellifera</i> (<b>Vm</b>) colonies in the Philippines and Vietnam. The genotypes of the mites sampled in Los Banos (<b>LB</b>) are shown in green (groups 1 and 2). The mites from Lipa city (<b>LC</b>, group 3) and sampled in <i>A</i>. <i>mellifera</i> from Dien Bien (<b>DB</b>, group 5) and Son La (<b>SL</b>, group 7) are represented in orange. The mites from <i>A</i>. <i>cerana</i> colonies located in Vietnam from Dien Bien (<b>DB</b>, group 4), Son La (<b>SL</b>, group 6) and Cat Ba (<b>CB</b>, group 8) are represented in blue. Each dot represents a distinct individual, and each inertia ellipsoid shows the population’s prediction ellipses for each group.</p

Overall information on the microsatellite loci used.

<p>For each of the six microsatellite markers used, the name of the locus (<b>Locus</b>), the overall number of Alleles (<b>N</b>_<b>A</b>) and the overall Allelic Richness (<b>R</b>) are represented.</p><p>Overall information on the microsatellite loci used.</p

Pairwise F_ST.

<p>Pairwise comparison of the genetic differentiation using the fixation index (F_ST) between the three main genetic clusters found in this study: the mites from the two honeybees species sampled in Los Banos (<b>Philippines</b>), the mites sampled in <i>A</i>. <i>mellifera</i> colonies in Vietnam and Lipa city (<b>Korea</b>) and the mites sampled in <i>A</i>. <i>cerana</i> colonies in Vietnam (<b>Vietnam</b>).</p><p>***: <i>p</i><0.001,</p><p>**: <i>p</i><0.01.</p><p>Pairwise F_ST.</p

Varroa <i>coxI</i> haplotype divergence in the Philippines and Vietnam.

<p>Network representing the amount of substitutions between the different <i>coxI</i> sequences obtained from different <i>Varroa</i> mites sampled in colonies of <i>A</i>. <i>mellifera</i> (in blue with white text) and <i>A</i>. <i>cerana</i> (in orange with black text). The mites were sampled in Vietnam in the surrounding of Dien Bien (<b>DB</b>), Son La (<b>SL</b>) and Cat Ba (<b>CB</b>) and in the Philippines in the city of Los Banos (<b>LB</b>) and Lipa City (<b>LC</b>). The sample size for each haplotype is written in italic between brackets below the sampling location. The grey components represent additional accessions generated by Navajas <i>et al</i>. (2010): AmK1-1 haplotype (<b>K1</b>, accession GQ379056), AcV1-1 (<b>V1</b>, accession GQ379061), AcC1-1 (<b>C1</b>, accession GQ379065) and AmJ1-6 together with AcJ1-4 (<b>J1</b>, accessions GQ379074.1 and GQ379072.1, respectively). Each diamond represents a substitution. The number indicated close to the dotted line represents the number of substitutions not represented on the figure.</p

Information on the genotyped individuals.

<p>Sample size and population genetic estimators for each sampling location (P for Philippines and V for Vietnam, followed by the closest city to the sampled apiaries) and host species (<i>A</i>.<i>c</i>: <i>A</i>. <i>cerana</i>, <i>A</i>.<i>m</i>: <i>A</i>. <i>mellifera</i>) on which the mites were collected, together with the clusters provided by the Principal Component Analysis. Number of apiaries <i>(</i><b><i>N</i></b>_{<b><i>A</i></b>}); Number of colonies (<b><i>N</i></b>_{<b><i>C</i></b>}); Number of individual mites (<b><i>N</i></b>_{<b><i>I</i></b>}); Mean number (± se) of alleles over the six microsatellite markers (<b>N</b>_<b>al</b>); allelic richness ± se; mean observed heterozygosity ± se (<b>H</b>_<b>O</b>).</p><p>Information on the genotyped individuals.</p