Thailand is the only place known where large populations of Mainland and Sundaland Apis cerana come into contact and where native strains of both Varroa destructor and V. jacobsoni occur. This provides a unique opportunity to investigate occurrence of Varroa species and mitochondrial lineages on different genetic lineages of A. cerana in a natural setting. We sampled Thai Varroa and A. cerana on a north to south transect, and identified mite and bee mtDNA haplotypes by RFLPs and COI sequence. Ranges of Mainland and Sundaland A. cerana meet at the Kra ecotone, between 10°34'N and 11°24'N. Varroa jacobsoni was found on both Sundaland and Mainland A. cerana; south of Kra ecotone (on Sundaland A. cerana) mites had the Malaysia haplotype, while north of Kra ecotone (on Mainland A. cerana) mites had the NorthThai1 or NorthThai2 haplotype. Varroa destructor was only found in Chiang Mai and Chiang Rai Provinces (18°36'N, 98°48'E) at altitudes above 1000 m. We found no evidence of V. destructor with so-called Japan-Thailand haplotype on Thai A. cerana. Apis cerana / Varroa / coevolution / mitochondrial DNA / Thailand

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Concordance between the mitochondrial haplotypes of the Eastern honey bee, Apis cerana, and its ectoparasitic Varroa mites across the Isthmus of Kra in Thailand has suggested that local host-pathogen co-evolution may be responsible for the geographic distribution of particular genotypes. To investigate nuclear microsatellites population structure in A. cerana, single workers of A. cerana colonies from Thailand were genotyped at 18 microsatellite loci. The loci showed intermediate to high levels of heterozygosity and a range of allele numbers. The analyses confirmed a fundamental subdivision of the Thai A. cerana population into the "Asia Mainland" and "Sundaland" regions at the Isthmus of Kra. However, the nuclear microsatellite differentiation was less distinct than mtDNA haplotype differences, suggesting male-biased dispersal and population admixture. Overall, samples showed a weak isolation-by-distance effect. The isolated population on Samui island was most differentiated from the other samples. The results do not support our initial hypothesis of local host-pathogen co-evolution, which predicts a strict correspondence between the nuclear genome and the lineage of parasitic Varroa mite of the A. cerana samples, because the gene flow indicated by our nuclear microsatellite markers should also mix potential Varroa resistance alleles among subpopulations. Instead, our study suggests that the coincidental distribution of Varroa lineages and A. cerana population structure in Thailand are the result of biogeographic history and current migration patterns Keywords: biogeography | co-evolution | local adaptation | microsatellites | population structure | apis cerana | honey bees | Thailand | Varroa mite association | social insects | biology Article: