MITOCHONDRIAL CONTROL REGION POLYMORPHISM REVEAL HIGH AMOUNT OF GENE FLOW IN FENNOSCANDIAN WILLOW TITS (PARUS-MONTANUS BOREALIS)

We studied the genetic variability and differentiation of two Fennosca ndian willow tit (Parus montanus borealis) populations located ca. 100 0 km apart in Finland and Sweden by using the control region sequences of the mitochondrial DNA. Individual variation in the control region was extensive since all the 13 Finnish and the 12 Swedish individuals had unique haplotypes and the mean pairwise genetic distance resulted in 0.0052 (range 0.0008-0.0109). In the minimum spanning network conne cting the genotypes, the two populations were completely intermingled. The mismatch distribution of the combined data set was very close to expected distribution of an expanding population. This result was supp orted by a significantly negative Tajima's D value. The sequence data indicate that (1) the long-term effective population size of the breed ing willow tits has been large (122,000 and 110,000 females for the Fi nnish and the Swedish populations, respectively); and that (2) the gen e exchange between distant localities is/has been extensive. Dispersal area for the Finnish females was estimated to be about 19,000-30,000 km(2) and for the Swedish, 22,000-28,000 km(2). Thus, the whole Fennos candian population can be regarded as one panmictic unit, without any subdivisions to local demes. The amount of gene flow is remarkable bec ause the willow tit has been considered a highly sedentary species. Pr ovided that adult birds are site-tenacious, the gene flow must occur t hrough juvenile summer dispersal, or irruptive autumn invasions, or bo th.