Aneuploids in the shrub birch Betula humilis populations in Poland

Shrub birch (Betula humilis Schrk.) is endangered glacial relict growing in natural and drained fens and transitional mires. At present study we examined karyotypes of 103 individuals of B. humilis, collected in six populations from eastern and northeastern Poland. We found 60% of diploid individuals with 2n = 28. The rest of studied plants were aneuploids with 26, 27, 29, 30 and 31 chromosomes in their karyotypes. High frequencies of aneuploids in Polish populations of B. humilis could be a consequence of: (i) hybridization with congeneric species, (ii) stress resulting from range fragmentation, (iii) karyotype instability of individuals with 2n ≠ 28, or (iv) vegetative reproduction.