Chromosomal polymorphism in Galerucella sagittariae (Gyll.) (Chrysomelidae, Coleoptera) in Finland

B‐chromosome polymorphism in 20 populations of Galerucella sagittariae in Finland and one population in Sweden was studied. 13 of the populations were living on Comarum palustre (southern Finland and Sweden), 7 on Rubus chamaemorus (northern Finland), and one on Alchemilla vulgaris. The distributions of the beetles on Comarum and Rubus do not overlap broadly. The standard chromosome number was found to be 2n = 34 (16 + Xy p ). In all but one Finnish population, also supernumerary B‐chromosomes were found in varying frequencies (1.6–58.6 %). In general, the B‐chromosome frequency of a population seemed to stay at a constant level from generation to generation. The distribution of the frequencies formed an increasing dine from southwest to northeast, indicating an adaptive significance of B‐chromosomes. The size of the B‐chromosomes varied from minute‐sized to autosome‐sized chromosomes. Being largely euchromatic, all B‐chromosomes were able to form chiasmatic bivalents in meiosis. They were isopycnotic during meiotic prophase and their segregational behavior was normal. Part of the B‐chromosomes were mitotically stable and another part mitotically unstable. The presence of B‐chromosomes could not be detected by inspecting the external phenotype of individuals. Obviously some genetic differentiation has taken place between southern and northern Galerucella sagittariae. The southern form is dependent on the immediate proximity of water while the northern form is able to live in considerably drier habitats, and thus the variety of suitable food plants is much broader.