Spatial and temporal distribution of supernumerary or B chromosomes were studied in natural populations of wood mice. A total of 859 individuals belonging to several species from Eurasia were examined. A very high percentage of individuals possessing B chromosomes was found in Apodemus peninsulae (97.9%, n = 47), a high percentage in A. flavicollis (42.5%, n = 362), and a low one in A. syluaticus (2.4%, n = 210). No B chromosomes were observed in A. uralensis (n = 10), A. fulvipectus {n = 19), A. hermonensis (n = 1), A. agrarius (n = 58), A. mystacinus (n = 7), and in Apodemus species collected in Turkey (n = 74), Israel (n = 1), Azerbaijan (n = 2), Tadjikistan (n = 24), and Kyrghyzstan (n = 44). A modest to high frequency of B chromosomes was thus observed particularly in the species confined to a forest environment, whereas no B's were found in the species dwelling in open, steppe-like or rocky habitats. Both in A. peninsulae and A. flavicollis the frequency of B chromosomes varied among local populations. Follow-up studies revealed stability in the respective proportions of B chromosomes during subsequent years. However, no unambiguous interrelationship could be found between the frequency of B chromosomes and various biological and/or ecological variables. Therefore it seems that the incidence of B chromosomes is mainly determined by stochastic effects operating within the specific genetic background of a population.
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