B-CHROMOSOMES IN THE WOOD MICE (GENUS APODEMUS)

Spatial and temporal distribution of supernumerary or B chromosomes we re studied in natural populations of wood mice. A total of 859 individ uals belonging to several species from Eurasia were examined. A very h igh percentage of individuals possessing B chromosomes was found in Ap odemus peninsulae (97.9%, n = 47), a high percentage in A. flavicollis (42.5%, n = 362), and a low one in A. sylvaticus (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 pa rticularly 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 c hromosomes varied among local populations. Follow-up studies revealed stability in the respective proportions of B chromosomes during subseq uent years. However, no unambiguous interrelationship could be found b etween the frequency of B chromosomes and various biological and/or ec ological variables. Therefore it seems that the incidence of B chromos omes is mainly determined by stochastic effects operating within the s pecific genetic background of a population.