Chromosomal investigations on some British wild rodents

The chromosomes of the two species of field mouse occurring in Britain, A. sylvaticus (L.) and A. flavicollis (Melchior) have not been widely studied. An investigation was therefore initiated to analyze the chromosomes of these species from different localities on the mainland of Britain. The chromosome characteristics studied at mitotic metaphase include the relative length of the chromosomes, G-banding, the presence and pattern of heterochromatic regions in the chromosomes (C-banding) and the position of the nucleolus organizers. In meiosis the type of association between the x and y and the mode of their segregation has been discussed.Species comparisons based on these chromosomal data showed a chromosomal difference between A. sylvaticus and A. flavicollis which support the separate identities of these two species. Intraspecific variation between different populations of A. sylvaticus has been reported. Chromosomal polymorphism due to B chromosomes has been detected in all the populations studied. Four classes of B chromosome have been identified in nine populations of A. sylvaticus from the mainland of Britain. They are distinguished by shape and size and according to their euchromatic or heterochromatic composition. Inter- and intra-individual variation (mosaicism) as well as interpopulational variation of the number and the nature of B chromosomes has been studied. Polymorphism resulting by a deletion, that is the loss of chromosomes or parts of chromosomes, has been found in the X chromosome of two females from different populations; one of them had XO and the other Xx. Intraspecific karyotype homology, as judged by G- and C-banding has been demonstrated between populations of the bank vole Clethrionomys qlareolus (Schreber) from different localities on the British mainland and from Skomer Island.In conclusion, the results of this investigation clearly indicate that studying karyotype evolution and variation between species, G- and C-banding and nucleolar organizer regions (NORs) are essential tools without which erroneous conclusions may be reached.