Fluorescent Approaches on the Origin of B Chromosomes of Apodemus argenteus hokkaidi

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The B chromosomes of the small Japanese field mouse, Apodemus argenteus hokkaidi, are reported for the first time, and their C-banding and fluorescent staining characteristics described in some detail. Three of seven specimens studied had 2n=47, in which one of the 47 chromosomes could be regarded as a B chromosome based on the totally-heterochromatic and deeply C-stained profiles : two females contained one minute acrocentric B, and one male one medium-sized metacentric B. The QM fluorescent analyses revealed that all C-band areas other than that of the Y, M2 and smallest acrocentrics, showed delayed fluorescence or dull-to-bright transition of QM-fluorescence with the lapse of time upon fluorescent observation under a microscope, whereas with DA/DAPI-staining only the X chromosome C-heterochromatin, termed as C-block, showed similar fluorescence behavior of dull-to-bright transition. The B chromosome C heterochromatin was apparently of the C-block type in fluorescence behavior, as it turned from dull to bright fluorescence with both QM and DA/DAPI staining. Since such unique behavior of fluorescence could be detected only in the C-block heterochromatin, but not in the autosomal C-heterochromatin, the B chromosomes of Apodemus argenteus hokkaidi might have been derived from X chromosome C-heterochromatin, supposing the fluorescence behavior was unaltered during and after the formation of the B chromosomes.
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