Do B chromosomes affect morphometric characters in yellow-necked mice Apodemus flavicollis (Rodentia, Mammalia)?

The presence of B chromosomes in the yellow-necked mouse Apodemus flavicoll is (Melchior, 1834) was analyzed in 320 adult animals from four localities in Yugoslavia. Our aim was to assess the effects of B chromosomes on certai n morphometric characteristics. The characters that were chosen are include d in the taxonomic discrimination of this particular species as well as the sibling species Apodemus sylvaticus (Linnaeus, 1758). B chromosomes were f ound in all the localities with frequencies ranging from 21.6% to 42.9%. It was ascertained that different frequencies of animals with B chromosomes c hange the slope of the regression curve of the length of foramina incisiva (FI) relative to the distance from the 3rd upper molar to the incisor (IM3) . This finding questions the validity of use of this relative value in taxo nomy without taking into consideration karyological data. It was also estab lished that the relative length of the tail (the body/tail ratio) significa ntly increase in the presence of B chromosomes. As this index demonstrates significant geographic variability the result could be taken as a trend onl y.