Chromosome patterns and Robertsonian polymorphism in Blarina brevicauda (Say) (Mammalia: Insectivora)

Abstract

A sample of 21 Blarina brevicauda, (Say) from seven localities in the Province of Ontario shows an intraspecific chromosomal polymorphism according to the rule of Robertson; 16 specimens have a homozygous karyotype, 2N = 50, composed of 48 acrocentric autosomes of regularly decreasing lengths and metacentric sex chromosomes, a large X. and a very small Y. Five specimens have a heterozygous karyotype, 2N = 49, showing one large submetacentric autosome arisen by centric fusion of two acrocentrics. The homozygous chromosomal complement, 2N = 48, has so far not been observed, but must exist in the population. It is impossible to determine exactly which are the chromosomes corresponding to the new submetacentric. The fundamental number, which is not affected by robertsonian mechanisms, is 52. The simplicity of the chromosomal set of B. brevicauda is discussed.The acrocentric nature of the autosomes allows for the possibility of a more extensive polymorphism, but this requires an analysis of numerous animals from several points of the distribution area of B. brevicauda. The exploration of intra-specific variation in number or morphology of chromosomes will require the analysis of considerably larger samples than has been customary in the study of mammalian karyotypes.