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Alfalfa Chromosomes. II. Pachytene Karyotype of a Tetraploid Medicago sativa L.¹

The chromosomes of a tetraploid Medicago sativa L. (2n=4x=32) were analysed from cells at the pachytene stage of meiosis. Mean chromosome lengths, arm ratios, and the distribution of chromatic areas were used to construct an idiogram showing the distinguishing features of the karyotype. The chromosomes ranged in length from 18.1 to 26.0 microns, and the haploid complement totaled 335.8 microns. The complement consisted of six acrocentric or subacrocentric chromosomes, eight metacentric chrmnosomes, and two nucleolus organizing chromosomes which were much more chromatic than the rest of the complement. The individual tetraploid chromosomes were much shorter and more chromatic than those of diploid M. sativa and M. falcata. The chromosomes of the tetraploid idiogram could be classified into pairs on the basis of morphological similarities, but only three pairs were observed as quadrivalents at pachytene. The tetraploid's chromosomes seemed more similar to those of diploid M. sativa than to those of diploid M. falcata.

Key Words: Idiogram • Pachytene chromosome morphology • Antotetraploidy • Allotetraploidy • Quadrivalents

² Graduate student, Department of Genetics, University of Alberta, Edmonton, Canada.

Received for publication October 15, 1969.