CYTOLOGICAL STUDIES IN ALFALFA POLYPLOIDS

Abstract

Normal tetraploid alfalfa, Medicago sativa L., is characterized by meiotic irregularities consisting of a low percentage of univalents, trivalents, and quadrivalents. While these irregularities might suggest an autoploid origin, their frequency is too low to be conclusive. Cytological studies of the induced octoploid and of the hexaploid, obtained from crossing tetraploid and octoploid, indicate that the two genomes in the tetraploid are only partially homologous. The partial homology is established by the meiotic behavior in the hexaploid in which a low univalent frequency indicates fairly complete pairing between the A and B genomes. The comparatively low frequency of quadrivalents in the octoploid indicates a correspondingly low chiasma frequency at pachytene. Nevertheless this quadrivalent frequency in the octoploid is more than three times as high as in the tetraploid which suggests a lack of complete homology between the two genomes. The theory is advanced that tetraploid species of Medicago originated from crosses between a series of diploid species fairly similar cytologically but differing in well marked, morphological characters. This affords an explanation for the inheritance of some characters in a disomic and others in a tetrasomic manner. Cytological and genetic evidence thus points to tetraploid alfalfa as originating as an alloploid from closely related diploid species.