Cytogenetic Research on Hexaploid Alfalfa,Medicago SativaL.

SUMMARYA spontaneously occurring hexaploid plant (2 n = 6x=48) was found among white-flowered tetraploid plants of Medicago sativa.This plant was studied from a morphological and cytological point of view. All the metaphases observed showed 48 chromosomes, six of which with large satellites. Total chromosome length ranged from μ 1.34 to μ 2.25 whereas that of tetraploid ranged from μ 2.08 to μ 2.83. Most diakinesis showed bivalents and 1 or, more frequently, 2 quadrivalents. Two or more univalents were observed in 56.4% of metaphases I and lagging chromosomes, varying in number from 1 to 4, were observed in 60% of anaphases II. Hexaploid pollen stainability (81%) was the same as in the tetraploid plants used as control (80%). The pollen grains were very variable in diameter (μ) and therefore two classes were distinguished: a smaller (diameter μ. 25.5) and a larger one (diameter μ 40.0). Such variability probably means that the hexaploid plant produces many unbalanced gametes which, in association with meiotic disorders may explain the very low fertility of the hexaploid plant. The importance of hexaploid for cytogenetic researches, for agronomic studies and for its significance in the evolution of Medicago sativa are discussed.