CYTOLOGICAL STUDIES IN MEDICAGO, MELILOTUS AND TRIGONELLA

Abstract

The investigation reported herein is an effort to secure information through the use of cytological methods, which may assist in providing a better understanding of the true relationships of species and varieties within the genus Medicago, and to compare cytologically, in a general way, this genus with the related genera Melilotus and Trigonella. In order to emphasize the present unsatisfactory taxonomic situation the morphological characters of the three genera are described and the taxonomic classifications of the species for each is reproduced as presented by Taubert (49). Classifications for Medicago by Dr. Candolle (11) and by Hegi (15) are also outlined for purposes of drawing attention to certain discrepancies between the classifications of Medicago by Taubert and by Hegi. Reference is made to specific taxonomic difficulties in Medicago. The general distributions of the three genera are given.In the section of the paper on "Cyto-taxonomic relations", an attempt is made, first, to point out five cytological principles which provide the philosophical foundation for cyto-taxonomic research; second, to summarize some of the background of literature which serves for the establishment of these principles; and third, to review a few papers reporting outstanding cyto-taxonomic investigations which demonstrate the applicability of the method. Previous cytological work on Medicago and Melilotus is then outlined. Details of observations concerning somatic chromosome morphology and numbers are recorded for 23 species and five varieties of two additional species of Medicago, for four species of Melilotus and for three species of Trigonella. A summarized statement of all cytological observations is given.In the discussion the following topics are considered in the light of the cytological data secured: general morphology of the somatic chromosomes and chromosome numbers found in the species studied, chromosome numbers and the annual or perennial habit in Medicago, different races in Medicago falcata, hybrid origin of Medicago media, taxonomic relations of Medicago lupulina, Medicago carstiensis, Medicago obscura, Medicago rigidula and Medicago arabica, and varieties of Medicago hispida. A few points of purely cytological interest are also discussed, viz., tetraploid chimeral areas, nucleolar remnants, and the peculiar anaphase chromosomes in somatic divisions.In general it is concluded that these cytological studies have yielded information which can be utilized very effectively in clearing up some of the taxonomic problems, particularly, in the genus Medicago. It is predicted that the cytological information will also be useful to plant breeders who may contemplate interspecific hybridization among any of the species studied.