Cytogenetic Studies of the Non-Dormant Alfalfa Germplasm Sources
Gary R. Bauchan and M. Azhar Hossain
USDA-ARS, Soybean & Alfalfa Research Lab., Beltsville, MD
Cytogenetic studies of the nine historical germplasm sources of tetraploid alfalfa in the U.S.(Barnes, et al. 1977) have been conducted using C-banding and image analysis techniques (Bauchan and Hossain, 1997). C-banding polymorphisms were detected in their numbers, positions and intensities of terminal and interstitial bands within each germplasm source and between germplasm sources. The similarity of the chromosome morphology and the C-banding pattern among homologous is not always perfect, however, there is enough resemblance between the chromosomes to group the alfalfa chromosomes into eight sets of four chromosomes. The morphometric measurements and C-banding studies reveal that alfalfa has nearly four identical sets of chromosomes which adds evidence that alfalfa is presumably an autotetraploid. The analysis of the non-dormant alfalfa which include'African'(PI 536539),'Chilean'(PI536534),'Peruvian'(PI536535)and'Indian'(PI 536536)germplasm sources reveals that they contain the largest number of heterochromatic DNA bands. The 'African' chromosomes possess the highest amount of heterochromatic DNA and resemble the banding pattern of diploid M. sativa ssp. caerulea. We consider the C-banding pattern of 'African' be the standard karyotype for tetraploid alfalfa for the development of additional karyotypes for diverse alfalfa populations as it contains the largest number of C-bands thus far studied. A standard karyotype of 'African' alfalfa consists of one set of satellited chromosomes (chromosome 8), four sets of submetacentric chromosomes (chromosomes 2-4), and three sets of small metacentric chromosomes (chromosome 5-7). All of the chromosomes have centromeric bands and a terminal band on their short arms with the exception of the satellited chromosome. Interstitial bands were observed on each of the short arms with the exception of chromosome 7. Chromosomes 1, 2, and 3 have interstitial bands on their long arms. The karyotype of 'Chilean' and 'Peruvian' chromosomes are similar, however, they have fewer C-bands than the 'African' germplasm source. 'Indian' chromosomes have the fewest number of bands when compared to the other non-dormant sources especially on the long arms of the chromosomes. The image analysis system utilized in this study has proven to be an efficient method of obtaining quality enhanced images of chromosomes and a rapid method of obtaining large amounts of data for the development of karyotypes. In addition, C-banding has become a powerful diagnostic tool for distinguishing the chromosomes of the alfalfa genome.
References
Barnes, DK, Bingham, ET, Murphy, RP, Hunt, OJ, Beard, DF, Skrdla WH, and Teuber, LR. 1977. Alfalfa Germplasm in the United States: Genetic Vulnerability, Use, Improvement and Maintenance. U.S. Department of Agriculture, U.S. Printing Office. Technical Bulletin No. 1571.
Bauchan GR and Hossain MA, 1997. Karyotypic analysis of C-banded chromosomes of diploid alfalfa: Medicago sativa ssp. caerulea and ssp. falcata and their hybrid. J Hered. 88:533-536.