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The Journal of Heredity 1994:85(1):18-22
© 1994 The American Genetic Association 85:18-22

research-article

Use of an Image Analysis System to Karyotype Diploid Alfalfa (Medicago sativa L.)

G. R. Bauchan, and T. A. Campbell

USDA/ARS, Soybean and Alfalfa Research Laboratory, Beltsville Agricultural Research Center Beltsville, Maryland 20705-2350

Abstract

Alfalfa somatic chromosomes are difficult to karyotype due to autotetraploidy, a relatively large number of chromosomes (2n = 4× = 32), small chromosome size (2 to 3 µm), and difficulty in identifying all eight sets of chromosomes because of their similar chromosome morphology. These difficulties can be surmounted by studying diploid (2n = 2× = 16) alfalfa and using a microcomputer-based image analysis system for karyotyping. Image analysis enhances the chromosome image by pseudocolorizing the image according to densitometric measurements and by enlarging the image, which enables the edges of the chromosomes to be distinguished. The chromosome arms from 26 cells were measured, and the relative chromosome lengths and arm ratios were computed. A randomized complete block design experiment showed that the relative length and total chromosome length measurement could be used to distinguish the eight homologous chromosome pairs. The video image was cut and pasted on the video screen according to the analyzed data to produce the karyotype. The diploid alfalfa karyotype consists of one pair of satellited chromosomes, one pair of large submetacentric chromosomes, three pairs of metasubmetacentric chromosomes, and three pairs of small metacentric chromosomes. This image analysis system is especially useful for plant species such as alfalfa that have been difficult to karyotype due to small chromosome size, similar chromosome morphology, and polyploidy. Other applications of this image analysis system are discussed.


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G. R. Bauchan and M. A. Hossain
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