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CELL BIOLOGY & MOLECULAR GENETICS

Distribution and Characterization of Heterochromatic DNA in the Tetraploid African Population Alfalfa Genome

USDA-ARS, Soybean Genomics and Improvement Laboratory, Beltsville Agricultural Research Center, Beltsville, MD 20705

* Corresponding author (BauchanG{at}ba.ars.usda.gov)

A reference karyotype of tetraploid alfalfa (Medicago sativa subsp. sativa, 2n = 4x = 32) African Population germplasm source (PI 536539) was constructed using the combined techniques of C-banding and image analysis. The image analysis system is a method of obtaining enhanced images of chromosomes in which morphological data can be obtained for the development of karyotypes. In addition, C-banding is a diagnostic tool for distinguishing the homologous chromosomes of alfalfa. Chromosome analysis of tetraploid alfalfa revealed that alfalfa has four similar series of chromosomes based on their chromosome morphology and C-banding. The karyotype of African Population alfalfa germplasm consists of one set of chromosomes with satellites (SATs, Chromosome 8), four sets of submetacentric chromosomes (Chromosomes 1–4), and three sets of metacentric chromosomes (Chromosomes 5–7). All of the chromosomes have centromeric bands and a terminal band on the short arm, with the exception of the SAT. Interstitial bands were observed on the short arm of each of the chromosomes, with the exception of Chromosome 7. Chromosomes 1, 2, and 3 have interstitial bands on their long arms. There exists considerable variability in the number, intensity, and location of the constitutive heterochromatic DNA; however, this variability is not sufficient to preclude recognition of the homologous chromosomes. The chromosome banding pattern of the African Population germplasm resembles the distribution of heterochromatic DNA C-bands of diploid M. sativa subsp. caerulea (Less. ex Ledeb.) Schmalh. The African Population karyotype of alfalfa developed in this study is suggested as the reference for the development of additional chromosome maps of diverse alfalfa populations.

Abbreviations: NOR, nucleolar organizer region • RFLP, restriction fragment length polymorphism • SAT, chromosome with a satellite

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