Genomic Organization Of Repeated Dna Sequences In Medicago Sativa Subspecies

E Falistocco

Dip. Biologia Vegetale e Biotecnologie Agroambientali, University of Perugia, Perugia, Italy
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    Medicago sativa L. includes forms with different morphology and chromosome number. Diploid taxa (2n = 2x = 16), with violet flowers and coiled pods, are classified as M. sativa subsp. coerulea, whereas those with yellow flowers and straight pods are known as M. sativa subsp. falcata. Cultivated alfalfa, M. sativa subsp. sativa (2n = 4x = 32), evolved by polyploidization from subsp. coerulea. Molecular cytogenetics is a valuable way to investigate the genomic organization of these taxa. Fluorescent in situ hybridization (FISH) has been used to analyze the distribution of ribosomal genes, but further information could be obtained by genomic in situ hybridization (GISH). The position of the fluorescent signals is also an indication of the presence of repetitive DNA along the chromosomes. In this study the GISH technique was used as a first approach to reveal genomic differences among the M. sativa subspecies taking into consideration the distribution of repetitive DNA sequences. Two different families of repetitive DNA sequences having different chromosome positions were found. One is located at the centromeric regions of all the subspecies and is the only one in subsp. falcata, whereas the other one is distributed at the telomeric and intercalary sites of chromosomes of subspp. coerulea and sativa. Studies are under way to investigate the evolutionary trend of the genome organization within M. sativa and to verify the implication of repetitive DNA with the genome size.