GENOMIC INSITU HYBRIDIZATION REVEALS THE ALLOPOLYPLOID NATURE OF MILIUM-MONTIANUM (GRAMINEAE)

Molecular techniques that "paint" chromosomes offer exciting new opportunities for testing genome relationships. Milium montianum (2n = 22) is a grass whose distinctive bimodal karyotype comprises 8 large (L-) and 14 smaller (S-) chromosomes. The proposal that M. montianum is an allotetraploid, with diploid Milium vernale (2n = 8) as the L-chromosome genome donor, has been impossible to confirm by classical means. To test this hypothesis, biotinylated total genomic DNA of diploid M. vernale (2n = 8) was hybridized in situ to root tip chromosomes of M. montianum. The M. vernale probe hybridized preferentially to all L-chromosomes, but not to the S-chromosomes. These results (i) confirm the allopolyploid nature of M. montianum, (ii) strongly support the theory that the L-chromosomes of M. montianum were donated by M. vernale, or a closely related genotype and (iii) show that subsequently the L-chromosomes have largely retained their genomic integrity in the new allopolyploid background. Clearly, genomic in situ hybridization (GISH) is a potentially powerful tool for studying genome evolution and biosystematics. It will often be useful for investigating the origins of wild and cultivated polyploid plant species, especially where conventional methods have failed, for studying introgression, and for understanding the mechanism(s) of origin of bimodal karyotypes.