SPATIAL SEPARATION OF ANCESTRAL GENOMES IN THE WILD GRASS MILIUM-MONTIANUM PARL

Previous work showed a strong tendency for genomes from two different parents to be spatially separated in cell nuclei of several man-made hybrids between grass species. An important question therefore is whether similar nonrandom genome disposition occurs in wild species. Milium montianum Pari. (2n = 22) is a naturally occurring allopolyploid grass combining two genetically dissimilar chromosome sets (V and M genomes), each originating from a different ancestral species. These two ancestral genomes were easily discriminated as all V genome chromosomes were larger than all M genome chromosomes. In two-dimensional spread preparations, the V genome derived from M. vernale Bieb. (In = 8), and the M genome (of different but uncertain origin) showed a highly significant tendency to he apart. Generally, the V chromosomes tended to surround the M chromosomes in both mitotic and meiotic nuclei suggesting that this arrangement persists throughout plant development. Such nuclear organization is probably under genetic control and may facilitate some independent behaviour of ancestral genomes in allopolyploids. Indeed it may play a significant role in plant evolution and speciation, especially if different intranuclear positions (e.g. central or peripheral) are correlated with preferential phenotypic expression of ancestral genes. © 1992 Annals of Botany Company.