Heterochromatin and ecological adaptation in Southern AfricanOrnithogalum(Liliaceae)

SUMMARYChromosome analysis of a group of Ornithogalum from Southern Africa, using Quinacrine fluorochrome staining and C-banding, has revealed a strong bias towards an increase in heterochromatin content and adaptation to extreme habitats. The group consists of chromosome races with equivalent karyotype length but with chromosome numbers varying from six to twelve. They have a chromosome complement of 6 large (L-) subterminal chromosomes and from 0 to 6 small (S-) submedian chromosomes. In the L-chromosomes the heterochromatin is located in intercalary positions and adjacent to the nucleolar organizers. The S-chromosomes are devoid of detectable heterochromatin. Habitats and climatic conditions of the eleven races studied, range from semi-desert with long droughts broken by periodic precipitations, to temperate scrubland. Each high-H race is endemic of a relatively small area, often consisting only of a small hillock or rock-scree. The adaptation of the high-H races includes a sequential vegetative cycle probably influenced, in its alternations and duration, by the different amount of heterochromatin. The existence and persistence of the races, each one a biological species in its own rights, may be due to effective reproductive isolation and to the great climatic stability, resulting from the distinctive geology of Southern Hemisphere, where glaciations, affecting the North, did not occur and were replaced by alternations of humid and droughty periods.