Showing papers by "G. Ledyard Stebbins published in 1976"

Species diversity, ecology and evolution in a primitive Angiosperm genus: Hibbertia ( Dilleniaceae )

Abstract: Among the approximately 130 species ofHibbertia found in Australia, there are tall shrubs, low or trailing shrubs and vines bearing a diversity of leaves as to shape and venation pattern. Flowers are solitary, in leafy cymes or in false spikes, and display various gradual and abrupt transitions from vegetative to reproductive appendages. In the androecium, stamen number is highly variable both between and within species. Some sections have radial symmetry, others bilateral symmetry of the androecium and gynoecium. Follicle number varies from 10 to 1. Basic chromosome numbers of n = 4, 5, 8, 9, 10, 12 and 13 have been found in various sections, and occasional higher numbers, up to n = 64, indicate the presence of polyploidy. Habitats vary from tropical savanna through rain forest margins, wet and dry sclerophyll forests, heaths, sphagnum swamps, and mallee scrub to desert margins. The principal center of diversity is southwestern Australia, less diverse centers are in southeastern and northern Australia. With respect to leaf size, structure and venation; floral symmetry; and chromosome numbers; the diversity found among the species ofHibbertia exceeds that found in all but a few genera of Angiosperms, and is greater than that in any other exclusively woody genus. Nevertheless, individual species are relatively constant with respect to both morphology and ecological preferences.

Chromosome, DNA and Plant Evolution*

Abstract: The past ten years have seen a remarkable rejuvenation in chromosome cytology. Cytologists are justified in distinguishing between the Old Karyology, which was concerned chiefly with chromosomes as taxonomic markers and with chromosome changes that influence genetic systems through their effects on linkage and recombination, and the New Karyology, which has a much broader base. Chromosomes are now recognized to be not only the carriers of genes in a linear sequence, but simultaneously they are highly complex organelles that contain many diverse mechanisms for controlling cellular proliferation, cell enlargement, and the differential action of genes during development. Some of these mechanisms are based upon the diverse nature of DNA with respect to both structure and function, while others, perhaps the majority of them, reside in the diverse proteins that are complexed with DNA to form the chromatin of chromosomes. These include both the histones, which are associated with the condensation of chromatin and nonspecific repression of DNA transcription (Arbuzova et al., 1968), and a much larger and more diverse assemblage of acidic proteins that perform a variety of functions, including specific activation of transcription (Cameron and Jeter, 1974; Stein et al., 1975ab). Consequently, a new concept that is highly relevant to evolutionists as well as to biologists in general is that of the nucleotype (Bennett, 1972, 1974). This includes not only the genic DNA, but in addition the entire battery of control systems that are built into the nucleus of eukaryote cells. The objective of the present paper is to review current knowledge of the nucleotype in plants, to point out its significance for understanding plant evolution, and to suggest the most promising lines of research for increasing our understanding of the role of the nucleotype in evolution.