No abstract

The reproductive biology of 28 native and four exotic species of Senecio is described in terms of their breeding system, longevity, habitat stability, seed size and number, dispersal potential and seedling establishment. Nineteen species (15 perennial, 2 annual, 2 ephemeral) are self-incompatible, 13 are self-compatible (4 perennial, 7 annual, 2 ephemeral) and nearly all occur in unstable environments. The success of the perennial species in unstable environments perhaps reflects the fact that they all reach sexual maturity within 1 year of germination and, provided populations of self-incompatible species are large enough, they can set seed in the first year.

Nuclear DNA amounts of 28 species of Senecio native to Australia and of four exotic species are reported. DNA amounts per 4C nucleus range from 4.22 pg to 42.90 pg, a 10-fold difference. Some of the variation is due to polyploidy as chromosome numbers range from n = 5 (by dysploid reduction) to n = 50, but DNA amounts per genome also vary from 0.84 pg to 4.69 pg, a 6-fold difference. DNA amounts correlate with cell size and, in some instances, the size of structures. There is also a correlation between DNA amount and generation length which corresponds with previous observations. The possible direction of change in DNA amount and the influence of natural selection on this process are discussed.

Absolute DNA amounts of individual chloroplasts from mesophyll and epidermal cells of developing spinach leaves were measured by microspectrofluorometry using the DNA-specific stain, 4,6-diamidino-2-phenyl indole, and the bacterium, Pediococcus damnosus, as an internal standard.Values obtained by this method showed that DNA amounts of individual chloroplasts from mesophyll cells fell within a normal distribution curve, although mean DNA amounts changed during leaf development and also differed from the levels in epidermal chloroplasts. There was no evidence in the data of plastids containing either the high or low levels of DNA which would be indicative of discontinuous polyploidy of plastids, or of division occurring in only a small subpopulation of chloroplasts. By contrast, the distribution of nuclear DNA amounts in the same leaf tissues in which cell division was known to be occurring showed a clear bimodal distribution. We consider that the distribution of chloroplast DNA in the plastid population shows that there is no S-phase of chloroplast DNA synthesis, all chloroplasts in the population in young leaf cells synthesize DNA, and all chloroplasts divide.There are a number of papers in the literature reporting changes in chloroplast DNA amounts determined by methods involving DNA extraction and cell and chloroplast counting (3,6,16,17

Absolute DNA amounts of individual chioroplasts were determined by measuring the fluorescence intensity of chloroplasts stained with 4',6-diamidino-2-phenylindole (DAPI) relative to that of the bacterium Pediococcus damnosus (cerevisiae) smeared on the same slide. An absolute DNA content of 7.7 x 10-i' g for a standard P damnosus cell type was calculated by comparing the relative fluorescence values and frequency ofeach stage ofcellular development in a culture to the average DNA content of all cell types determined by chemical methods. Chlorophyll was extracted from the chloroplasts during fixation so that chlorophyll autofluorescence was not present when DAN fluorescence was measured.