The timing of mitosis and meiosis in Allium ascalonicum: A problem of differentiation

Abstract

I. THE ABNORMAL PLANTTOGETHER with a fertile crop of onions raised at Benares, one plant was seen to set no seed in the glasshouse. The bulb was split and all the sets flowered in England. Their anthers, however, never burst. When ripe they turned green instead of yellow. Sections cut by the freezing microtome showed much chlorophyll in the anther wall. Within the anther the pollen grains dried up and died after their first mitosis. The development of the embryo sac was normal. With foreign pollen, moreover, the plants set seed which germinated well.Thus the sterility of our plant, or clone as it now is, is due to failure of the pollen to develop to maturity. This decisive and total disability is preceded, however, by certain small and sporadic errors much earlier in the development of the pollen-forming tissue. From a study of these errors there came to light an instructive group or syndrome of defects whose origin we have attempted to trace. METHODSAnthers and root tips were fixed in fresh acetic-alcohol (i 3) for ten minutes and stored overnight in 95 per cent, alcohol before staining by the Feulgen method (La Cour, 1947). Equally good results were also obtained by fixing anthers overnight in acetic-alcohol with a trace of ferric chloride followed by staining in weak acetocarmine (P. T. Thomas, unpub.).For studying their development the ovules and anthers were fixed in La Cour's 2 BE after a short treatment with Carnoy's fluid. The material was embedded and sections cut at 10-14 were stained by the Feulgen method. Light green was used as a counterstain for the cytoplasm and the cell walls. MEIOSIS IN THE ANTHERSThe chromosome complement as seen in the root tips consists as normally in A ilium ascalonicum of eight pairs of chromosomes of which two pairs-one longer than the other-had median and the rest sub-median centromeres. The latter class included a satellited pair carrying the nucleolar organiser (plate, fig. i).In the anthers of most of the flowers from different inflorescences meiosis is regular. In the remainder, about one-third, up to 20 per cent, of the pollen mother cells in every anther show a group of meiotic abnormalities including asynapsis.

THE EFFECT OF ACTIDIONE ON MITOSIS IN THE SLIME MOLD PHYSARUM POLYCEPHALUM

“…Protein was analysed by a modified Folin procedure (Lowry et al, 1951 Two observations suggest that the nucleolus may play a more than passive part in the mitotic process: (a) mitosis became relatively resistant to Actidione treatment at some time shortly after the beginning of nucleolar dissolution, and (b) nucleolar reconstruction proceeded (albeit at times at a reduced rate) even though the synthesis of nuclear proteins was inhibited. A relationship between the nucleolus and mitosis has frequently been suggested from earlier experiments based on direct observation (Robinow, 1963), the behavior of plant micronuclei (La Cour, 1953;McLeish, 1954;Darlington and Hague, 1955), genetic ex-periments (Philp and Huskins, 1931;McLeish, 1955;and McClintock, 1934), and ultraviolet microbeam irradiation experiments (Gaulden and Perry, 1955). On the other hand, embryos of frog (Elsdale, Fischberg, and Smith, 1958) and insect (yon Borstel and Rekemeyer, 1958) went through several cleavage divisions when nucleolar organizing regions of the chromosomes were absent.…”

Anomalous chromosome behaviour in the germ line of Schistocerca gregaria

“…Here it would appear that chromosome segments which pair first are preferred in chiasma formation presumably because they reach the appropriate stage for exchange before the later paired regions. In at least some instances of asynapsis or desynapsis there are definite indications that general disturbances of timing relations are involved (Rees, 1952;Darlington and Haque, 1955). Certainly alterations in the time available for pairing appears to be one of the prime variables of meiosis (Darlington, 1940).…”

Genetic systems in Allium

“…But the former reproduces by bulbils and the latter apomictically (Gustafsson,'947). An interesting case of sterility owing to genotypic unbalance has been shown in A. ascalonicum (Darlington and Haque, 1955). It usually reproduces vegetatively including by bulbils.…”

“…Correlation between polyploidy and apomixis has been pointed out by Gustafsson (1947) but this is not necessarily a casual one. It is instead more a case of pre-adaption (Darlington, 1956;Lewis and John, 1963). Pseudogamy in the diploid as well as tetraploid A.…”

“…Such factors have not been studied in A ilium. Similarly, B-chromosomes increasing the amount of variability in a population (Darlington, 1956) are also important and some Allium species are known to carry them (see Ved Brat, 1965). An interesting suggestion but based on too meagre data is presented by A. paniculatum.…”