Cytological observations on Phytophthora cactorum, P. erythroseptica and an Achlya species indicate that these organisms are diploid with meiosis occurring in the sex organs just before gamete formation. In P. cactorum and P. er ythroseptica it has been shown that two nuclear divisions occur in the gametangia, the first division having a long prophase and that the resulting nuclei are about half the size of the vegetative nuclei. There is no growth in size of the nuclei between the first and second divisions. The observation of a bridge and fragment, presumably resulting from crossing over in an inversion, at anaphase of P. cactorum, and of multivalents in polyploid nuclei found after camphor treatment of oogonia and antheridia of P. cactorum, P. erythroseptica and an Achlya sp. are taken to be critical cytological evidence that the divisions in the gametangia are meiotic divisions. In P. cactorum dead oospores have been reported interspersed among healthy oospores. If these are due to segregation, they would not be expected to occur in a homothallic species. In P. erythroseptica about 10% of the oogonia fail to produce oospores. In this case segregation must have occurred before the division of the oospore nucleus, in fact during gamete production. The thesis that the Oomycetes as a whole are diploid is discussed briefly in relation to its genetical and phylogenetical implications. The problems of the behaviour of chromosomes of fungi during vegetative divisions is also discussed.
Nuclear constituents were degraded by nucleases and proteinases. Patterns of depolymerization are described in interphasic and in mitotic stages of Euglena gracilis var. bacillaris treated “in vivo” with DNAse, RNAse, trypsin and α-chymotrypsin for exposure extended from 2 to 6 hours. DNAse degraded chromatin at all stages, and it also hydrolyzed a nucleolar constituent. RNAse moderately degraded one nucleolar component and certain regions in the prophase chromosomes. Observations on control cells and RNAsetreated cells showed that the nucleolus possesses highly coiled filamentous structures which resemble chromonemata and extend into the nucleolar chromosomes. Treatment by nuclease followed by proteinase demonstrated that the nucleolar filamentous structure resembling chromonemata was degraded by DNAse and by either proteinase but was RNAse-resistant. This nucleolar structure is not regarded as the nucleolonema but as the nucleolar organizer proper, and as such chromosomal in nature.
1. In spite of the many and varied reports on spontaneous mutations the factors responsible for their widespread occurrence in nature have not so far been clearly understood. Investigations carried out in the present study showed that decaying organic substances like compost, cowdung and oil cakes when present in high concentrations in the soil are capable of inducing mutations of different magnitudes. Chromosome changes induced by these substances and their possible role in the production of mutations in nature are discussed in this paper. 2. High concentrations of compost, cowdung and oil cake in sand exert a distinctly harmful effect as regards germination and growth. Very high concentrations of bassia oil cake (1 oil cake: 5 sand) inhibited germination. Though germination was observed in fresh cowdung the plants did not survive. No such inhibition in germination was observed in compost unmixed with sand. Lower concentrations of these substances have been found to be beneficial. 3. Cytological changes induced by compost, cowdung and oil cakes were almost similar. Breakage of chromosomes followed by reunion resutling in repatterning of chromosomes (translocation and inversion), chromosome doubling leading to polyploidy and spindle abnormalities resulting in the formation of aneuploid cells were commonly observed. Chromosome clumping, cytomixis, asynapsis, erosion of chromosomes etc. were some of the other abnormalities observed. 4. Quantitative analysis of the data showing the frequencies of the different abnormalities in the different treatments showed that the frequency of abnormalities increases with an increase in the concentration of the substances in the sand. 5. There is a gradual decrease in the percentage of aberrations with longer duration in compost, this relation being found in the pollen mother cells also. It is suggested that this decrease may possibly be due to the gradual fall in the content of nitrogen or other substances in compost causing these aberrations. 6. Cytological changes induced by a few fertilizers like ammonium sulphate, ammonium nitrate, ammonium phosphate and potassium chloride were studied with a view to comparing their effects with those of the organic manures used for the study. Abnormalities in cell division induced in the root tip cells of onion following treatment with these chemical fertilizers were almost similar to these observed in treatments with organic manures like compost, cowdung and oil cakes. The results of the studies appear to show that nitrogen, phosphorus and potassium can act as mutagenic factors in compost. The differences exhibited by compost, bassia oil cake and cowdung might probably be due to the variation in the relative concentrations of nitrogen, phosphorus and potassium present in them. 7. There is striking similarity between the aberrations induced by decaying organic substances like compost, cowdung and oil cakes and those reported as occurring spontaneously. It is suggested that decaying organic matter which is present in plant environment under natural conditions in varying concentrations, and those added as manures in agricultural practices, are responsible for some at least of such spontaneous mutations which play an important part in the evolution of species.
Roots of Allium cepa (Var. Giza 6) were treated for different periods of time with solutions of pure and formulated Sevin prepared at 22° and 60°. The end-effect of both forms of the insecticide depended mainly on the temperature of preparation of the agent solution. Solutions prepared at 22° possessed merokinetic tendencies, while those prepared at 60° showed stathmokinetic tendencies. Continuous treatment with the insecticide for 24 hours nearly arrested mitosis. However, the roots recovered after replacement in water for 48 hours and the rate of mitosis became normal again.
The branching was observed to occur in fern gametophytes cultured in various light conditions (strong white light of 2, 500 lux, red light and darkness). The nuclear volume, amounts of total nitrogen and total nucleic acid phosphorus, and other cytochemically detected elements suggested strongly a distinct apico-basal gradient of the physiological activity in the gametophyte, and the growth was carried out only at the apical region likely of the greatest activity. Light conditions as mentioned above seem to destroy this gradient, to alter the developmental axis and consequently to induce the branching.
The two species, Isopsera sp. and Letana atomifera belonging to the family Locustidae, a group of long-horned grasshoppers, were found to have unusual sex chromosome constitutions namely, neo-X+neo-Y and X1X2 +neo-Y respectively, in the male individuals. At the present status of our knowledge of chromosomes in the Locustidae, it is impossible to draw any conclusion regarding the nature of the probable evolutionary pathway in these insects.
Interspecific hybrids were obtained between H. depressum (4X) and the allotetraploids (H. pusillum No. 53×H. compressum), (H. marinum No. 56-39×H. pusillum×H. pusillum No. 53) and (H. marinum No. 56-39×H. compressum). These hybrids were all self-sterile. Interspecific hybrids between H. depressum and allotetraploid (H. pusillum No. 53×H. compressum) produced seed through backcrossing to the male parent. Cytological examinations indicated that at least some of the chromosomes of H. depressum pair with the chromosomes of H. pusillum.
Autotetraploid plants were induced in Trigonella foenum-graecurn by means of colchicine. Seed treatment appears to be quite lethal, but seedling treatment resulted in the recovery of some polyploid plants. Comparative morphological studies between tetraploid and hexaploid plants have been made. The tetraploid plant manifested gigas characters for most of the morphological characters. In addition, the tetraploid plants possessed a higher number of branches. The cytology of the tetraploid plant has been studied in detail. The high percentage of pollen sterility accompanied with very poor setting of seed has been attributed, mainly, to the abnormalities recorded at anaphase I and II. The practical value of such induced autotetraploids has been discussed.
By means of phase cinematography the behavior of the daughter chromosome groups was investigated in silkworm spermatocytes exposed at telophase to ultraviolet microbeam partial irradiation. Information was obtained that in cells received microbeam irradiation in a restricted part at telophase the daughter chromosome group moved rapidly from the irradiated pole toward the non-irradiated opposite pole. Exposure to several other parts or to two poles in telophasic cells induced no such an unusual behavior of chromosomes.
X. fenesterata has been found to be arrhenotokously parthenogenetic, and the haploid male/diploid female chromosomal relationship is 16/32. The male meiosis I is abortive but a cytoplasmic bud, usually considered as the vestigial secondary spermatocyte, is not produced. The second spermatocyte division is also anomalous as only one functional spermatid is formed.
A natural triploid isolated from a diploid population of T. 1242 has been used for establishing trisomics for linkage group studies. The triploid plant when cytologically analysed at pachytene showed chromosomal differences indicating its allo-polyploid nature. The triploid was used as a female parent in triploid-diploid crosses for the synthesis of trisomics. Morphological variations of the trisomics corresponding to chromosomal differences have been studied. Trisomics showed morphological variations with partial sterility. These variations have been found to be associated with specific chromosomes present as extra in these plants. Five out of twelve (n=12) types of chromosomes have been associated with certain diagnostic morphological traits indicating their relationships with specific linkage groups. Correlations of the morphological traits of triploid-diploid progenies in the F2 with their respective chromosomal set up have also been computed with a view to detecting linkage relationships.