Mitosis and cell division in some cereal rust fungi. II. The processes of mitosis and cytokinesis

Before mitosis in intercellular Puccinia graminis f. sp. avenae, P. coronata f. sp. avenue, and axenic P. graminis f. sp. tritici and P. coronata, the nuclei were reduced in size by nucleolar extrusion and (or) partitioning of variable portions of the nucleus. Also there was increased vesiculation in the cytoplasm with a corresponding increase in lipid and carbohydrate storage material.The mitotic spindle first formed in one corner of the nucleus, then elongated until the spindle poles were oriented at either end of the nucleus. During the intermediate stages of mitosis the chromatin was arranged around the periphery of the spindle, which consisted mostly of chromosomal fibres. In the later stages the nucleus elongated and became dumbbell-shaped, with long straight fibres passing through the nucleus from pole to pole. The end of mitosis was marked by the chromatin assuming a ‘two-track’ configuration at the poles on either side of the intranuclear fibres and by the breakdown of the nuclear envelope in the constricted region of the dumbbell-shaped nucleus.After the daughter nuclei had separated, they migrated into new hyphal branches and septum synthesis was subsequently initiated. The septa grew by centripetal invagination in both the intercellular and the axenic hyphal states. There were often accumulations of mitochondria in the region of septal growth. Mature septa of intercellular P. coronata and axenic P. coronata and P. graminis tritici were typical of those found elsewhere in the rust fungi.

Mitosis and cell division in some cereal rust fungi. I. Fine structure of the interphase and premitotic nuclei

The detailed structure of the nucleus and its associated suborganelles in the rust fungi Puccinia graminis tritici, P. graminis avenue, P. recondita, and P. coronata is described. The non-mitotic nuclei in intercellular hyphae of all of the fungi examined were irregularly oval in shape and had prominent nucleoli, and except for P. recondita, heterochromatin was usually uniformly dispersed. In P. recondita, densely staining patches occurred throughout the nucleus, and this was a distinguishing feature of this species. The nuclei in monokaryotic axenic cultures of P. graminis tritici and P. coronata were larger than those in their respective dikaryotic parasitic hyphae or in a dikaryotic axenic culture of P. graminis tritici.The nucleoli varied in size and composition, depending on the physiologic condition or type of cell. In senescing cells the nucleoli occupied about 10% of the nuclear volume, while in young active cells the respective volume occupied was up to 60%. In haustoria the nucleoli were smaller in size and were composed mainly of fibrillar material. In active intercellular hyphae of all of the fungi examined the nucleoli consisted of about equal granular and fibrillar regions. There was a lighter-staining central region and similar light spaces in the fibrillar zones. These light areas were similar to the nucleoplasm in appearance and were interpreted as lacunae possibly continuous with the nucleoplasm. The rust fungal nucleoli and those of some higher plants were compared.A bipolar spindle pole body (SPB) was a regular feature of non-mitotic nuclei. The SPB consisted of two disc-like structures located some distance apart on a layer of amorphous substance. The SPB was located outside the nucleus in a depression of the nuclear envelope, usually toward one side of the nucleus. Subtending the SPB in the nucleus and joined to the SPB via a large pore in the nuclear envelope was a moderately dense region which consisted of an amorphous layer from which loose threads radiated into the nucleus. Occasionally a thread connected this region to the nucleolus.In several cells the nuclei were deformed with concomitant extrusion of the nucleoli. Nucleolar extrusion was seen in all material examined, and the process is described. Nuclear deformation and nucleolar extrusion were considered to be indicators of premitotic nuclei.

Memoirs: The Oogenesis of Calanus finmarchicus

Three regions can be recognized in the ovary: a multiplication zone containing oogonia undergoing mitosis, a synapsis zone containing the first formed oocytes in the prophases of the maturation division, and a growth zone containing oocytes in a series of growth phases with the nucleus in a ‘resting condition’. The oogonial nuclei contain two or three nucleoli--plasmosome and karyosomes. In the oocytes a single nucleolus is present; this is formed by the fusion of the plasmosome and at least one karyosome and is therefore an amphinucleolus. The chromatin in the oogonia and young oocytes is arranged round the periphery of the nucleus andis aggregated in knots (pp. 196-200). Nucleolar extrusion begins in the young oocyte and continues throughout the growth period. It is most marked in the young oocytes and in oocytes about to undergo maturation (pp. 200-3). In the older oocytes the chromatin is in the form of a tangled thread surrounding the nucleolus. Immediately before maturation this condenses and circular chromosomes emerge: these form tetrads (pp. 203-5). The mitochondria are present in the oogonia and very young oocytes in the form of a cap lying upon the surface of the nuclear membrane. The mitochondrial elements spread and multiply until they surround the nucleus as a ring; afterwards they disperse and are distributed evenly throughout the cytoplasm. They swell up and finally yolk-droplets appear in their place (pp. 207-11). Yolk-formation usually begins in half-grown oocytes, but is sometimes earlier. The formation of yolk-droplets begins at the periphery of the cell and proceeds inwards. It is suggested that yolk is formed by transformation of the mitochondria and the deposition in them of substances derived from the cytoplasm and the nucleolus. The cytoplasm is flocculent in the young oocytes, granular in the half-grown oocytes, and filled with fluid vacuoles in mature oocytes. It passes from a primary condition of oxyphily to basophily and finally back to a secondary oxyphil condition in mature oocytes (pp .211-13). In the young oocytes deeply-staining spherical structures were seen adjacent to the mitochondrial cap. From their appearance it is possible these bodies represent the Golgi apparatus, but Da Pano fixation failed to demonstrate them. In half-grown oocytes the apparatus was visible in the complex condition at one side of the cell. As growth proceeds it passes from a complex to a diffuse condition and in mature oocytes the Golgi elements are uniformly distributed throughout the cytoplasm (pp. 213-15).

VIII.—The Oogenesis of Carcinus mœnas Penn., with Special Reference to Yolk Formation

The present paper is the third of a series recording the results of investigations into the relations of the structures present in the oocyte during the growth period, particularly in their bearing on yolk formation. A Crustacean was selected for various reasons: firstly, as representing a large class about the oogenesis of which little is known, this in contrast to most other Arthropod groups which have been studied by several authors; secondly, in the hope that it would prove to have no yolk nucleus, and would exhibit nucleolar extrusion, a hope which has been fairly satisfactorily justified; thirdly, for the utilitarian reason that it is a common animal, easily obtainable, and able to live under laboratory conditions during considerable periods.