Fertility and meiotic mechanisms of hybrids between chromosome autoduplication tetraploid wheats and Aegilops species

During the production of Triticum-Aegilops amphidiploids, it was observed that some F1 hybrids between Triticum ssp. and Aegilops ssp. were self-fertile, with seed set ranging from 0.97 to 39.72%. Seeds from fertile F1's were spontaneous amphidiploids, as determined by root-tip chromosome counts. The present study revealed that the autoduplication of chromosomes in these F1's was caused by the tetraploid wheat parents T. persicum PS5 and T. durum DR147. Detailed meiotic studies of T. persicum PS5 crossed with Ae. squarrosa showed that during meiosis, there were two ways by which unreduced gametes could be formed: first, some of the PMCs omitted the first meiotic division and only underwent the second meiotic division and second, first division restitution with premature cytokinesis, which resulted in an asymmetrical dyad composed of one anucleate cell and one cell with a restitution nucleus. This cell produced two unreduced microspores by a normal second division. The use of this new knowledge has resulted in the production of numerous spontaneous amphiploids of T. persicum crossed with Ae. squarrosa, Ae. speltoides, Ae. umbellulata, Ae. comosa, Ae. ovata, Ae. ventricosa, Ae. crassa, and Ae. triuncialis. Many of these have been produced for the first time.Key words: Triticum, Aegilops, chromosome autoduplication, unreduced gametes, amphidiploids.

Electron microscopic study of oospore maturation and germination in an emasculate isolate of Saprolegnia ferax. 4. Nuclear cytology

Changes in nuclear ultrastructure during the formation, maturation, and germination of oospores produced by an agamospermous (emasculate) isolate of Saprolegnia ferax are described. Oogonium nuclei undergo synchronous meiotic divisions which appear identical to those previously reported in antheridiate species. Fusion of two typical gametic nuclei result in the formation of a single, unusually large, restitution nucleus within delimited oospheres. During the activation phase of germination the nuclei undergo three or four successive, near synchronous, mitotic divisions which appear similar to those described in other oomycetes. There was no evidence to support the occurrence of zygotic meiosis. Some nuclei in germinating oospores contain unusual spheroidal and lamellar inclusions, whereas others have abnormally large, irregularly lobed, profiles. Nuclear autolysis was also a common event in germlings. These results are compared with those in other oomycetes and the evidence supporting automictic sexuality discussed.

An apomictic mutant for sticky chromosomes in the fern Ceratopteris

A meiotic mutant has been derived from a triploid hybrid of the fern genus Ceratopteris. The mutant behavior involves chromosome stickiness that is regularly observed during both meiotic divisions. Chromosome stickiness is also infrequently observed during mitotic division. In addition, the first meiotic division frequently fails and results in the formation of a restitution nucleus. A single second meiotic division follows restitution and results in the production of a dyad of unreduced spores. Viable unreduced spores germinate to produce gametophytes containing the sporophytic number of chromosomes. Spermatozoids produced by the gametophytes are non-functional. However, sporophytes regularly develop apogamously from the gametophytes. Because meiotic reduction and syngamy are absent, the mutants are, reproductively, obligate apomicts.

Meiosis and sporogenesis in a haploid plant of Pellaea glabella var. occidentalis

In spore mother cells of a sporophyte of Pellaea glabella var. occidentalis (E. Nelson) Butters resulting from induced apogamy, the chromosomes did not pair during prophase I and separated randomly, usually into two groups. Meiosis II generally produced tetrads with nuclei of unequal sizes. Spore walls began to form around these nuclei, but most of the spores eventually aborted. One apparently normal spore, which later germinated, is assumed to have resulted from the formation of a restitution nucleus at the end of meiosis I.The total lack of pairing among the 29 chromosomes of this plant is evidence that this number represents the basic haploid complement of the species.

A Note on the Effects of some Cytotoxic Substances on Amphibian Embryos

In connexion with the experiments of Jurand (1958) on the action of TEM on avian embryos, it seems worthwhile describing very shortly some observations on the effects of this substance, and also of Myleran, on early amphibian embryos. Bodenstein (1954 and earlier) has described some of the cytological results of the administration of various ‘radio-mimetic’ substances to amphibian embryos, including nitrogen mustards and TEM. The effects on the development of the eye were particularly considered, and they have also been discussed and illustrated by Töndury (1955). Two main types of damage were noted, the formation of giant cells, and of cellular degeneration accompanied by nuclear breakdown and the formation of deeply staining cytoplasmic lumps. The giant cells presumably arise from the suppression of cell-division followed by the formation of a restitution nucleus. These nuclei stain rather palely, and appear to be lacking in DNA.