Single Gametes Research Articles

Genetics of the polycross

Rates and patterns of male gamete incorporation for a polycross mating design were studied for two independent years of pollination in Norway spruce, Picea abies (L) Karst. Segregation distortion in a subset of maternal clones was documented for one locus. We have proposed a model, involving the existence of a linked lethal allele, which accounts for these observations. Significant temporal and maternal clonal differences were observed in the rates at which single locus and multilocus gametes were incorporated. Striking differences in apparent fertility existed among four clones which produced unique multilocus gametes. One clone, in particular, was shown to be contributing three times as many gametes to the next generation as predicted by the hypothesis of equal clonal male contribution. These deviations from expectation were also detected in the genotypic distributions of the resultant filial generation. Ramifications of these results on family structures in the filial generation, effective size of the male population, and possible bias in inferences of genetic differences and parameter estimation are discussed.

Kinetics of adhesion and de-adhesion of Chlamydomonas gametes.

In medium with low nitrogen content, vegetative strains of the unicellular biflagellate alga Chlamydomonas reinhardi form gametes. Mating type plus (mt+) and mating type minus (mt-) gametes adhere via their flagella to give aggregates in which the gametes eventually fuse to form zygotes. A quantitative assay has now been developed which measures aggregation and fusion by use of a Coulter electronic particle counter to determine loss of single gametes as they form aggregates in suspension. Determination of the rate and extent of cell fusion by microscopy agrees with the results obtained with the more rapid and convenient Coulter counter assay. By use of the assay it was found that aggregation and fusion occur at the same rate and to the same extent at 12 degrees C and 25 degrees C. Flagella from one of the mating types can specifically substitute for the corresponding live gametes; more than 70% of the gametes were aggregated and the extent of aggregation was proportional to the number of flagella added, until the ratio of cells to flagella exceeded 2. At 22 degrees C, in the flagella/gamete mixtures, adhesion was complete in less than 5 min, but at 5 to 10 min, gametes began to de-adhere from the clusters and, depending on the number of flagella added, essentially all of the gametes detached from the aggregates in 10 to 50 min. The gametes in such mixtures were fully competent to aggregate again, whereas the flagella recovered from such mixtures were shown by use of a radioactive flagella-binding assay to be inactive with fresh gametes. Inactivation of the flagella was temperature-dependent, was not catalyzed by soluble factors, and required adhesion of flagella to gametes of the opposite mating type. The potential physiological functions of the de-adhesion process are discussed.

THE DEVELOPMENT OF CYTOLOGICAL THEORY IN THE OOMYCETES

Summary1. Meiosis in the Oomycetes is gametangial.2. The life‐cycle of the Oomycetes is therefore haplobiontic, type B.3. The gametangia are multinucleate prior to septation. Vegetative nuclear divisions may occur in the hyphae subtending the gametangia, but there is no evidence for such divisions occurring in the gametangial primordia nor is there any indication that nuclei may move out of the primordium against any cytoplasmic flow.4. Some abortion of supernumerary nuclei probably occurs after the gametangium is cut off from the vegetative thallus by the septum. Meiosis then takes place.5. The spindle of the first metaphase is almost certainly within a persistent nuclear membrane, but there remains some doubt as to whether this membrane persists to the second telophase in all Oomycetes.6. In the higher Peronosporales, and possibly the Rhipidiaceae, meiosis is accompanied or preceded by zonation into the periplasm and ooplasm. Spindle orientation and the timing of zonation movements probably account for the differences in the number of presumptive oosphere nuclei recorded between many Peronosporales. In some Albuginaceae, at least, it is possible that only one nucleus completes the meiotic division, but this needs confirmation.7. A smaller number of nuclei enter the male gametangium and undergo a more or less simultaneous meiosis.8. Some variation in the pattern and degree of synchrony of meiotic division within and between gametangia occurs in different species.9. Nuclear abortions may precede, accompany or follow meiosis, but only in a few instances (Pythium debaryanum, P. deliense, Phytophthora himalayensis, Aphanomyces laevis) does the male gametangium finally contain only a single gamete nucleus.10. Cytoplasmic cleavage, involving the tonoplast and central vacuole of the oogonium, occurs after meiosis in the Saprolegniales, thus offering an alternative mechanism to zonation movements for the production of uninucleate oospheres. The presence (Edson, 1915; Patterson, 1927b; Murphy, 1918) or absence (Trow, 1901; Saskena, 1936) or an homologous central vacuole in the Pythiaceae is disputed.11. Karyogamy must follow antheridial penetration in those species which are not agamospermous, but the degree of facultative agamospermy is unknown. The timing of karyogamy, as opposed to somatogamy, is apparently variable between and within species (Wager, 1899; Arens, 1929, Moreau & Moreau, 1935; McDonough, 1937; Flanagan, 1970; Win‐Tin, 1972). There are a few indications that karyogamy may be precocious and other evidence that it may be considerably delayed, even after the oospore has achieved morphological maturity, and exceptionally until germination.12. It would appear that the majority of the oospores of most Oomycetes eventually contain only one fusion or diploid nucleus, but there are exceptions (Albugo bliti, A. platensis, A. portulacae and Aplanopsis terrestris respectively) and without further study it would be unwise to assume that this is necessarily true even for closely related species.13. Mitosis immediately following karyogamy is reported as occurring in some species of Albugo, but in most Oomycetes it is delayed until the period immediately preceding any cytoplasmic or morphological change at the start of germination.14. The nuclear divisions of the germinating oospore are mitotic, but they may differ in the detailed morphology of the spindle apparatus or the degree of condensation of the chromosomes.15. Interpretations of the cytology of the small nuclei of the Oomycetes have been profoundly influenced by the prevailing climates of scientific opinion. In particular, the development of studies of meiosis and the science of genetics on the one hand, and the appreciation of the polyphyletic origin of the fungi, especially the algal origins of the Oomycetes on the other hand, have necessitated a re‐evaluation of much of the older literature.

Heterogeneity, complexity, and repetition of the chloroplast DNA of Chlamydomonas reinhardtii.

The chloroplast DNA of wild-type Chlamydomonas reinhardtii was isolated in a CsCl density gradient as a single, homogeneous density class with a mean density of 1.695 g/cm(3). Irrespective of sheared size, denatured chloroplast DNA renatured as a single homogeneous species. Compositional heterogeneity, presumably intramolecular, was revealed by the absorbance-temperature profile. The complement of unique nucleotide sequences of the chloroplast DNA, as determined by the rate of renaturation, was 1.94 x 10(8) daltons. This kinetic complexity is 26-fold less than the DNA content of a single gamete chloroplast, and 52-fold less than the chloroplast of a vegetative cell, which indicates that the chloroplast of C. reinhardtii possesses at least 26 copies of a unique nucleotide sequence.

DIE SEXUELLE FORTPFLANZUNG DER PENNATEN DIATOMEEN

DIE SEXUELLE FORTPFLANZUNG DER PENNATEN DIATOMEEN