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. portulacaeandAplanopsis terrestrisrespectively) 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 ofAlbugo, 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 liter
展开▼
