Meiosis in Diploid and Polyploid Sex Organs of <i>Phytophthora</i> and <i>Achlya</i>

Abstract

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.