In a cytological study, some isolates of Phytophthora meadii were found to be predominantly polyploid and to show normal oospore development, while others were predominantly diploid. The diploids produced mainly small oogonia, which degenerated during meiosis with the production of many empty oogonia; but also some larger oogonia with polyploid nuclei and normal oospore development. It is therefore suggested that P. meadii is derived from a species hybrid in which meiosis is upset by the genetical imbalance; and that chromosomal doubling during vegetative growth, followed by selection, leads to the production of tetraploid forms which can undergo normal meiosis. No differences were found in the soluble protein patterns of diploid and polyploid isolates. While both A1 and A2 compatibility types were found in each ploidy group, the polyploid A2's (presumed compatibility type heterozygotes) tended to be weakly self-fertile, and one of them gave rise to a self-sterile polyploid A1 culture. This demonstrates the difficulty of maintaining the heterozygous condition during vegetative growth in a polyploid. The possible role of polyploidy in the apparent decline of the A2 type in Sri Lankan P. meadii populations is discussed. © 1991, British Mycological Society. All rights reserved.
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