Genetics Of Phytophthora Boehmeriae

The inheritance and variation of the biological characters including colony morphology, growth rate, homothallism, pathogenicity, metalaxyl-resistance and antheridium attachment mode of Phytophthora boehmeriae Sawada, the important homothallic Phytophthora species causing cotton and ramie blights in China, were reporteed in this paper.Based on the results of studies on the inheritance of colony morphology and growth rate of P. boehmeriae isolates from different localities and various host plants ,the genetic diversity of the two characters above was found for the first time in P.boehmeriae, i.e. there were at last three patterns on the inheritance of the two characters as follows. (1) The two characters were inherited steadily in both zoospore and selfed oospore progenies -, (2) Colony morphology and growth rate were inherited stably in zoospore generations but showed obvious variation in first selfed oospore progeny (S1); (3)The two characters displayed obviously variation both in zoospore generations and in S1 progenyr It was suggested that colony morphology and growth rate of P. boehmeriae might be controlled by nuclear homozygous genes or by nuclear heterozygous ones and or by cytoplasmic genetic factors However, homothallic character was inherited stably in asexual and sexual progenies, which suggested that the genetic factors controlling the character was homozygous in P- boehmeriae.The studies on inheritance and variation of colony morphology and growth iate of S1 single oospore cultures of isolate JS - 5 of P. boehmeriae obtainedfrom cotton boll rot tissue in Jiangsu provinces in zoospore generations indicated that about 2/3 of Si progenis showed continuing variation in colony morphology and growth rate in 3--4 successive single-zoospore generations, but the parental isolate JS -5 not, suggesting that except for causing gene recombination , the sexual reproduction induced a new mechanism which may be involved in regulating and controlling the inheritance and variation of the characters. Futhermore, it was proven that the above mechanism was not associated with heterokaryosis. The phenomenon of continuing variation in asexual generations induced by sexual reproduction may be an important channel to genetic diversity of biological characters of P- boehmeriae-The inheritance of pathogenicity of isolate JS-5 of P. boehmeriae to cotton seedlings in zoospore and oospore progenies was investigated by culture-block-wound inoculation. It was reported for the first time that the pathogenicity of the pathogenic oomycete to cotton seedlings was stably inherited in two successive zoospore generations but showed obviously variation in selfed oospore progenies , indicating that the pathogenicity of P. boehmeriae to cotton seedlings might be determined by nuclear heterozygous polygenes and that the sexual reproduction played an important part in the differentiation of pathogenicity in the pathogenic organism.The studies on characteristics and inheritance of metalatyl-resistance of P. boehmeriae indicated that the oomycete developed the resistance to meta-laxyl easily, and the resistance level was over 1800 times higher than that of the wild-type and metalaxy-sensitive parent isolate, but that the resistance to metalaxyl of the metalaxyl-resistant mutants reduced or lost gradually in the condition of not contacting metalaxyl, and was unstably inherited, i. e. they reverted to metalaxyl-sensitive or showed continous variation in successive zoospore progenies. It was suggested that the resistance to metalaxyl in /'. boehmeriae might be controlled by the unstably inherited mitochondrial genes in cytoplasm. Based on the results a strategy for managing metalaxyl resistance of the pathogen was suggested.The phenomenon of oogonia with paragynous antheridia(PaA ) and the in-heritance of antheridium attachment mode were studied with the isolates of P. boehmeriae from different regions and various hosts as the microorganisms tested. It was proven for the first time that the paragynous phenomenon was common in the isolates of P. boehmeriae in China, and that the paragynous antheridium character could be inherited in both asexual and sexual progenies. However, the percentages of oogonia with PaA displayed considerable variation among different isolates and among the monosporous cultures derived from the same parental isolates, and the variation ranges were obviously greater in the sexual progenies than in the asexual progenies derived from the same parental isolates. The results suggested that character of antheridium attachment mode in P. boehmeriae might be controlled by cytoplasmic factors and associated partly with nuclear genes. Furthermore, temperature, illumination and cultural condition of agar medium or broth had all no significant influences on the character. However, the medium nutrient had very significant effects on the character. The test isolates formed predominantly amphigynous an-theridia on some media, such as lima been agar (LBA), and produced mostly paragynous antheridia on certain media, such as lecithin-basal-salt-solution a-gar (LSA). And the PaA percentage increased with a decrease in LBA concentration, and decreased with an increase in LSA concentration. It was suggested that the medium nutrient had significant effects on the expression of paragynous and/or amphigynous character (s) in P. boehmeriae. However, the effects could not inherited. The results mentioned above were important both for the description of morphological characteristics of P. boehmeriae and for the re-vealment of genetic background of antheridium attachment mode in P. boehmeriae and other related oomycetes.In addition, the effects of light, humidity and medium on the oospore production (OP) of P â–  boehmeriae were investigated by applying three-factor randomized block design. The results showed that medium, illumination and the all interactions of the above three factors had significant effects on OP- Among the different combinations, OP from the combination of low humidity+continuous darkness+SL A was the highest and that from low humidity+continuousillumination+ V6B was the lowest. The results were valuable both to the genetic studies on P- boehmeriae and to the control of the plant bights it causes-...