Evidence for mitochondrial gene control of mating types in Phytophthora

When protoplasts carrying metalaxyl-resistant (Mr) nuclei from the A1 isolate of Phytophthora parasitica were fused with protoplasts carrying chloroneb-resistant (Cnr) nuclei from the A2 isolate of the same species, fusion products carrying Mr nuclei were either the A2 or A1A2 type, while those carrying Cnr nuclei were the A1, A2, or A1A2 type. Fusion products carrying Mr and Cnr nuclei also behaved as the A1, A2, or A1A2 type. The result refutes the hypothesis that mating types in Phytophthora are controlled by nuclear genes. When nuclei from the A1 isolate of P. parasitica were fused with protoplasts from the A2 isolate of the same species and vice versa, all of the nuclear hybrids expressed the mating type characteristics of the protoplast parent. The same was true when the nuclei from the A1 isolate of P. parasitica were fused with the protoplasts from the A0 isolate of Phytophthora capsici and vice versa. These results confirm the observation that mating type genes are not located in the nuclei and suggest the presence of mating type genes in the cytoplasms of the recipient protoplasts. When mitochondria from the A1 isolate of P. parasitica were fused with protoplasts from the A2 isolate of the same species, the mating type of three out of five regenerated protoplasts was changed to the A1 type. The result demonstrated the decisive effect of mitochondrial donor sexuality on mating type characteristics of mitochondrial hybrids and suggested the presence of mating type genes in mitochondria. All of the mitochondrial hybrids resulting from the transfer of mitochondria from the A0 isolate of P. capsici into protoplasts from the A1 isolate of P. parasitica were all of the A0 type. The result supports the hypothesis of the presence of mating type genes in mitochondria in Phytophthora.

Non-Mendelian inheritance revealed in a genetic analysis of sexual progeny of Phytophthora cinnamomi with microsatellite markers

We report the development of four microsatellite loci into genetic markers for the diploid oomycete plant pathogen Phytophthora cinnamomi and that (AC)(n) and (AG)(n) microsatellites are significantly less frequent than in plant and mammal genomes. A minisatellite motif 14 bp long was also discovered. The four microsatellite loci were used to analyze sexual progeny from four separate crosses of P. cinnamomi. A large proportion of non-Mendelian inheritance was observed across all loci in all four crosses, including inheritance of more than two alleles at a locus and noninheritance of alleles from either parent at a locus. The aberrant inheritance is best explained by nondisjunction at meiosis in both the A1 parent and the A2 trisomic parents, resulting in aneuploid progeny. Two loci on the putative trisomic chromosome showed linkage and no loci were linked to mating type. One aneuploid offspring was shown to have lost alleles at two loci following subculture over 4 years, indicating that aneuploid progeny may not be mitotically stable.

Creation of hybrid vigor through nuclear transplantation in Phytophthora

When isolated nuclei of a diploid oomycete, Phytophthora parasitica, were fused with protoplasts of another strain of the same species, the regenerated nuclear hybrids grew faster than the parental isolates. Such a phenomenon did not occur in hybrids regenerated from mitochondrion-protoplast or protoplast-protoplast fusion products between these two strains. These results indicate that hybrid vigor is the result of the interaction between two different kinds of nuclei, but not between mitochondria, and they suggest that the presence of mitochondria from nuclear donor cells represses the expression of increased vigor. The nuclear hybrids also expressed increased fungicide resistance and propagule production. Increased vigor in growth was also observed in the interspecific nuclear hybrids when isolated nuclei of P. parasitica were transferred into protoplasts of Phytophthora capsici, and vice versa. This phenomenon may have potential applications, such as the creation of superior fungal strains and plant cultivars with improved commercial traits for usage in industry and agriculture.

Transplantation and subsequent behavior of mitochondria in cells of Phytophthora

Mitochondria isolated from streptomycin-resistant (S(r)) protoplasts of Phytophthora parasitica were transferred into chloramphenicol-resistant (Cpr) protoplasts of P. parasitica or Phytophthora capsici with an average successful rate of 1.7 x 10(-4), using a selective medium containing streptomycin. No colonies appeared when self-fusion products of donor mitochondria or recipient protoplasts were exposed to the selective medium. Mitochondria isolated from Cpr protoplasts of P. capsici were also transferred into S(r) protoplasts of P. parasitica with a similar success rate using a selective medium containing chloramphenicol. Zoospores produced by the Cpr + S(r) intraspecific mitochondrial hybrid gave rise to S(r) and Cpr + S(r) cultures. The second generation zoospores produced by S(r) and Cpr + S(r) cultures also gave rise to S(r) and Cpr + S(r) cultures, suggesting the possible occurrence of fusion between some of the Cpr mitochondria and S(r) mitochondria, and the displacement of non-fused Cpr mitochondria in the receptor protoplast by the donor S(r) mitochondria. Zoospores produced by the interspecific mitochondrial hybrid gave rise to Cpr, S(r), Cpr + S(r), and Cps + Ss cultures. The second generation zoospores produced by Cpr + S(r) or S(r) cultures also gave rise to the same four types of cultures, suggesting the existence of residual antibiotic-sensitive mitochondria (Cps + Ss) in the parental isolates and the random distribution of Cpr, S(r), and Cps + Ss mitochondria during asexual reproduction. Results suggest that the phenotype of antibiotic resistance/sensitivity was the end result of the interactions among the three types of mitochondria.

Segregation following interspecific transfer of isolated nuclei between Phytophthora parasitica and P. capsici

Nuclei isolated from metalaxyl-resistant (MR) protoplasts of Phytophthora parasitica were transferred into chloroneb-resistant (CnR) protoplasts of Phytophthora capsici and vice versa, with an average success rate of 2.6 x 10(-4) (protoplasts with donor nuclei/regenerated protoplasts), using a selective medium containing only the fungicide tolerated by the nuclear donor. No colonies appeared when self-fusion products of donor nuclei or recipient protoplasts were exposed to the selective medium. Colonies produced by the nuclear transfer formed sectors commonly, and differed from the parental types in appearance. All the zoospores produced by the nuclear hybrids were of normal size, and one-fifth of them contained both MR and CnR genes. Since zoospores are mostly uninucleate, these results indicated the occurrence of chromosome re-assortment or mitotic crossing-over following the production of transitory tetraploids, followed by diploidization during zoosporogenesis, thus suggesting the completion of events leading to a parasexual cycle. Hyphal fragment cultures from a nuclear hybrid tested showed considerable variation in growth rate, mycelial morphology, and level of resistance to metalaxyl, indicating uneven distribution and continuous segregation of different types of nuclei in mycelia during vegetative growth.