Natural Hybrids of Phytophthora nicotianae and Phytophthora cactorum Demonstrated by Isozyme Analysis and Random Amplified Polymorphic DNA

Abstract

Three similar isolates of Phytophthora (Phytophthora sp-h) were obtained from diseased Spathiphyllum and Primula plants. Cultural characteristics did not fit any known description of Phytophthora species. The Phytophthora sp-h isolates are papillate, are homothallic, possess 80 to 86% amphigynous antheridia, and have a maximum temperature for growth of 36.5 degrees C. Isozyme analysis of the Phytophthora sp-h isolates revealed a three-banded pattern with malic enzyme and a three-banded pattern with malate dehydrogenase on the second putative locus. The fastest band at both enzyme loci comigrated with the single P. nicotianae band, the slowest band comigrated with the single P. cactorum (and also P. pseudotsugae) band, and one band in between was concluded to represent the heterodimeric isozyme. The random amplified polymorphic DNA patterns of the Phytophthora sp-h isolates almost exclusively consisted of bands that were also present in either P. nicotianae or P. cactorum. Southern hybridization showed that bands specific for P. nicotianae were present as comigrating bands in the Phytophthora sp-h isolates. The same was found for species-specific bands of P. cactorum. It is concluded that the three Phytophthora sp-h isolates represent interspecific hybrids, P. nicotianae being the one parent and P. cactorum the other. Analysis of mito-chondrial DNA with restriction enzymes revealed banding patterns in all the Phytophthora sp-h isolates identical with those of P. nicotianae, confirming that indeed P. nicotianae was one of the parents.

Genetic Diversity and Origins of the Homoploid-Type Hybrid Phytophthora ×alni

“…In this regard, the predominance of the M over the U mtDNA in P. ϫalni is also consistent with the pattern found by Ioos et al (15). Despite the fact that during sexual hybridization both parents could act as the antheridial strain, it seems that one is often favored, leading to an unequal transmission of the mtDNA (38,39). Bringing together this evidence, we estimated that at least 13 different hybridization events occurred during the genesis of P. ϫalni.…”

Natural hybrids of resident and introduced Phytophthora species proliferating on multiple new hosts

“…Also mitochondrial DNA of three P. nicotianae  P. cactorum hybrid strains was analysed and contained mtDNA of the introduced P. nicotianae (Man in Ôt Veld et al, 1998). The predominant presence in the hybrids of mtDNA of the introduced species may lend some support to a theory developed by Olson and Stenlid (2001).…”

“…Additional hybrid strains have been detected on Rhododendron since the present study was completed, indicating that they are still successfully proliferating on this host. Among phytopathogenic Oomycetes, as mentioned above, natural hybrids have been discovered previously (Man in Ôt Veld et al, 1998, Brasier et al, 1999. Unlike those hybrids, the present hybrids were obtained from multiple new hosts not colonized by either parent (Erwin and Ribeiro, 1996), including monocots (Allium spp.)…”

Phytophthora ×stagnum nothosp. nov., a New Hybrid from Irrigation Reservoirs at Ornamental Plant Nurseries in Virginia

“…Many known Phytophthora hybrids have close association with aquatic environments. Examples include the four subclade 6b hybrids recovered from river and riparian ecosystems [38]; Phytophthora alni and its variants associated with riparian Alnus trees [31], [32]; and Phytophthora × pelgrandis initially recovered from horticultural plants grown in hydroponic systems [28]. The fact that most natural Phytophthora hybrid species were initially identified from aquatic environments is interesting.…”

“…Similarly, hundreds of ornamental plants are grown in nurseries using hydroponic or recycling irrigation systems which greatly increase the chance of close contact between species. Phytophthora × pelgrandis , P. × serendipita , and P. × stagnum may have formed in these systems by mating or anastomosis [28]. Second, newly formed Phytophthora hybrids may have a better opportunity to survive and adapt to aquatic ecosystems that contain a diverse variety of plant species.…”