Genetic Diversity of Phytophthora infestans sensu lato in Ecuador Provides New Insight Into the Origin of This Important Plant Pathogen

Abstract

The metapopulation structure of Phytophthora infestans sensu lato is genetically diverse in the highlands of Ecuador. Previous reports documented the diversity associated with four putative clonal lineages of the pathogen collected from various hosts in the genus Solanum. This paper simultaneously analyzes diversity of the complete collection of isolates, including a large number that had not yet been reported. This analysis confirmed the existence of three pathogen populations, which all appear to be clonal lineages, and that correspond to those previously reported as US-1, EC-1, and EC-3. No evidence was found from the analyses of recently collected isolates that would contradict earlier reports about these three lineages. In contrast, new data from a group of isolates from several similar hosts caused us to modify the previous description of clonal lineage EC-2 and its previously proposed hosts, S. brevifolium and S. tetrapetalum. Given the uncertainty associated with the identification of these hosts, which all belong to the section Anarrhichomenum, we refer to them as the Anarrhichomenum complex, pending further taxonomic clarification. New pathogen genotypes associated with the Anarrhichomenum complex were isolated recently that are A1 mating type and Ia mitochondrial DNA (mtDNA) haplotype, and therefore differ from the previously described EC-2 lineage, which is A2 and Ic, respectively. Because of uncertainty on host identification, we do not know if the new genotypes are limited to one host species and therefore represent yet another host-adapted clonal lineage. For now, we refer to the new genotypes and previously described EC-2 genotypes, together, as the pathogen group attacking the Anarrhichomenum complex. Two A2 isolates identical to the previously described EC-2 archetype were collected from severely infected plants of pear melon (S. muricatum). Pear melon is generally attacked by US-1, and this is the first clear case we have documented in which two distinct pathogen genotypes have caused severe epidemics on the same host. Based on presence of unique marker alleles (restriction fragment length polymorphism [RFLP] and mtDNA) and genetic similarity analysis using RFLP and amplified fragment length polymorphism data, EC-3 and isolates from the Anarrhichomenum complex are genetically distinct from all genotypes of P. infestans that have been reported previously. No current theory of historical migrations for this pathogen can adequately support a Mexican origin for EC-3 and genotypes of the Anarrhichomenum complex and they may, therefore, be palaeoendemic to the Andean highlands. To date, we have identified 15 hosts in the genus Solanum, in addition to the Anarrhichomenum complex, and some unidentified species of P. infestans sensu lato in Ecuador. Five of the Solanum hosts are cultivated. One isolate was collected from Brugmansia sanguinea, which represents the first report from Ecuador of a host of this pathogen that is not in the genus Solanum. However, P. infestans sensu lato was only foun...

Genetic Structure of the Population of Phytophthora infestans Attacking Solanum ochranthum in the Highlands of Ecuador

“…Three of these appear to be clonal lineages, each characterized by a particular mating type, mitochondrial DNA (mtDNA) haplotype, nuclear RFLP fingerprint and isoenzyme electrophoretic pattern. Each of the three lineages has been described in detail: EC-1 (Forbes et al, 1997), US-1 (Oyarzun et a1., 1998), and EC-3 (Adler et al, 2004). Another pathogen group was described as the EC-2 clonal lineage (Ordon˜ez et al, 2000) but we have since identified new genotypes of P. infestans similar to EC-2 but differing in mating type and mtDNA haplotype.…”

“…In recent studies in the tropical highlands of Ecuador we identified at least 18 host species in the Solanaceae with blight-like symptoms in nature (Adler et al, 2004). Some of the pathogen genotypes attacking these hosts are atypical and may eventually be described as one or more new species of Phytophthora (Adler et al 2004).…”

“…In Ecuador, five groups of P. infestans have been detected (Erselius et al, 1999;Chaco´n et al, 2002;Adler et al, 2004). Three of these appear to be clonal lineages, each characterized by a particular mating type, mitochondrial DNA (mtDNA) haplotype, nuclear RFLP fingerprint and isoenzyme electrophoretic pattern.…”

“…Another pathogen group was described as the EC-2 clonal lineage (Ordon˜ez et al, 2000) but we have since identified new genotypes of P. infestans similar to EC-2 but differing in mating type and mtDNA haplotype. Clearly, the EC-2 group with its two mating type subgroups does not fit the definition of a clonal lineage and this population has subsequently been referred to as the Anarrhichomenum group, in reference to the series in the genus Solanum comprising the varied and poorly defined plant species that these pathogens attack (Adler et al, 2004).…”

“…Re-inoculation of these isolates on pear melon demonstrated that they were highly aggressive . US-1 and the Anarrhichomenum group are genetically distant and have different mating types (Adler et al, 2004).…”