Doctoral Dissertations

Date of Award

8-2008

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Plants, Soils, and Insects

Major Professor

Kurt H. Lamour

Committee Members

Feng Chen, John K. Moulton, Beth Mullin

Abstract

The genus Phytophthora includes more than 90 described species infecting over 1000 plant species. Population studies were conducted to investigate the survival and spread of P. capsici in the Peruvian coastal region. A total of 227 P. capsici isolates, recovered at widely distant localities from 2005-2007, were fingerprinted with AFLPs and SNP genotyping. A clonal population (PcPE-1) represented by 221 isolates was found to be distributed throughout the country. Atypical isolates of P. nicotianae were isolated from loquat trees in Peru and nuclear (internal transcribed spacer [ITS], the phenol acid carboxylase gene, and AFLPs) and mitochondrial genotyping (cytochrome oxidase gene [coxI]) identified this species as a hybrid between P. nicotianae and P. cactorum A comparison of five Phytophthora hybrid isolates from Peru and Taiwan (also infecting loquat trees) suggested that isolates from Peru likely originated from a single hybridization event and that the two isolates from Taiwan originated through different hybridization events.

The generation of genetic resources for the study of complex genetic traits in P. capsici was initiated by studying its inbreeding up to the sixth generation. A total of 692 oospore-derived isolates were fingerprinted and a subset was characterized for pathogenicity in cucumber and jalapeno fruits and for segregation of the mating type. The traits tested revealed no-Mendelian segregation, and apomixis were observed to be more prevalent (100%) in deep (fifth generation) inbreeding crosses. Inbreeding was measured by studying the segregation of 20 AFLP markers, which indicated a loss of heterozygosity of ~75% by the sixth generation. The seminal cross from this study was used as a mapping population (F1) for generating a genetic linkage framework with 189 AFLP and 18 SNP markers. A total of 18 linkage groups were produced for each parental isolate using 65 and 42 markers for CBS121657 and CBS121656 isolates respectively covering 409 cM. SNP markers FL5 and FL6 were used for estimating the genome size of P. capsici and precision of the genome assembly.

In order to conduct functional studies in P. capsici, we tested the efficacy of the polyethylene glycol mediated transformation. We regenerated up to 30 antibiotic resistant isolates and 53% of them were stable after three months of subculturing.

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