Date of Award

Winter 1989

Project Type

Dissertation

Program or Major

Botany

Degree Name

Doctor of Philosophy

First Advisor

Thomas C Harrington

Abstract

The objectives of this research were to determine genetic variation in Leptographium and how it corresponds to the taxonomy of the genus.

The similarity of 88 strains of 27 species of Leptographium was studied using enzyme electrophoresis. UGPMA cluster analysis of similarity matrices (Nei genetic identity, I) generated from data of 267 electrophoretic forms (electromorphs) of 15 enzymes showed a close correspondence between morphology and electrophoretic similarity. Strains of a species clustered at I $\geq$ 0.60, but in two cases, taxa clustered at I $>$ 0.60, suggesting conspecificity.

Additional isozyme studies were made of 76 isolates of Leptographium wageneri representing three host-specialized varieties. Of 21 enzymes tested, 10 were polymorphic, having from two to six electromorphs. Only 14 combinations (electrophoretic types) of the 29 electromorphs were found; each electrophoretic type was restricted to a single variety. Within each variety, one electrophoretic type was abundant and broadly distributed; additional types were geographically isolated or restricted. Ordination of Nei genetic distance (D) among strains revealed three discrete clusters that corresponded to the three varieties. Nei gene diversity (H) in each variety was low (0.017 to 0.040), but differentiation between varieties was high; the Nei coefficient of gene differentiation (G$\sb{\rm st})$ for the species was 0.860.

Vegetative compatibility was tested among the same L. wageneri isolates by pairing auxotrophic, nitrate non-utilizing mutants on nitrate media. The development of dense hyphal growth in the zone of confrontation between complementing phenotypes indicated compatibility. Heterokaryons were recovered from hyphal tips and conidiophores of complementing pairings. Only fourteen groups of vegetatively compatible isolates (VC groups) were detected. Each contained isolates that were of similar electrophoretic types, and most had unique geographic ranges. No intervarietal complementation occurred. The indications of low genetic diversity in Leptographium wageneri (i.e., few electrophoretic types and VC groups), the unique geographic distributions of the electrophoretic and compatibility phenotypes, and the correlation between electrophoretic type and VC group may be due to a lack of recombination, to strong clonal selection and to founder effects.

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