Date of Award


Degree Type


Degree Name

Doctor of Philosophy


The delimitation of phenotypically similar species is often problematic. Increasingly, molecular methods of taxonomy are called upon to resolve closely related taxa. DNA reassociation is a method capable of distinguishing between sibling species. This method has been used to clarify the genetic group boundaries that exist within the Sporopachydermia cereana species complex.;The yeast genus Sporopachydermia currently comprises three species, Sp. lactativora, Sp. quercuum, and Sp. cereana. These yeasts are unusual ascomycetes that assimilate inositol, lack the ability to ferment sugars, and may give a positive diazonium blue B reaction. Preliminary studies indicated that Sp. cereana contained 4 or 5 genetically distinct species by mol% G + C and DNA reassociation experiments (Phaff, unpublished). Standard taxonomic methods, such as assimilation and resistance profiles, were not sufficient to define the apparent genetic groups.;The elucidation of the genetic groups that comprise the Sp. cereana species complex was achieved by DNA reassociation. Six distinct genetic groups can now be identified within the Sp. cereana complex. Of the six groups, 3 are possible new species, 2 are probable varieties, and the sixth is Sp. cereana sensu stricto. The DNA reassociation data are supported by Phaff's (unpublished) preliminary results.;Electrokaryotyping was investigated as a method of studying the genetic variation that exists in the species complex. Chromosome sizes and numbers were shown to vary throughout the species complex. These characters were not sufficient to clarify the genetic groups independent of the DNA reassociation method. Electrokaryotypic patterns were characteristic for some genetic groups, but variable within others. Assimilation profiles, growth characteristics, mating and killing patterns were investigated as potential taxonomic characters. Mating ability was found to be limited within the Sp. cereana complex. Extensive mating studies throughout the complex established that self-fertile and asexual strains are common. Definition of reproductively isolated populations was not possible for this reason. Killing experiments were conducted to investigate intra- and inter-group interactions. These experiments uncovered a unique system in which mating type and killing ability are associated. However, neither mating nor killing results were found to correlate to the genetic boundaries indicated by the DNA reassociation data. Informative assimilation and resistance tests, growth characteristics, and electrokaryotypes, are summarized for each genetic group, and provide the basis for future descriptions of the new species and varieties in this intriguing genus.



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