Date of Graduation
Davis College of Agriculture, Natural Resources and Design
Joseph B Morton
James D Bever
Jonathan R Cumming
Daniel G Panaccione
Alan J Sexstone
Arbuscular mycorrhizal fungi (AMF) in the phylum Glomeromycota are globally distributed biotrophic fungi that form symbiotic associations with a majority of terrestrial plants. Their global beneficial role in plant and soil ecology warrants a better understanding of their evolutionary history and taxonomic relationships. This project focused on species relationships in two deeply rooted families Ambisporaceae and Archaeosporaceae. In one study, analysis of morphology and 28S ribosomal DNA (LSU) sequences resulted in two major changes that redefined Ambispora to include only species that were dimorphic for both acaulosporoid and glomoid spore types. First, species described as producing only glomoid spores (Am. leptoticha, Am. fecundispora, and Am. callosa), only acaulosporoid spores (Am. jimgerdemannii), or both spore morphotypes (Am. appendicula) were synonymized with Ambispora leptoticha. LSU and more conserved 18S (SSU) sequences indicated little divergence between species. Second, Ambispora fennica was synonymized with Ambispora gerdemannii based on morphological and LSU sequence variation equivalent to that in the sister clade Am. leptoticha. With this analysis, Ambispora was reduced to three species: Am. leptoticha, Am. gerdemannii, and Am. granatensis. Morphological and molecular characters were given equal treatment in this study, as each data set informed and clarified grouping and ranking decisions. The two inner layers of the acaulosporoid spore wall were the only structural characters uniquely defining each of these three species; all other characters were shared. Phenotypes of glomoid spores were indistinguishable between species, and thus were informative only at the genus level. Evolution of sporogenous saccule was a convergent trait because it also evolved in other AMF families. In a second study, morphology and LSU sequence variation were compared in the only two Archaeospora species, Ar. trappei and Ar. schenckii. Both species form a precursor sporogenous saccule, but spores develop laterally (acaulosporoid) or within (entrophosporoid) the subtending hypha, respectively. In all other morphological characters, the two species are identical. One Ar. schenckii and ten Ar. trappei strains grouped all taxa into one highly supported monophyletic clade. However, LSU sequences of Ar. trappei strains segregated into two subclades, with Ar. schenckii grouping in one of the Ar. trappei clades. This evidence indicates the entrophosporoid mode does not represent a speciation event, but rather is a stable dimorphism within one or more populations of a single species. Therefore, Ar. schenckii and Ar. trappei were considered conspecific and synonymized as Ar. trappei. The combination of molecular and morphological evidence exposed the entrophosporoid mode as a polymorphism that likely was fixed in a founder asexual population and then was heritable and stable in the absence of recombination or negative selection pressures. Spore dimorphism is not a rare event, occurring also in the previous study. LSU sequence divergence between the two Ar. trappei clades was greater than that within the clade containing Ar. trappei and Ar. schenckii strains. This distinct subclade structure suggests fixation of discrete variants typical of clonal reproduction and possible retention of polymorphisms in rDNA repeats. The broader implication from this result is that clade topology alone may not provide an adequate criterion for ranking at the species level.
Bills, Robert J., "Phylogenetic relationships among arbuscular mycorrhizal fungal species in the deeply rooted archaeosporales (Glomeromycota)" (2015). Graduate Theses, Dissertations, and Problem Reports. 5211.