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Four Nostocacean species from desert soils of the western United States, including the phycobiont of the lichen Collema tenax, were studied. Our strains could be forced into morphospecies previously described from Europe, but phylogenetic analysis indicated that they belonged in separate, distinct, and previously undescribed taxa. Partial 16S rRNA sequences of the strains CM1-VF10, CM1-VF14-, CNP-AK1 and JT2-VF2 were determined and aligned with published Nostoc sequences from GenBank and our lab, as well as other Nostocales. All aligned sequences were analysed using parsimony, distance, and maximum likelihood methods, and trees based on three separate data sets were generated. Full 16S-23S internal transcribed spacer (ITS) regions were also characterized for our strains, and secondary structures of the ITS region were compared among these and N. commune and N. punctiforme. Intragenomic variability was documented among ITS regions in different opérons for these taxa. One of the four strains (JT2-VF2) is distinct from Nostoc by both morphological and molecular criteria and is described as Mojavia pulchra gen. et sp. nov. Two other strains (CM1-VF10 and CM1-VF14) are described as Nostoc indistinguendum sp. nov. and Nostoc desertorum sp. nov., respectively. According to both morphological and molecular characteristics, the phycobiont of C. tenax is not N. commune, N. sphaericum or N. punctiforme as variously suggested in the lichenological literature, and the older name for this taxon, Nostoc lichenoides, is consequently validated in this paper.
KEY WORDS: Collema, Cyanobacteria, Clark Mountains, ITS secondary structure, Microbiotic soil crust, Mojavia, Nostoc, rRNA operons
INTRODUCTION
Nostoc is a well-known, widespread genus within the cyanobacteria. It occurs in freshwater aquatic, hydroterrestrial, and euterrestrial habitats throughout the world (Geitler 1932). Geoffroy is credited with the first use of the name by Drouet (1978 - as Nostoc Geoffroy ex Linnaeus), but due to Article 13.1 of the International Code of Botanical Nomenclature (Greuter et al. 2000), valid publication for the heterocytous cyanobacteria begins with Bornet and Flahault (1886-1888). This ruling makes Nostoc Vaucher ex Bornet et Flahault the valid name for the genus, with Nostoc commune Vaucher (1803) ex Bornet et Flahault (1886-1888) serving as the generitype.
The genus is problematic due to an absence of derived characteristics with which to define species, as well as an at times complicated life history that is seldom described...