Molecular markers offer new opportunities for breeding for disease resistance. Resistance gene pyramiding in a single cultivar, as a strategy for durable resistance, can be facilitated by marker-assisted selection (MAS). A RAPD marker, ROH20(450), linked to the Mesoamerican Co-2 anthracnose resistance gene, was previously transformed into a SCAR marker, SCH20. In the present paper we have further characterized the relevance of the SCH20 SCAR marker in different genetic backgrounds. Since this SCAR marker was found to be useful mainly in the Andean gene pool, we identified a new PCR-based marker (SCAreoli) for indirect scoring of the presence of the Co-2 gene. The SCAreoli SCAR marker is polymorphic in the Mesoamerican as well as in the Andean gene pool and should be useful in MAS. We also report that PvH20, the cloned sequence corresponding to the 450-bp RAPD marker ROH20(450), contains six imperfect leucine-rich repeats, and reveals a family of related sequences in the vicinity of the Co-2 locus. These results are discussed in the context of the recent cloning of some plant resistance genes.

To investigate the genetic diversity of the genus Arthrospira and to compare it with other cyanobacteria, sequences of 670 nt from the phycocyanin operon were determined for 23 natural, cultivated or commercial strains of Arthrospira and compared with sequences from 20 other non-Arthrospira cyanobacterial strains. The sequenced DNA fragment comprises the last 255 nt of cpcB, the cpcB-cpcA spacer and the first 304 nt of cpcA. The resulting phylogenetic tree confirms that the genus Arthrospira is not related to Spirulina. So far, cpcB-cpcA data suggest that the closest relative of Arthrospira is Planktothrix. Based on this locus, the genus Arthrospira consists of three genetically clustered lineages. However, the distribution of nucleotide substitutions indicates that these three lineages are not the result of a simple cladogenesis characterized by the accumulation of independent substitutions. Instead, the observed clustering is the result of horizontal transfers of blocks of sequences. Analysis of the distribution of substitutions in the sequenced fragment indicates a point of intragenic recombination close to the stop codon of cpcB. The capacity of exchange of genetic material among strains probably explains why morphology and geographical origin do not correlate with the cpcB-cpcA clusters. Nevertheless, this study shows for the first time that the genus Arthrospira, represented here by cultivated and wild specimens, is clearly monophyletic. Moreover, the cpcB-cpcA DNA fragment, comprising both highly and moderately variable regions, allows (1) a strict differentiation of the taxon Arthrospira from other cyanobacteria (using the coding regions only) and (2) the study of relationships inside Arthrospira (using both the coding and non-coding regions).

Here we report the whole-genome shotgun sequence of a Peruvian strain of Arthrospira platensis (Paraca), a cultivated and edible haloalkaliphilic cyanobacterium of great scientific, technical, and economic potential.