Three Substrains of the Cyanobacterium Anabaena sp. Strain PCC 7120 Display Divergence in Genomic Sequences and Function.

sp. strain PCC 7120 is a model strain for molecular studies of cell differentiation and patterning in heterocyst-forming cyanobacteria. Subtle differences in heterocyst development have been noticed in different laboratories working on the same organism. In this study, 360 mutations, including single nucleotide polymorphisms (SNPs), small insertion/deletions (indels; 1 to 3 bp), fragment deletions, and transpositions, were identified in the genomes of three substrains. Heterogeneous/heterozygous bases were also identified due to the polyploidy nature of the genome and the multicellular morphology but could be completely segregated when plated after filament fragmentation by sonication. is a gene upregulated in developing cells during heterocyst formation in sp. strain PCC 7120 and found in approximately half of other heterocyst-forming cyanobacteria. Inactivation of in 3 substrains of sp. PCC 7120 led to different phenotypes: the formation of heterocysts, differentiating cells that keep dividing, or the presence of both heterocysts and dividing differentiating cells. The expression of P - in these mutants also showed different patterns of green fluorescent protein (GFP) fluorescence. Thus, the function of is influenced by the genomic background and epistasis and constitutes an example of evolution under way. Our knowledge about the molecular genetics of heterocyst formation, an important cell differentiation process for global N fixation, is mostly based on studies with sp. strain PCC 7120. Here, we show that rapid microevolution is under way in this strain, leading to phenotypic variations for certain genes related to heterocyst development, such as This study provides an example for ongoing microevolution, marked by multiple heterogeneous/heterozygous single nucleotide polymorphisms (SNPs), in a multicellular multicopy-genome microorganism.