Genome evolution of the cyanobacterium Nostoc linckia under sharp microclimatic divergence at "Evolution Canyon," Israel

We describe the genomic DNA diversity and divergence of the cyanobacterium Nostoc linckia from "Evolution Canyon," a microsite consisting of ecologically contrasting slopes, south-facing slope (SFS) and north-facing slope (NFS), at lower Nahal Oren, Mt. Carmel, Israel. The opposing slopes share their limestone lithology but vary greatly in their ecology, primarily because of different levels of solar radiation (which is six times higher on the SFS than on the NFS). The warm and xeric SFS displays a tropical African savanna, whereas the cool and mesic NFS displays a temperate South European Mediterranean live-oak maquis shrub forest. The cyanobacterium Nostoc linckia tested here is a sessile microorganism, growing as a carpet on rock surfaces and constantly exposed to environmental fluctuations of solar radiation, temperature, and desiccation. We demonstrate remarkable interslope and intraslope genetic divergence of the genome (including both coding and noncoding regions) of Nostoc linckia, by using 211 AFLP (amplified fragment length polymorphism) DNA molecular marker loci. Genetic polymorphism of N. linckia subpopulations on the ecologically harsher SFS was significantly (p < 0.05) higher (P = 99.53%) than was that of the subpopulations on the climatically milder NFS (P = 85.78%). Genetic polymorphism (P) and gene diversity (He) were significantly correlated with variables influencing aridity stress: solar radiation (Sr) (r_p = 0.956; p = 0.046), temperature (Tm) (r_p = 0.993; p = 0.0068), and day-night temperature difference (Tdd) (r_p = 0.975; p = 0.025). As in other tested organisms from "Evolution Canyon", but even more exceptionally because of its completely sedentary nature, we suggest that the climatically stressed SFS environment is responsible for this marked increase of genetic polymorphism, which is maintained by the combined evolutionary forces of diversifying and balancing selection. This could highlight the importance of ecological stress and selection in evolution and its remarkable effect on the genetic system across the prokaryotic genome.