Diversity of the Cosmopolitan Thermophile Mastigocladus laminosus at Global, Regional and Local Scales

A recent theme in the study of microbial diversity has been the issue of whether and how the genetic and phenotypic variation of microorganisms is distributed along a geographic transect. Population genetic theory and the results of experimental evolution studies in the laboratory (e.g., Wright, 1931; Atwood et al., 1951; Bennett and Lenski, 1993) suggest that spatially structured microbial populations in nature should rapidly diverge from each other, provided that migratory gene flow among them is low, thereby creating geographic patterns of variation. Recent reports confirm that divergence of geographically isolated populations indeed outpaces the homogenization of genetic variation by migration, indicating the presence of dispersal barriers for microorganisms (e.g., Miller and Castenholz, 2000; Papke et al., 2003; Whitaker et al., 2003; Miller et al., 2006). These observations run counter to the longstanding idea that the abundance of a microorganism at a location is not limited by dispersal but is determined solely by environmental factors (Baas- Becking, 1934), a view that has been recently championed for eukaryotic microorganisms on the basis of morphological criteria (Finlay, 2002). Here, I will summarize our recent investigations of the biogeography of the moderately thermophilic, filamentous cyanobacterium, Mastigocladus (Fischerella) laminosus.