Weighted morphology: a new approach towards phylogenetic assessment of Nostocales (Cyanobacteria).

The classification of order Nostocales (Cyanobacteria) and inter relationships of morphologically similar taxa is still debatable due to ever changing morphological features. No attempt has been made to improve the morphological taxonomy despite the fact that it is the morphology that represents the totality of genes. To test the validity of morphological taxonomy and fine tune the phylogenetic relationships within the order Nostocales a new weighted morphology approach was applied by using 76 isolates and their 16S rRNA gene sequences. Further, the study was extended with morphological data set of the remaining 232 taxa for which no molecular data are yet available. Trichome aggregation, heterocyst shape, and akinete shape are suggested as important and stable features for identification. At 30% weight assignment to the selected morphological characters, morphological taxonomy found 36% compatible with 16S tree. Adding weight to the morphological characters considerably improved the congruence between the morphology and 16S rRNA-based phylogenetic trees of the order Nostocales. When the weighting procedure was extended to all the Nostocalean members irrespective of molecular data availability, it was found that Nostoc sphaericum and Nostoc microscopicum closely assembled in a single clade. Closer arrangement of Aulosira and Nodularia represent the subfamily aulosirae (Bornet and Flahault Ann Sci Nat Bot 7:223-224, 1888) while taxonomic affiliation of Cylindrospermum with Nostoc, Anabaena, and Raphidiopsis representing the subfamily anabaenae (Bornet and Flahault Ann Sci Nat Bot 7:223-224, 1888) was resolved.