Genome Evolution of Filamentous Cyanobacterium Nostoc Species: From Facultative Symbiosis to Free Living

In contrast to obligate bacteria, facultative symbiotic bacteria are mainly characterized by genome enlargement. However, the underlying relationship of this feature with adaptations to various habitats remains unclear. In this study, we used the global genome data of Nostoc strains, including 10 novel genomes sequenced in this study and 26 genomes available from public databases, and analyzed their evolutionary history. The evolutionary boundary of the real clade of Nostoc species was identified and was found to be consistent with the results of polyphasic taxonomy. The initial ancestral species of Nostoc was demonstrated to be consistent with a facultative symbiotic population. Further analyses revealed that Nostoc strains tended to shift from facultative symbiosis to a free-living one, along with an increase in genome sizes during the dispersal of each exterior branch. Intracellular symbiosis was proved to be essentially related to Nostoc evolution, and the adaptation of its members to free-living environments was coupled with a large preference for gene acquisition involved in gene repair and recombination. These findings provided unique evidence of genomic mechanisms by which homologous microbes adapt to distinct life manners and external environments.

What can genome analysis offer for bacteria?

This book chapter is organized as follows: (i) the main approaches to the philosophy of taxonomic classification are recapitulated; (ii) the paradigm of polyphasic taxonomy is discussed in this context; (iii) the causes of conflict between previous classifications and genome-scale analyses are investigated, using examples from recent phylum-wide studies, with a discussion of how markers used in polyphasic taxonomy can be replaced by genome-derived ones; and (iv) the challenges in assigning taxonomic ranks using genome-scale or other data are revisited. The conclusion assesses the chances, or lack thereof, of reconciling taxonomic classifications. Phenetic and phylogenetic thinking still compete with each other on the classification of bacteria, with potentially conflicting and confusing results. Some causes of problematic taxonomic classifications are independent of the type and number of characters that can be used and can only be mitigated if, for example, taxon sampling and branch support are more appropriately taken into account. It may be possible to devise objective criteria for separating bacterial species, but the currently dominating approaches for microbial species delineation may be inadequate. It is even harder to delineate higher taxa; in contrast to claims in the literature, it may prove to be impossible to objectively assign taxonomic ranks above species level.

New species in Aspergillus section Usti and an overview of Aspergillus section Cavernicolarum

Aspergillus sections Usti and Cavernicolarum are accommodated in the subgenus Nidulantes. In the present study, a polyphasic approach using morphology and multi-gene phylogeny was applied to investigate the taxonomy of these two sections. Based on the phylogenetic analysis, Aspergillus section Usti includes 25 species, which can be assigned to four series: Calidousti, Deflecti, Monodiorum and Usti. Aspergillus sigarelli is newly described in this section and this species was isolated from a cigarette from PR China and belongs to series Calidousti. It is clearly distinct from other members in this series based on ITS, BenA, CaM and RPB2 sequences. Aspergillus section Usti members like A. calidoustus and A. granulosus are important opportunistic pathogens, it is speculative that more pathogenetic species will be found by using polyphasic taxonomy approaches. Aspergillus section Cavernicolarum includes five species, the growth rates on agar media and size and ornamentation of conidia are important characters for differentiating species in section Cavernicolarum.