Abstract
The genetic and evolutionary relationships among floral nectar-dwelling Pseudomonas ‘sensu stricto’ isolates associated to South African and Mediterranean plants were investigated by multilocus sequence analysis (MLSA) of four core housekeeping genes (rrs, gyrB, rpoB and rpoD). A total of 35 different sequence types were found for the 38 nectar bacterial isolates characterised. Phylogenetic analyses resulted in the identification of three main clades [nectar groups (NGs) 1, 2 and 3] of nectar pseudomonads, which were closely related to five intrageneric groups: Pseudomonas oryzihabitans (NG 1); P. fluorescens, P. lutea and P. syringae (NG 2); and P. rhizosphaerae (NG 3). Linkage disequilibrium analysis pointed to a mostly clonal population structure, even when the analysis was restricted to isolates from the same floristic region or belonging to the same NG. Nevertheless, signatures of recombination were observed for NG 3, which exclusively included isolates retrieved from the floral nectar of insect-pollinated Mediterranean plants. In contrast, the other two NGs comprised both South African and Mediterranean isolates. Analyses relating diversification to floristic region and pollinator type revealed that there has been more unique evolution of the nectar pseudomonads within the Mediterranean region than would be expected by chance. This is the first work analysing the sequence of multiple loci to reveal geno- and ecotypes of nectar bacteria.
Highlights
Microorganisms find in plants heterogeneous and highly dynamic habitats which can differ widely with regard to nutrient availability and physico-chemical conditions at macro- and microscales [1]
We explored the phylogenetic and evolutionary relationships among nectar-inhabiting pseudomonads associated to Mediterranean and South African insect-pollinated plants
In the case of Pseudomonas, it seems that a long evolutionary history and continual exposure to a range of spatially and temporally complex environments have resulted in an astonishing diversity of genetic lineages and ecotypes [31], some of which are well adapted to thrive in close association to plants [35,74,75,76]
Summary
Microorganisms find in plants heterogeneous and highly dynamic habitats which can differ widely with regard to nutrient availability and physico-chemical conditions at macro- and microscales [1]. The importance of other plant parts as microbial habitats remains yet underexplored, but important advances in their study have been made in recent years In this sense, an emerging focus of plant research is the role of floral nectar as a natural habitat for microorganisms, and their ecological function. An emerging focus of plant research is the role of floral nectar as a natural habitat for microorganisms, and their ecological function Most of these studies have dealt with nectardwelling yeasts [4,8,9,10,11,12,13,14,15], but the role of nectar as a reservoir for bacterial diversity, and the possible influence of bacteria on plant fitness, have been considered recently [16,17,18,19,20,21]
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