THE DISTRIBUTION OF WOODY LEGUMES IN NEOTROPICAL DRY FORESTS: THE PLEISTOCENE ARC THEORY 20 YEARS ON

The Pleistocene Arc Theory (PAT) suggests that present-day disjunct fragments of dry forests in central tropical South America give evidence of a previously more continuous distribution during the Pleistocene that has been disrupted by dry-cold vs. humid-warm climatic cycles. This Arc extends from NE Brazil to NE Argentina and eastern Paraguay, through the Chiquitanía to NW Argentina and SW Bolivia and into the dry inter-Andean valleys in Peru and Ecuador, with intrusions into the Great Chaco. Seasonally Dry Tropical Forests (SDTFs) are floristically and physiognomically dominated by woody legumes, mostly deciduous in the dry season. In the last two decades field collection and research on legume taxa has greatly increased, with a significant number of taxonomic revisions and molecular phylogenetic studies, together with some paleoclimatic modelling studies. The evidence accumulated in the last 23 years has confirmed the integrity of the Chaco and Caatingas phytogeographical provinces, with an impressive and increasing level of botanical endemism discovered. The PAT pattern has also been supported, specifically through the mapping of five selected woody Leguminosae species (Anadenanthera colubrina, Enterolobium contortisiliquum, Pterogyne nitens, Amburana cearensis and Piptadenia viridiflora). The pre-existing nuclei of South American SDTF (Caatingas, Misiones and Piedmont) are now increased to four with the postulation of the Chiquitanía Nucleus in south-eastern Bolivia and bordering Paraguay. Some new endemisms are compiled from recent literature and mapped for the Misiones and Chiquitanía nuclei. The need for more botanical collections and further taxonomic, phylogenetic and demographic studies of South American legumes is emphasised.

Phylogenetic multilocus sequence analysis identifies seven novel Ensifer genospecies isolated from a less-well-explored biogeographical region in East Africa

The diversity of 71 rhizobial strains belonging to the genus Ensifer , isolated from root nodules of woody legumes growing in southern Ethiopia, was studied using multilocus sequence analysis (MLSA) and phenotypic approaches. Phylogenetic analyses based on core genes revealed that 43 strains were clustered in seven distinct and consistent positions (genospecies I–VII), while another 25 strains were also distinct but were discrepant in their placement on the different gene trees. The remaining three strains occupied the same phylogenetic branches as defined Ensifer species and thus were not distinct. Irrespective of their chromosomal background, the majority of the test strains were highly related with respect to their nifH and nodC gene sequences, suggesting that these symbionts might have acquired these genes recently from a common origin. On the nifH phylogenetic tree, the branch containing the test strains and reference species isolated from woody legumes in Africa was clearly separate from those isolated outside the continent, suggesting that these symbionts have a long history of separate evolution within Ensifer for this gene. A cross-inoculation study showed that our strains were capable of eliciting effective nodulation on the homologous host and on other host species. This suggests a potential to improve nitrogen fixation by selecting for broad-host-range inoculants. Our study confirms the presence of a wide diversity of Ensifer in East Africa and, while contributing to the general knowledge of the biodiversity within the genus, also highlights the need to focus on previously less-well-explored biogeographical regions to unravel as-yet-unidentified rhizobial resources.