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SummaryThe legume genus Aeschynomene is notable in the ability of certain semiaquatic species to develop nitrogen-fixing stem nodules. These species are distributed in two clades. In the first clade, all the species are characterized by the use of a unique Nod-independent symbiotic process. In the second clade, the species use a Nod-dependent symbiotic process and some of them display a profuse stem nodulation as exemplified in the African Aeschynomene afraspera.To facilitate the molecular analysis of the symbiotic characteristics of such legumes, we took an integrated molecular and cytogenetic approach to track occurrences of polyploidy events and to analyze their impact on the evolution of the African species of Aeschynomene.Our results revealed two rounds of polyploidy: a paleopolyploid event predating the African group and two neopolyploid speciations, along with significant chromosomal variations. Hence, we found that A. afraspera (8x) has inherited the contrasted genomic properties and the stem-nodulation habit of its parental lineages (4x).This study reveals a comprehensive picture of African Aeschynomene diversification. It notably evidences a history that is distinct from the diploid Nod-independent clade, providing clues for the identification of the specific determinants of the Nod-dependent and Nodindependent symbiotic processes, and for comparative analysis of stem nodulation.

BackgroundAmong semi-aquatic species of the legume genus Aeschynomene, some have the property of being nodulated by photosynthetic Bradyrhizobium lacking the nodABC genes necessary for the synthesis of Nod factors. Knowledge of the specificities underlying this Nod-independent symbiosis has been gained from the model legume Aeschynomene evenia but our understanding remains limited due to the lack of comparative genetics with related taxa using a Nod factor-dependent process. To fill this gap, we combined different approaches to perform a thorough comparative analysis in the genus Aeschynomene.ResultsThis study significantly broadened previous taxon sampling, including in allied genera, in order to construct a comprehensive phylogeny. In the phylogenetic tree, five main lineages were delineated, including a novel lineage, the Nod-independent clade and another one containing a polytomy that comprised several Aeschynomene groups and all the allied genera. This phylogeny was matched with data on chromosome number, genome size and low-copy nuclear gene sequences to reveal the diploid species and a polytomy containing mostly polyploid taxa. For these taxa, a single allopolyploid origin was inferred and the putative parental lineages were identified. Finally, nodulation tests with different Bradyrhizobium strains revealed new nodulation behaviours and the diploid species outside of the Nod-independent clade were compared for their experimental tractability and genetic diversity.ConclusionsThe extended knowledge of the genetics and biology of the different lineages sheds new light of the evolutionary history of the genus Aeschynomene and they provide a solid framework to exploit efficiently the diversity encountered in Aeschynomene legumes. Notably, our backbone tree contains all the species that are diploid and it clarifies the genetic relationships between the Nod-independent clade and the Nod-dependent lineages. This study enabled the identification of A. americana and A. patula as the most suitable species to undertake a comparative genetic study of the Nod-independent and Nod-dependent symbioses.Electronic supplementary materialThe online version of this article (10.1186/s12870-018-1567-z) contains supplementary material, which is available to authorized users.

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This study concerns post-cultural dynamics in the semi-arid South-West of Madagascar (Analabo area, near the Mikea Forest). A synchronic comparison was performed on a set of abandoned fields plots aged from 2 to 30 years and on forest and savanna reference ecosystems, located on cambic arenosols developed from same Quaternary dune sands. The studied parameters concerned mainly a few physical and chemical soil properties (density, permeability, compaction, texture, C, N, K, P content). Important changes occur in the post-cultural succession: an increase of the soil density and compaction, and decrease of the soil upper layer permeability. Consequently, slash and burn cultivation leads to a packing and an induration of the soil surface. Results about edaphic indicators have shown that the physical parameters used better discriminate various stages of evolution than the chemical parameters do. The multivariate analysis of soil indicators shows that vegetative succession over 30 years in a forest ecosystem cleared, burnt, cultivated and left, does not lead to features corresponding to a closed-canopy forest but rather to those of a tree savanna with open, mixed woody-herbaceous vegetation. The primary dense dry deciduous regeneration of the primary forest is very low or nil. In the semi-arid SW Madagascar (Analabo region), post-cultural dynamics conditions consists in a savannaformation process, controlled by: (1) the intensity and duration of the disturbance (during the cultivation phase); and (2) the more drastic environmental conditions (including both climate and soil).