Molecular evidence for reticulate speciation in Astragalus (Fabaceae) as revealed by a case study from sect. Dissitiflori

Abstract

Although hybridization has long been recognized as a major force driving speciation in land plants, it has not yet been evidenced in Astragalus, the largest angiosperm genus. Here, we reveal the possible contribution of hybridization to speciation in Astragalus by employing cloning of the nrDNA ITS region and sampling three plastid regions (ycf1, ndhF-rpl32, and rpl32-trnL) in taxa belonging to sect. Dissitiflori. Phylogenetic network and tree analyses uncovered various levels of intra-individual and intraspecific polymorphism of ITS in most of the taxa investigated. Two distantly related ribotype groups were found to be shared by the closely related polyploids Astragalus pallescens M.Bieb., Astragalus peterfii Jáv., and Astragalus pseudoglaucus Klokov suggesting ancient hybridization followed by incomplete lineage sorting (i.e., shared ancestral polymorphism) in nrDNA ITS. Reticulation is also invoked as an underlying evolutionary process behind the statistically highly supported incongruent placement of A. pseudoglaucus and Astragalus vesicarius subsp. pastellianus (Pollini) Arcang. in nuclear versus plastid phylogenies. The phylogenetic results also shed light on taxonomic controversies in the section, such as the false synonimization of A. peterfii under A. vesicarius s.l. Our results provide evidence for the (at least past) existence of speciation processes driven by hybridization in Astragalus.Key words: hybridization, ycf1, nrDNA ITS cloning, allopolyploidy, reticulation.Résumé : Bien qu'on ait reconnu depuis longtemps l'hybridation comme force majeure soutenant la spéciation chez les plantes terrestres, on n'en pas encore la preuve chez les Astragalus spp, le plus important genre des angiospermes. On évoque ici la contribution possible de l'hybridation à la spéciation des Astragalus spp., en utilisant le clonage de la région ITS du nrADN et en échantillonnant trois régions plastidiques (ycf1, ndhF-rpl32 et rpl32-trnL) chez des taxons de la section Dissitiflori. Le réseau phylogénétique et trois analyses révèlent divers degrés de polymorphisme intra-individuel et intraspecifique de l'ITS, chez la plupart des taxons examinés. On a repéré deux groupes de ribotypes partagés par l'Astragalus pallescens M. Bieb., l'Astragalus peterfii Jáv., et l'Astragalus pseudoglaucus Klokov, des polyploïdes étroitement reliées, ce qui suggère d'anciennes hybridations suivies de sélections de lignées incomplètes (i. e. polymorphisme ancestral partagé) dans l'ITS nrADN. On évoque la réticulation comme processus évolutif sous-jacent, également à la source de la localisation incongrue, fortement supportée statistiquement, de l'A. pseudoglaucus et de l'Astragalus vesicarius subsp. pastellianus (Pollini) Arcang. dans les phylogénies nucléiques versus plastidiques. Les résultats phylogénétiques éclairent les controverses sur la section, tel la fausse synonymie de l'A. peterfii sous l'A. vesicarius s.l. Les résultats fournissent la preuve (du moins dans le passé) de l'existence de processus de spéciation activés par l'...

The first complete chloroplast genome of the Genistoid legume Lupinus luteus: evidence for a novel major lineage-specific rearrangement and new insights regarding plastome evolution in the legume family

“…Interestingly, most of these variable regions have not been or have rarely been employed in legume phylogeny [e.g. the trnS-trnG region in Lupinus by Drummond (2008); ycf1 in Astragalus by Bartha et al (2013)], and thus represent a new set of markers to explore evolutionary relationships within legumes. Each of these sequences needs to be tested in order to evaluate at which taxonomic level and in which lineages they could be more informative and useful.…”

Molecular phylogeny of Astragalus section Anthylloidei (Fabaceae) inferred from nrDNA ITS and plastid rpl32-trnL(UAG) sequence data

“…This is the best noncoding region for low-level molecular studies (Shaw et al, 2007;Dong et al, 2012). To our knowledge, the rpl32-trnL (UAG) region has rarely been used in molecular phylogenetic investigations on Astragalus (Bartha et al, 2013).…”

Molecular phylogeny and divergence times of Astragalus section Hymenostegis: An analysis of a rapidly diversifying species group in Fabaceae

“…This could be due to ( i ) the inappropriate choice of molecular marker regions selected. In our study we used, however, with the nrDNA ITS region the fastest evolving universal nuclear marker, and the ycf1 gene that provided the best resolution in earlier Astragalus studies 38 . Therefore, we assume that ( ii ) we deal here with a very rapid radiation, i.e.…”

“…PCR amplification and sequencing followed Bartha et al . 38 . When due to low DNA quality not the entire gene region could be amplified by a single PCR, we used internal primers to amplify the locus in two separate parts.…”