Evolutionary relationships among Ascochyta species infecting wild and cultivated hosts in the legume tribes Cicereae and Vicieae

Abstract

Evolutionary relationships were inferred among a worldwide sample of Ascochyta fungi from wild and cultivated legume hosts based on phylogenetic analyses of DNA sequences from the ribosomal internal transcribed spacer regions (ITS), as well as portions of three protein-coding genes: glyceraldehyde-3-phosphate-dehydrogenase (G3PD), translation elongation factor 1-alpha (EF) and chitin synthase 1 (CHS). All legume-associated Ascochyta species had nearly identical ITS sequences and clustered with other Ascochyta, Phoma and Didymella species from legume and nonlegume hosts. Ascochyta pinodes (teleomorph: Mycosphaerella pinodes [Berk. & Blox.] Vestergen) clustered with Didymella species and not with well characterized Mycosphaerella species from other hosts and we propose that the name Didymella pinodes (Berk. & Blox.) Petrak (anamorph: Ascochyta pinodes L.K. Jones) be used to describe this fungus. Analysis of G3PD revealed two major clades among legume-associated Ascochyta fungi with members of both clades infecting pea ("Ascochyta complex"). Analysis of the combined CHS, EF and G3PD datasets revealed that isolates from cultivated pea (P. sativum), lentil (Lens culinaris), faba bean (Vicia faba) and chickpea (Cicer arietinum) from diverse geographic locations each had identical or similar sequences at all loci. Isolates from these hosts clustered in well supported clades specific for each host, suggesting a co-evolutionary history between pathogen and cultivated host. A. pisi, A. lentis, A. fabae and A. rabiei represent phylogenetic species infecting pea, lentil, faba bean and chickpea, respectively. Ascochyta spp. from wild relatives of pea and chickpea clustered with isolates from related cultivated hosts. Isolates sampled from big-flower vetch (Vicia grandiflora) were polyphyletic suggesting that either this host is colonized by phylogenetically distinct lineages of Ascochyta or that the hosts are polyphyletic and infected by distinct evolutionary lineages of the pathogen. Phylogenetic species identified among legume-associated Ascochyta spp. were fully concordant with previously described morphological and biological species.

Identification and Genetic Diversity Analysis of Ascochyta Species Associated with Blight Complex of Pea in a Northwestern Hill State of India

“…Lawyer [28] clearly differentiated the three species with tan colored spots delineated by distinct margin as A. pisi, concentric lesions/spots as A. pinodes, and the one causing foot rot as A. pinodella. These symptoms resemble the description made by various workers [25,28,30,36,41]. It was interesting to note that of the four isolates placed in group I exhibited two types of leaf spots, i.e., small tan colored (2-6 mm) to large irregular up to 15 mm spots with concentric rings and resembled group II; in addition, these isolates showed marked variation in symptoms under artificial and natural conditions, and are thus categorized as A. pisi and A. pinodella.…”

“…ITS sequences were clearly heterogenous enough to separate A. pisi from other two species. FarisMokaiesh et al [18] on the basis of PCR-RFLP of IGS whereas Barve et al [7] and Peever et al [36] on the basis of ITS sequences also differentiated A. pisi from A. pinodes and A. pinodella. Moreover, A. pisi showed lower mean/net genetic distance with A. rabiei group than A. pinodes/A.…”

“…Based on HMG phylogeny, Barve et al [7] found that A. pisi was closer to A. rabiei clade than other two species infecting pea. On the basis of G3PD sequencing, Peever et al [36] found A. rabiei and A. pisi to be in the same clade.…”

“…This means that ITS region does not separate these two species and there is a need to sequence other regions of their genome. In fact, [7,36] were unable to differentiate these two species on the basis of ITS ribotyping, but could easily differentiate into separate clades on the basis of high mobility group (HMG) domain of MAT gene and glyceraldehyde-3-phosphate dehydrogenase (G3PD) phylogeny. The third species in the Ascochyta blight complex of peas, A. pisi formed a separate clade II which also included As42 (FJ032642), an Ascochyta spp.…”

“…Thus, direct sequence determination provides the most robust data from which phylogenetic reconstruction can proceed with confidence. The most contemporary phylogenetic studies have been based on ITS, mtSSU, mtLSU, b-tubulin, glyceraldehyde-3-phosphate dehydrogenase gene (G3PD), chitin synthase 1 gene (CHS), translation elongation factor 1 alpha gene (EF) and MAT locus sequence analysis, and have supported the organization of Ascochyta species into distinct species groups [7,12,13,20,36].…”