Gradual polyploid genome evolution revealed by pan-genomic analysis of Brachypodium hybridum and its diploid progenitors.

Our understanding of polyploid genome evolution is constrained because we cannot know the exact founders of a particular polyploid. To differentiate between founder effects and post polyploidization evolution, we use a pan-genomic approach to study the allotetraploid Brachypodium hybridum and its diploid progenitors. Comparative analysis suggests that most B. hybridum whole gene presence/absence variation is part of the standing variation in its diploid progenitors. Analysis of nuclear single nucleotide variants, plastomes and k-mers associated with retrotransposons reveals two independent origins for B. hybridum, ~1.4 and ~0.14 million years ago. Examination of gene expression in the younger B. hybridum lineage reveals no bias in overall subgenome expression. Our results are consistent with a gradual accumulation of genomic changes after polyploidization and a lack of subgenome expression dominance. Significantly, if we did not use a pan-genomic approach, we would grossly overestimate the number of genomic changes attributable to post polyploidization evolution.

Reconstructing the origins and the biogeography of species' genomes in the highly reticulate allopolyploid-rich model grass genus Brachypodium using minimum evolution, coalescence and maximum likelihood approaches.

The identification of homeologous genomes and the biogeographical analyses of highly reticulate allopolyploid-rich groups face the challenge of incorrectly inferring the genomic origins and the biogeographical patterns of the polyploids from unreliable strictly bifurcating trees. Here we reconstruct a plausible evolutionary scenario of the diverging and merging genomes inherited by the diploid and allopolyploid species and cytotypes of the model grass genus Brachypodium. We have identified the ancestral Brachypodium genomes and inferred the paleogeographical ranges for potential hybridization events that originated its allopolyploid taxa. We also constructed a comprehensive phylogeny of Brachypodium from five nuclear and plastid genes using Species Tree Minimum Evolution allele grafting and Species Network analysis. The divergence ages of the lineages were estimated from a consensus maximum clade credibility tree using fossil calibrations, whereas ages of origin of the diploid and allopolyploid species were inferred from coalescence Bayesian methods. The biogeographical events of the genomes were reconstructed using a stratified Dispersal-Extinction-Colonization model with three temporal windows. Our combined Minimum Evolution-coalescence-Bayesian approach allowed us to infer the origins and the identities of the homeologous genomes of the Brachypodium allopolyploids, matching the expected ploidy levels of the hybrids. To date, the current extant progenitor genomes (species) are only known for B. hybridum. Putative ancestral homeologous genome have been inherited by B. mexicanum, ancestral and recent genomes by B. boissieri, and only recently evolved genomes by B. retusum and the core perennial clade allopolyploids (B. phoenicoides, B. pinnatum 4x, B. rupestre 4x). We dissected the complex spatio-temporal evolution of ancestral and recent genomes and have detected successive splitting, dispersal and merging events for dysploid homeologous genomes in diverse geographical scenarios that have led to the current extant taxa. Our data support Mid-Miocene splits of the Holarctic ancestral genomes that preceded the Late Miocene origins of Brachypodium ancestors of the modern diploid species. Successive divergences of the annual B. stacei and B. distachyon diploid genomes were implied to have occurred in the Mediterranean region during the Late Miocene-Pliocene. By contrast, a profusion of splits, range expansions and different genome mergings were inferred for the perennial diploid genomes in the Mediterranean and Eurasian regions, with sporadic colonizations and further mergings in other continents during the Quaternary. A reliable biogeographical scenario was obtained for the Brachypodium genomes and allopolyploids where homeologous genomes split from their respective diploid counterpart lineages in the same ancestral areas, showing similar or distinct dispersals. By contrast, the allopolyploid taxa remained in the same ancestral ranges after hybridization and genome doubling events. Our approach should have utility in deciphering the genomic composition and the historical biogeography of other allopolyploid-rich organismal groups, which are predominant in eukaryotes.

Evolution of the beta-amylase gene in the temperate grasses: Non-purifying selection, recombination, semiparalogy, homeology and phylogenetic signal.

Low-copy nuclear genes (LCNGs) have complex genetic architectures and evolutionary dynamics. However, unlike multicopy nuclear genes, LCNGs are rarely subject to gene conversion or concerted evolution, and they have higher mutation rates than organellar or nuclear ribosomal DNA markers, so they have great potential for improving the robustness of phylogenetic reconstructions at all taxonomic levels. In this study, our first objective is to evaluate the evolutionary dynamics of the LCNG ß-amylase by testing for potential pseudogenization, paralogy, homeology, recombination, and phylogenetic incongruence within a broad representation of the main Pooideae lineages. Our second objective is to determine whether ß-amylase shows sufficient phylogenetic signal to reconstruct the evolutionary history of the Pooid grasses. A multigenic (ITS, matK, ndhF, trnTL, and trnLF) tree of the study group provided a framework for assessing the ß-amylase phylogeny. Eight accessions showed complete absence of selection, suggesting putative pseudogenic copies or other relaxed selection pressures; resolution of Vulpia alopecuros 2x clones indicated its potential (semi) paralogy; and homeologous copies of allopolyploid species Festuca simensis, F. fenas, and F. arundinacea tracked their Mediterranean origin. Two recombination events were found within early-diverged Pooideae lineages, and five within the PACCMAD clade. The unexpected phylogenetic relationships of 37 grass species (26% of the sampled species) highlight the frequent occurrence of non-treelike evolutionary events, so this LCNG should be used with caution as a phylogenetic marker. However, once the pitfalls are identified and removed, the phylogenetic reconstruction of the grasses based on the ß-amylase exon+intron positions is optimal at all taxonomic levels.

Multiple colonizations, in situ speciation, and volcanism-associated stepping-stone dispersals shaped the phylogeography of the Macaronesian red fescues (Festuca L., Gramineae).

Whereas examples of insular speciation within the endemic-rich Macaronesian hotspot flora have been documented, the phylogeography of recently evolved plants in the region has received little attention. The Macaronesian red fescues constitute a narrow and recent radiation of four closely related diploid species distributed in the Canary Islands (F. agustinii), Madeira (F. jubata), and the Azores (F. francoi and F. petraea), with a single extant relative distributed in mainland southwest Europe (F. rivularis). Bayesian structure and priority consensus tree approaches and population spatial correlations between genetic, geographical, and dispersal distances were used to elucidate the phylogeographical patterns of these grasses. Independent versus related origins and dispersal versus isolation by distance (IBD) hypotheses were tested to explain the genetic differentiation of species and populations, respectively. Genetic structure was found to be geographically distributed among the archipelagos and the islands endemics. The high number of shared AFLP fragments in all four species suggests a recent single origin from a continental Pliocene ancestor. However, the strong allelic structure detected among the Canarian, Madeiran, and Azorean endemics and the significant standardized residual values obtained from structured Bayesian analysis for pairwise related origin hypotheses strongly supported the existence of three independent continental-oceanic colonization events. The Canarian F. agustinii, the Madeiran F. jubata, and the two sister F. francoi and F. petraea Azorean species likely evolved from different continental founders in their respective archipelagos. Despite the short span of time elapsed since colonization, the two sympatric Azorean species probably diverged in situ, following ecological adaptation, from a common ancestor that arrived from the near mainland. Simple dispersal hypotheses explained most of the genetic variation at the species level better than IBD models. The optimal dispersal model for F. agustinii was a bidirectional centripetal stepping-stone colonization pattern, an eastern-to-western volcanism-associated dispersion was favored for F. francoi, whereas for the recently derived F. petraea a counterintuitive direction of colonization (west-to-east) was suggested. The population-based phylogeographical trends deduced from our study could be used as predictive models for other Macaronesian plant endemics with similar distribution areas and dispersal abilities.