The evolutionary history of PDR in Brachypodium distachyon polyploids

Environmental aridity is associated with cytotype segregation and polyploidy occurrence in Brachypodium distachyon (Poaceae)

• The ecological and adaptive significance of plant polyploidization is not well understood and no clear pattern of association between polyploid frequency and environment has emerged. Climatic factors are expected to predict cytotype distribution. However, the relationship among climate, cytotype distribution and variation of abiotic stress tolerance traits has rarely been examined. • Here, we use flow cytometry and root-tip squashes to examine the cytotype distribution in the temperate annual grass Brachypodium distachyon in 57 natural populations distributed across an aridity gradient in the Iberian Peninsula. We further investigate the link between environmental aridity, ploidy, and variation of drought tolerance and drought avoidance (flowering time) traits. • Distribution of diploids (2n = 10) and allotetraploids (2n = 30) in this species is geographically structured throughout its range in the Iberian Peninsula, and is associated with aridity gradients. Importantly, after controlling for geographic and altitudinal effects, the link between aridity and polyploidization occurrence persisted. Water-use efficiency varied between ploidy levels, with tetraploids being more efficient in the use of water than diploids under water-restricted growing conditions. • Our results indicate that aridity is an important predictor of polyploid occurrence in B. distachyon, suggesting a possible adaptive origin of the cytotype segregation.

Evolution and taxonomic split of the model grass Brachypodium distachyon

BACKGROUND AND AIMS

Brachypodium distachyon is being widely investigated across the world as a model plant for temperate cereals. This annual plant has three cytotypes (2n = 10, 20, 30) that are still regarded as part of a single species. Here, a multidisciplinary study has been conducted on a representative sampling of the three cytotypes to investigate their evolutionary relationships and origins, and to elucidate if they represent separate species.

METHODS

Statistical analyses of 15 selected phenotypic traits were conducted in individuals from 36 lines or populations. Cytogenetic analyses were performed through flow cytometry, fluorescence in situ hybridization (FISH) with genomic (GISH) and multiple DNA sequences as probes, and comparative chromosome painting (CCP). Phylogenetic analyses were based on two plastid (ndhF, trnLF) and five nuclear (ITS, ETS, CAL, DGAT, GI) genes from different Brachypodium lineages, whose divergence times and evolutionary rates were estimated.

KEY RESULTS

The phenotypic analyses detected significant differences between the three cytotypes and demonstrated stability of characters in natural populations. Genome size estimations, GISH, FISH and CCP confirmed that the 2n = 10 and 2n = 20 cytotypes represent two different diploid taxa, whereas the 2n = 30 cytotype represents the allotetraploid derived from them. Phylogenetic analysis demonstrated that the 2n = 20 and 2n = 10 cytotypes emerged from two independent lineages that were, respectively, the maternal and paternal genome donors of the 2n = 30 cytotype. The 2n = 20 lineage was older and mutated significantly faster than the 2n = 10 lineage and all the core perennial Brachypodium species.

CONCLUSIONS

The substantial phenotypic, cytogenetic and molecular differences detected among the three B. distachyon sensu lato cytotypes are indicative of major speciation processes within this complex that allow their taxonomic separation into three distinct species. We have kept the name B. distachyon for the 2n = 10 cytotype and have described two novel species as B. stacei and B. hybridum for, respectively, the 2n = 20 and 2n = 30 cytotypes.

Karyotype analysis and genetic variation of a mutant in Siraitia grosvenorii

This study analyzed the differences in karyotype and genetic variation between a mutant and wild-type Siraitia grosvenorii. Genetic variation included changes in genome and gene expression by SRAP molecular markers. Results showed that wild-type S. grosvenorii was diploid, with a chromosome number of 2n = 2x = 28, whereas the mutant was tetraploid with a chromosome number of 2n = 4x = 56. 4573 DNA bands were obtained using 189 different primer combinations, 577 of which were polymorphic, averaging 3.1 bands for each primer pair, while 1998 pairs were identical. There were no apparent differences on bands amplified by most primer pairs. After comparing the diploid and tetraploid strains, the data generally indicated that the polymorphism would be quite low. 2917 cDNA bands were generated using 133 primer combinations, and stable and clearly differential fragments were sorted out, cloned and sequenced. Ninety-two differentially expressed fragments were successfully sequenced. Sequence analysis showed that most fragments had significant homologous nucleotide sequences with resistant to stress and photosynthesis genes, including ribulose-1,5-bisphosphate carboxylase/oxygenase, phosphoenolpyruvate carboxykinase, pyruvate kinase, peroxisomal membrane transporter, NBS-LRR type resistance protein, protein phosphatase and others. The results revealed that the tetraploid strain has more resistant and photosynthesis ability than its diploid relatives, which providing reference information and resources for molecular breeding and seedless Luohanguo.