Flowering time diversification and dispersal in central Eurasian wild wheat Aegilops tauschii Coss.: genealogical and ecological framework

Abstract

Timing of flowering is a reproductive trait that has significant impact on fitness in plants. In contrast to recent advances in understanding the molecular basis of floral transition, few empirical studies have addressed questions concerning population processes of flowering time diversification within species. We analyzed chloroplast DNA genealogical structure of flowering time variation in central Eurasian wild wheat Aegilops tauschii Coss. using 200 accessions that represent the entire species range. Flowering time measured as days from germination to flowering varied from 144.0 to 190.0 days (average 161.3 days) among accessions in a common garden/greenhouse experiment. Subsequent genealogical and statistical analyses showed that (1) there exist significant longitudinal and latitudinal clines in flowering time at the species level, (2) the early-flowering phenotype evolved in two intraspecific lineages, (3) in Asia, winter temperature was an environmental factor that affected the longitudinal clinal pattern of flowering time variation, and (4) in Transcaucasus-Middle East, some latitudinal factors affected the geographic pattern of flowering time variation. On the basis of palaeoclimatic, biogeographic, and genetic evidence, the northern part of current species' range [which was within the temperate desert vegetation (TDV) zone at the Last Glacial Maximum] is hypothesized to have harbored species refugia. Postglacial southward dispersal from the TDV zone seems to have been driven by lineages that evolved short-flowering-time phenotypes through different genetic mechanisms in Transcaucasus-Middle East and Asia.

Figure 1. Geographic distribution of the 200 Ae. tauschii accessions.

Circles, triangles, stars, and asterisks denote, respectively, accessions of the HG7 lineage, HG9 lineage, HG16 lineage, and HG17 lineage. For each accession, flowering time phenotype is colored according to the key. The belt area where fairly undisturbed habitats exist is shaded –. The dashed yellow line indicates the southern limit of the temperate desert vegetation zone at the Last Glacial Maximum . The species distribution was divided into two regions in the analyses: Transcaucasus-Middle East (longitude <60°E) and Asia (longitude ≥60°E).

Figure 2. Reduced-median network for chloroplast DNA haplogroups.

In total, 21 biallelic sites were used to define the haplogroups (Table S2). Circles denote haplogroups (with diameters proportional to the number of accessions). Mutations that define haplogroups are shown on the branches. mv denotes a hypothetical haplogroup not found in this study. Grey scale indicates geographic origin proportions: grey for western habitats (longitude <60°E) and black for eastern habitats (longitude ≥60°E).

Figure 3. Flowering time variation in the 200 Ae. tauschii accessions.

Flowering time for each accession is the mean of thrice-replicated data of days from germination to flowering.

Figure 4. Geographic clines of flowering time.

A) The relationship between latitude of origin and flowering time (regression coefficient±SE = 1.61±0.14, P<0.0001). B) The relationship between longitude of origin and flowering time (regression coefficient±SE = −0.34±0.05, P<0.0001).

Figure 5. Graph of the first two axes from a principle component analysis.

The fifty-two HG7 and HG7 haplotypes based on 14 polymorphic loci were used (Table S3). The first component (x) explains 25.3% and the second (y) 14.5% of the total variation. Circles, squares, and diamonds indicate the western HG7 haplotypes, eastern HG7 haplotypes, and HG16 haplotypes, respectively.