Multilocus species tree analyses resolve the ancient radiation of the subtribe Zizaniinae (Poaceae).

The phylogeny of the subtribe Zizaniinae of rice tribe (Oryzeae) has not been well resolved, particularly for the monotypic Hygroryza whose systematic position was inconsistent in previous studies. Here, we used the concatenation approach and coalescent-based species tree methods to reconstruct the phylogeny of Zizaniinae based on sequences of 14 nuclear single-copy loci and concatenated chloroplast fragments. Despite the low resolution of the tree from concatenated data and substantial topological incongruence of individual gene trees, the species trees inferred from three coalescent-based methods were fully concordant and highly supported. Importantly, the genus Hygroryza was consistently recovered with strong support by all coalescent-based methods. Further various phylogenetic analyses indicated that incomplete lineage sorting was the most likely process that generated pervasive discordance among individual gene trees, although hybridization and introgression cannot be excluded completely. Our species tree inferences based on multilocus data successfully resolved the phylogenetic relationships of the Zizaniinae lineages and confirmed that ancient rapid radiation has taken place in the diversification history of Zizaniinae. This study demonstrates that coalescent-based species tree approaches outperformed the concatenation method and could effectively decipher ancient rapid radiations as long as well resolved individual gene trees were sufficiently sampled.

Selection on grain shattering genes and rates of rice domestication.

Molecular cloning of major quantitative trait loci (QTLs) responsible for the reduction of rice grain shattering, a hallmark of cereal domestication, provided opportunities for in-depth investigation of domestication processes. Here, we studied nucleotide variation at the shattering loci, sh4 and qSH1, for cultivated rice, Oryza sativa ssp. indica and Oryza sativa ssp. japonica, and the wild progenitors, Oryza nivara andOryza rufipogon. The nonshattering sh4 allele was fixed in all rice cultivars, with levels of sequence polymorphism significantly reduced in both indica and japonica cultivars relative to the wild progenitors. The sh4 phylogeny together with the neutrality tests and coalescent simulations suggested that sh4 had a single origin and was fixed by artificial selection during the domestication of rice. Selection on qSH1 was not detected in indica and remained unclear in japonica. Selection on sh4 could be strong enough to have driven its fixation in a population of cultivated rice within a period of c. 100 yr. The slow fixation of the nonshattering phenotype observed at the archeological sites might be a result of relatively weak selection on mutations other than sh4 in early rice cultivation. The fixation of sh4 could have been achieved later through strong selection for the optimal phenotype.

Multilocus analysis of nucleotide variation and speciation in Oryza officinalis and its close relatives.

Nucleotide variation in 10 unlinked nuclear genes was investigated in species-wide samples of Oryza officinalis and its close relatives (Oryza eichingeri and Oryza rhizomatis). Average estimates of nucleotide diversity were the lowest in O. rhizomatis ((sil) = 0.0038) and the highest in O. eichingeri ((sil) = 0.0057) that is disjunctly distributed in Africa and Sri Lanka. These wild rice species appeared to harbor relatively low levels of nucleotide variation relative to other plant species because the diversity level of O. eichingeri is only 23-46% of those in Zea species and 35% of that in Arabidopsis thaliana. The lower nucleotide diversity in these Oryza species could be best explained by their smaller historic effective population sizes. The speciation model test indicated that O. officinalis and its close relatives might have undergone a process of population contraction since divergence from their ancestor. Incongruent topologies among 10 gene trees, particularly regarding the positions of O. eichingeri and O. rhizomatis accessions might be attributed to lineage sorting arising from ancient polymorphism and hybridization/introgression between the Sri Lankan O. eichingeri and O. rhizomatis. However, the null hypothesis of the isolation model was not rejected for any contrast between taxa, which suggested that no subsequent gene flow shaped the present patterns of nucleotide variation since their divergence and that introgression was not pervasive in this group of species. Our molecular dating provides an approximate divergence time of 0.37 Myr between 2 geographical races of O. eichingeri, much more recent compared with the times of other speciation events in this group (0.63-0.68 Myr). A long-distance dispersal from West Africa to Sri Lanka was more likely to play a role in the disjunct distribution of O. eichingeri.