Summary Numerous studies have argued that environmental variations may contribute to evolution through the generation of novel heritable variations via meiotic recombination, which plays a crucial role in crop domestication and improvement. Rice is one of the most important staple crops, but no direct estimate of recombination events has yet been made at a fine scale. Here, we address this limitation by sequencing 41 rice individuals with high sequencing coverage and c. 900 000 accurate markers. An average of 33.9 crossover (c. 4.53 cM Mb−1) and 2.47 non‐crossover events were detected per F2 plant, which is similar to the values in Arabidopsis. Although not all samples in the stress treatment group showed an increased number of crossover events, environmental stress increased the recombination rate in c. 28.5% of samples. Interestingly, the crossovers showed a highly uneven distribution among and along chromosomes, with c. 13.9% of the entire genome devoid of crossovers, including 11 of the 12 centromere regions, and c. 0.72% of the genome containing large numbers of crossovers (> 50 cM Mb−1). The gene ontology (GO) categories showed that genes clustered within the recombination hot spot regions primarily tended to be involved in responses to environmental stimuli, suggesting that recombination plays an important role for adaptive evolution in rapidly changing environments.

BackgroundRice blast fungus Magnaporthe oryzae is one of the most devastating pathogens in rice. Avirulence genes in this fungus share a gene-for-gene relationship with the resistance genes in its host rice. Although numerous studies have shown that rice blast R-genes are extremely diverse and evolve rapidly in their host populations, little is known about the evolutionary patterns of the Avr-genes in the pathogens.ResultsHere, six well-characterized Avr-genes and seven randomly selected non-Avr control genes were used to investigate the genetic variations in 62 rice blast strains from different parts of China. Frequent presence/absence polymorphisms, high levels of nucleotide variation (~10-fold higher than non-Avr genes), high non-synonymous to synonymous substitution ratios, and frequent shared non-synonymous substitution were observed in the Avr-genes of these diversified blast strains. In addition, most Avr-genes are closely associated with diverse repeated sequences, which may partially explain the frequent presence/absence polymorphisms in Avr-genes.ConclusionThe frequent deletion and gain of Avr-genes and rapid non-synonymous variations might be the primary mechanisms underlying rapid adaptive evolution of pathogens toward virulence to their host plants, and these features can be used as the indicators for identifying additional Avr-genes. The high number of nucleotide polymorphisms among Avr-gene alleles could also be used to distinguish genetic groups among different strains.

Extensive studies have focused on the largest class of disease resistance genes (nucleotide binding site-leucine-rich repeat, NBS-LRR) in various plants. However, no research on the dynamic evolution of these genes in domesticated species and their progenitors has been reported. Recently published genome sequences of bread wheat and its two ancestors provide a good opportunity for comparing NBS-encoding genes between ancestors and their progeny. Over 2000 NBS-encoding genes have been identified in bread wheat, which is the largest number having been reported so far. Compared with other grass species, its two progenitors also contained more NBS-encoding genes, indicating that there was an expansion of these genes in their common ancestor. Interestingly, the inherited relationships of NBS-LRR genes among the bread wheat and its two progenitors were ambiguous and only 3 % single-copy orthologues retained gene order in three-way genome comparisons of the three genomes. Lots of NBS-encoding genes present in the either ancestor could not be found in the bread wheat. These results indicated that NBS-LRR genes in bread wheat might have evolved rapidly through a rapid loss of ancestor genes.

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SWEET/MtN3/saliva genes are prevalent in cellular organisms and play diverse roles in plants. These genes are widely considered as evolutionarily conserved genes, which is inconsistent with their extensive expansion and functional diversity. In this study, SWEET genes were identified from 31 representative plant species, and exhibited remarkable expansion and diversification ranging from aquatic to land plants. Duplication detection indicated that the sharp increase in the number of SWEET genes in higher plants was largely due to tandem and segmental duplication, under purifying selection. In addition, phylogeny reconstruction of SWEET genes was performed using the maximum-likelihood (ML) method; the genes were grouped into four clades, and further classified into 10 monocot and 11 dicot subfamilies. Furthermore, selection pressure of SWEET genes in different subfamilies was investigated via different strategies (classical and Bayesian maximum likelihood (Datamonkey/PAML)). The average dN/dS for each group were lower than one, indicating purifying selection. Individual positive selection sites were detected within 4 of the 21 sub-families by both two methods, including two monocot subfamilies in Clade III, harboring five rice SWEET homologs characterized to confer resistance to rice bacterial blight disease. Finally, we traced evolutionary fate of SWEET genes in clade III for functional characterization in future.