Pentaploidization Enriches the Genetic Diversity of Wheat by Enhancing the Recombination of AB Genomes

Abstract

Allohexaploidization and continuous introgression play a key role in the origin and evolution of bread wheat. The genetic bottleneck of bread wheat resulting from limited germplasms involved in the origin and modern breeding may be compensated by gene flow from tetraploid wheat through introgressive hybridization. The inter-ploidy hybridization between hexaploid and tetraploid wheat generates pentaploid hybrids first, which absorbed genetic variations both from hexaploid and tetraploid wheat and have great potential for re-evolution and improvement in bread wheat. Therefore, understanding the effects of the pentaploid hybrid is of apparent significance in our understanding of the historic introgression and in informing breeding. In the current study, two sets of F₂ populations of synthetic pentaploid wheat (SPW1 and SPW2) and synthetic hexaploid wheat (SHW1 and SHW2) were created to analyze differences in recombination frequency (RF) of AB genomes and distorted segregation of polymorphic SNP markers through SNP genotyping. Results suggested that (1) the recombination of AB genomes in the SPW populations was about 3- to 4-fold higher than that in the SHW populations, resulting from the significantly (P < 0.01) increased RF between adjacent and linked SNP loci, especially the variations that occurred in a pericentromeric region which would further enrich genetic diversity; (2) the crosses of hexaploid × tetraploid wheat could be an efficient way to produce pentaploid derivatives than the crosses of tetraploid × hexaploid wheat according to the higher germination rate found in the former crosses; (3) the high proportion of distorted segregation loci that skewed in favor of the female parent genotype/allele in the SPW populations might associate with the fitness and survival of the offspring. Based on the presented data, we propose that pentaploid hybrids should increasingly be used in wheat breeding. In addition, the contribution of gene flow from tetraploid wheat to bread wheat mediated by pentaploid introgressive hybridization also was discussed in the re-evolution of bread wheat.

Keywords: adaptive evolution; chromosome recombination; genetic bottleneck; linkage drag; pentaploid.

FIGURE 1

Procedures followed to create mapping populations. (A) CM42 and LS665 were used as the maternal donor to generate SPW1 and SHW1 populations, respectively. (B) LS783 was used as the maternal donor to generate SPW2 and SHW2 populations.

FIGURE 2

Comparison of the genetic maps of each chromosome between pentaploid and hexaploid populations. The position of SNP markers was aligned with the IWGSC genome (version 1.0). Dotted lines indicate the physical location of the centromere on each chromosome.

FIGURE 3

Comparison of the variation in recombination frequency between adjacent and linked SNP loci in pentaploid and hexaploid populations. Dotted lines indicate the physical location of the centromere on each chromosome. (A) SPW1 and SHW1 populations. (B) SPW2 and SHW2 populations.

FIGURE 4

Statistical of genotype frequency in pentaploid and hexaploid populations. 0, genotype frequency of the female parent; 1, genotype frequency of the heterozygous; 2, genotype frequency of the male parent; -1, missing rate. (A) SPW1 population. (B) SHW1 population. (C) SPW2 population. (D) SHW2 population.

FIGURE 5

The distorted segregation (A,C) and parental gene bias (B,D) of SNP markers of AB genomes in SPW and SHW populations. P-value was calculated by Chi-square test with a theoretical genotype frequency of 1:2:1 (AA: AB: BB). If the [log0.05 (P-value) - 1] > 0 or 0.54 at a SNP site, significant distorted segregation was inferred at this locus at P = 0.05 or 0.01 level, respectively. p (A) means gene frequency of the female parent; p (B) means gene frequency of the male parent. If [p (A)/p (B)–1] > 0, the gene frequency of the female parent was higher than that of the male parent in the F₂ recombination populations, and the genotype was biased toward the female parent at this locus.