The Origin And Selective Breeding Of Chinese Waxy Maize

Modern crops have usually experienced domestication selection and subsequent genetic improvement (post-domestication selection). Waxy maize, which originated from non-glutinous domesticated maize (Zea mays ssp. mays), provides a unique model for investigating the post-domestication selection (selection breeding) of maize. In this study, the genotypes of waxy (wx) gene and its geographical distribution in325waxy maize was investigated to identify the waxy origin events and then the genetic diversity and neutrality of wx and other five key genes (shl, sh2, sul, eal, bt2)in the starch pathway was investigated in two glutinous populations with main waxy genotype. The main results are as follow:Genotyping of wx showed that, out of325,315(96.9%) waxy corns hold the wx-D7(D7) or wx-DIO (D10) genotype (88.7%inbred lines,86.6%commercial hybrids and15.6%landrances hold D7, others hold D10), and remaining10accessions (5inbred lines and5landrances) are not genomic but transcriptional mutations, including three unique genotypes (wx-Nl(N1)/wx-N2(N2)/wx-N3(N3)) and transcriptional D7and D10mutants. No accession with both the D7and D10mutations was observed. These indicated that D7and D10are main wx genotypes in Chinese waxy maize and applied in modern breeding program.AAC determination shows that the mean AAC values in commercial hybrids and inbred lines were similar to each other (2.15%and2.13%), while the value in landrances was20%higher (2.57%). D10accessions have relatively diverse AAC, and the means (2.65%) is21%higher than that in D7(2.18%). The results suggested that D7might be more able to maintain a waxy phenotype than D10. This may explain why the D7genotype is more popular in Chinese breeding programs for waxy maize among current commercial hybrids and inbred lines.Geographical distribution of D7and D10in landrances shows that corns from SW China hold D10mutation while D7ones from East China. All of the11accessions from Thailand, which is close to SW China, had the D10genotype, while the5accessions from S. Korea, which is northeast China, had the D7genotype. It indicated that at least two independent origins or genetic improvement events happened in waxy maize.Apparent genetic diversity reductions in waxy maize compared with non-glutinous maize was observed. On the six neutral genes, a reduction of over15%(average16.6%and19.1%for D10and D7subgroups in total sites of π) was found. A greater loss of genetic diversity (29.6%and37.8%) was seen in the six starch genes. The wx experienced the greatest reduction (24.9and53.2fold in the D10and D7subgroups). The results indicate that Chinese waxy maize experienced a genetic bottleneck during its improvement, and improvement selection combined with a demographic effect should have contributed to the loss of genetic diversity on wx gene. Significant positive selection was detected in wx by all of four neutrality tests (Tajima's D, HKA, CS and Fst) but not ael, sul and bt2in the glutinous population, in complete contrast to the findings in non-glutinous maize, which indicated a shift in the selection target from ael, bt2and sul to wx during the improvement of Chinese waxy maize. Our results suggest that an agronomic trait can be quickly improved into a target trait with changes in the selection target among genes in a crop pathway.