Since its first cultivation in Mexico around 10,000 years ago, maize has become one of the world's most widely-produced crops.
To better understand the genetic changes associated with crop-breeding, scientists at the Chinese Agricultural University (CAU) in Beijing sampled 278 inbred maize lines and published their findings earlier this month (3 June) in Nature Genetics.
The sampled lines included 36 public lines from the United States, 90 lines whose Plant Variety Protection Act (PVPA) certifications had expired, and 152 elite Chinese lines, bringing together a broad variety of temperate inbred maize lines.
Jinsheng Lai, the CAU geneticist leading the research, told SciDev.Net that US public inbred maize lines had been fostered in the 1970s, while the ex-PVP (plant variety protected) and Chinese lines were bred in the 1990s, providing a breeding timeline of 25 years.
Genetic re-sequencing revealed a steady decline in nucleotide diversity the range of variation within a single species during that period. Not surprisingly, the ex-PVPA species were significantly less diverse than their US public line ancestors.
However the researchers also observed an increase in the percentage of rare alleles, or mutations, in the genetic makeup of later crops. Rare alleles are important sources of the genetic improvements sought by plant breeders, Lai said.
He added that the relative percentage of such mutations in a given species could help plant breeders reduce the time and effort spent on large-scale field tests.
The researchers also identified hundreds of target regions in the genes of the 278 maize lines, where genetic variation had been reduced or eliminated altogether. These could also serve as reference points for future breeding programs, as well as for the biotechnology industry, the paper's authors noted.
Bin Han, director of the National Center for Gene Research, told SciDev.Net that the research provided a useful map of deliberately-introduced genetic changes, and that this could guide future efforts to improve maize.
Zhang Shihuang, a maize expert at the Chinese Academy of Agricultural Sciences, said the research was solid but warned against rapid implementation of genome-based maize-breeding programmes, saying China did not have a strong domestic seed development industry.
He said Chinese maize research should prioritise specific domestic needs.