vavilov domestication south central mexico maize origins nectarless flowers pollinators flower heteranthery exchange conservation germplasm cgiar genetic relationship marker gene phenotype mutant map based gene cloning dna markers virulent pathogen bacterial leaf blight rice disease resistance marker assisted selection cooking qualities conventional breeding marker assisted breeding microarrays or real-time pcr snp epigenetic markers genotyping nucleic acid sequence chromosomes genomic dna plants methods applications principls probes gish in-vitro cultures heterotic grouping stress quality breeding maize genesis
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