Molecular evaluation of two methods for developing maize synthetic varieties
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A generalized way to obtain a maize (Zea mays L.) synthetic variety is to randomly mate all possible crosses among inbred lines, but, when the number of lines is large, it would save time and effort to use other mating methods. This work is aimed to determine the utility of two alternative methods, assuming that an equal contribution of each inbred line to the synthetic is the goal (; 1993). Eight Reid inbred lines originated from the U.S. Corn Belt population ‘Reid’, and eight non-Reid unrelated inbreds were the base meterial for synthetic verieties EPS20 and EPS21, respectively . Each variety was built using both alternative methods: i) controlling the balanced contribution of each parental inbred (convergent cross method), and ii) randomly mating a seed mixture from the eight inbreds (random method). In the convergent cross method, single crosses between inbreds, and double crosses were performed in 1995, and 1996, respectively, as the first step to create EPS20c and EPS21c. Finally, in 1997, about 40 plants from each double-cross hybrid were mated only once (as male or female) in plant-to-plant crosses between double cross-hybrids, to generate about 20 ears that provided the base material for synthetics EPS20c and EPS21c. In the random method, a total of 304 seeds from the eight inbred lines, 38 seeds from each one, were bulked and sown in ten rows of 15 two-kernel hills per row in 1998 to form EPS20r and EPS21r. After thinning, 150 plants were left, and random plant-to plant crosses were made (where each plant was used only once as male or female), that resulted in 38 and 39 ears, respectively. The two crossing schemes were followed by two generations of recombination. A sample of 40 individuals from each synthetic variety was genotyped with 17 SSR (simple sequence repeat) markers loci. Primer sequences and chromosomal locations are available at the MaizeDB (www.agron.missouri.edu/ssr.html). Genetic drift due to the relatively small samples used to constitute balanced synthetic varieties, and/or natural selection could modify the original frequencies, but did not cause allele or heterozygosity losses. Therefore, the convergent cross method appeared to be appropriate for developing synthetic varieties either from related or non related inbreds. The random method should be used with caution, especially when inbred parents are genetically different, since relative adaptive advantages of some inbred lines could cause loss of genetic variability.
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