ARS | Publication request: Comparison of the performance of synthetic maize varieties created based on genetic distance or general combining ability of the parent

Submitted to: Maydica
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 27, 2012
Publication Date: N/A

Interpretive Summary: Synthetic corn varieties are often grown by farmers in developing countries because unlike hybrid corn varieties, the seeds of synthetics can be saved from the current crop and planted the following year. Synthetics yield much higher than unimproved landrace varieties many of the more recently released Open Pollinated Varieties. Synthetics are also used by breeders to develop new corn varieties. The creation of synthetic corn populations requires prior knowledge on how each corn inbred line behaves in hybrid crosses with other inbreds, which is called general combining ability (GCA). Typically, 8 – 12 inbred lines with good GCA are intercrossed to form a new synthetic. The measurement of GCA has traditionally been done by making test crosses between each inbred to be measured and various testers, and growing the resulting hybrids in replicated fields over different locations and years. This is a slow and expensive process. A much faster proxy for GCA measurements may be the use of molecular genetic markers, which can estimate the genetic relatedness (or conversely, the genetic distance, GD) between any pair of inbred lines in the study. If 8 – 12 inbreds with high GD can be intercrossed to form synthetics, instead of having to measure GCA, many years and considerable money can be saved in the process. In this study, we compared synthetic varieties created based on GD to synthetics formed based on GCA for yield and other important traits in the field. Two synthetics were formed with low GD, and two with high GD. The two synthetics formed based on high GD performed better than all other synthetics in yield and most agronomic traits. The synthetics formed based low GD performed worst for yield and most other traits. The synthetics formed based on the best GCA were generally intermediate for all traits in this study. Response of the synthetics to environmental variation and efficiencies gained via the use of molecular markers in synthetic formation is discussed.

Technical Abstract: Synthetics can be grown by farmers or used by breeders to select new inbred lines. In countries with inadequate infrastructure to market hybrids, use of synthetics leads to major yield improvements. Synthetics are derived from intercrossing fixed lines known to possess high general combining ability (GCA) as measured via top crosses between a line and various testers. These top crosses are phenotyped for yield in multiple environments. Genetic similarity (GS) between lines may be more efficient to estimate GCA. Although the prediction of specific combining ability (SCA) between lines via molecular markers has not been successful, their use to predict the suitability of lines to form a synthetic variety may work better. This has not been reported, and thus, the objective of this research was to compare the performance of 4 synthetic maize varieties developed using SSR markers to calculate genetic similarities between parents with the performance of synthetics developed using GCA based on yield. Synthetics were phenotyped for yield and other important agronomic traits in replicated field trials in several environments. The two synthetics formed based on low GS (0.34 and 0.33) performed better than all other synthetics in yield and most agronomic traits. The synthetics formed based high GS (0.77 and 0.53), performed worst for all or nearly all traits, respectively. The synthetics formed based on the best GCA were generally intermediate for all traits in this study, including two synthetics formed based on GCA that had GS levels as low as the best synthetics formed based on markers. Response of the synthetics to environmental variation and efficiencies gained via the use of molecular markers in synthetic formation is discussed.