Abstract
As the plant variety protection (PVP) of commercial inbred lines expire, public breeding programs gain a wealth of genetic materials that have undergone many years of intense selection; however, the value of these inbred lines is only fully realized when they have been well characterized and are used in hybrid combinations. Additionally, while yield is the primary trait by which hybrids are evaluated, new phenotyping technologies, such as ear photometry (EP), may provide an assessment of yield components that can be scaled to breeding programs. The objective of this experiment was to use EP to describe the testcross performance of inbred lines from temperate and tropical origins. We evaluated the performance of 298 public and ex-PVP inbred lines and 274 Drought Tolerant Maize for Africa (DTMA) inbred lines when crossed to Iodent (PHP02) and/or Stiff Stalk (2FACC) testers for 25 yield-related traits. Kernel weight, kernels per ear, and grain yield predicted by EP were correlated with their reference traits with r = 0.49, r = 0.88, and r = 0.75, respectively. The testcross performance of each maize inbred line was tester dependent. When lines were crossed to a tester within the heterotic group, many yield components related to ear size and kernels per ear were significantly reduced, but kernel size was rarely impacted. Thus, the effect of heterosis was more noticeable on traits that increased kernels per ear rather than kernel size. Hybrids of DTMA inbred lines crossed to PHP02 exhibited phenotypes similar to testcrosses of Stiff Stalk and Non-Stiff Stalk heterotic groups for yield due to significant increases in kernel size to compensate for a reduction in kernels per ear. Kernels per ear and ear length were correlated (r = 0.89 and r = 0.84, respectively) with and more heritable than yield, suggesting these traits could be useful for inbred selection.
Highlights
Maize production in the United States totaled 363.2 billion kg on 33.5 million hectares with an average yield of 10,860 kg ha−1 in the five growing seasons from 2015 to 2019; an incredible feat considering the nationwide average yield of 2,950 kg ha−1 in 1956, which was the beginning of the single-cross hybrid maize era (USDA NASS)
Grain yield is a composite of many yield-related traits known as yield components
While yield components generally are found to be more heritable (Table 1), phenotyping these traits has historically been timeconsuming, labor-intensive, prone to error, and difficult to scale in a large breeding program (Bernardo, 2014; Cooper et al, 2014)
Summary
Maize production in the United States totaled 363.2 billion kg on 33.5 million hectares with an average yield of 10,860 kg ha−1 in the five growing seasons from 2015 to 2019; an incredible feat considering the nationwide average yield of 2,950 kg ha−1 in 1956, which was the beginning of the single-cross hybrid maize era (USDA NASS). Since the implementation of double-cross and subsequently single-cross hybrids, maize yields have increased at a rate of 48 and 119 kg ha−1, respectively (USDA NASS). Duvick (2005) attributes about 50% of the increases in maize yields to hybrid maize breeding and the development of superior genetics, while optimized genotype placement and management practices are other key factors. In 1970, the Plant Variety Protection Act (PVP) allowed commercial breeding programs to register varieties and inbred lines as intellectual property restricting the use, sale, and importation of this material for 20 years (Beckett et al, 2017). Inbred lines selected in commercial breeding programs are highly advanced and have undergone many rounds of intense selection. As PVPs expire, public breeding programs have access to these highly advanced inbred lines that can be used to quickly incorporate useful alleles into breeding programs. Many studies have used genomic and pedigree information to characterize exPVP and founder temperate inbred lines (Mikel, 2006, 2008, 2011; Mikel and Dudley, 2006; Nelson et al, 2008; White et al, 2020); phenotypic information describing these temperate inbred lines is less common
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