Heterotic Patterns of Temperate and Tropical Maize by Ear Photometry

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.

Keywords: ear photometry; heterotic groups; high-throughput phenotyping; hybrid breeding; multivariate analysis; tropical maize.

FIGURE 1

Linear regression and Pearson correlation between reference kernel weight (REFKW) (x-axis) to photometry-estimated kernel weight (KERWGT) (y-axis) in 2017 (n = 1,413) (A). Linear regression and Pearson correlation between reference kernels per ear (REFKPE) (x-axis) to photometry-estimated kernels per ear (PHTKPE) (y-axis) in 2017 (n = 1,413) (B). Linear regression and Pearson correlation between reference yield (REFYLD) (x-axis) to photometry-estimated yield (PHTYLD) (y-axis) in 2017 (n = 1,413) (C). Linear regression and Pearson correlation between reference yield on a plot basis (REFYLD18) and the fitted values for PHTYLD adjusted for stand count in modeling REFYLD18 (REFYLD18 = PHTYLD + Stand Count) in 2018 (n = 1,568) (D).

FIGURE 2

Principal Component Analysis (PCA) to visualize the ear photometry (EP) traits and their correlation to photometry-estimated yield (PHTYLD) (A). Dendrogram displaying the hierarchical clustering of EP traits using Ward’s Minimum Variance. The Ball-Hall index was used to determine the correct number of groups (5) among the EP traits (B).

FIGURE 3

Principal component analysis (PCA) for 533 inbred lines in this study using 755,339 Single Nucleotide Polymorphisms (SNPs). PC1 (x-axis) explains 20.2% of the variation of the SNP data while PC2 (y-axis) explains 16.6% of the variation of the SNP data. Testers in this experiment (2FACC and PHP02) are individually labeled with distinct shapes while heterotic groups are differentiated based on color. A, B, and AB are DTMA heterotic groups, while Iodent (IO), Non-Stiff Stalk (NS), and Stiff Stalk (SS) are temperate heterotic groups.

FIGURE 4

Representative ears for each of the heterotic groups in hybrid combination with 2FACC. Selected ears had an average yield and yield components for their heterotic group combination. Abbreviations for heterotic groups include: NS (Non-Stiff Stalk), IO (Iodent), and SS (Stiff Stalk).

FIGURE 5

Representative ears for each of the heterotic groups in hybrid combination with PHP02. Selected ears had an average yield and yield components for their heterotic group combination. Abbreviations for heterotic groups include: NS (Non-Stiff Stalk), IO (Iodent), SS (Stiff Stalk), and DTMA (Drought Tolerant Maize for Africa).

FIGURE 6

Visual description of hybrid breeding. Inbred line 2FACC, on the left, from the Stiff Stalk heterotic group and inbred line PHP02, on the right, from the Iodent heterotic group show reduced vigor due to inbreeding depression. The hybrid of these inbred lines displays greater yield potential than either inbred parent due to hybrid vigor. *2FACC inbred was not produced in this study. As the progenitor of 2FACC, PB80 produced a highly inbred line when crossed to 2FACC.