Upper and lower heritability estimates in guayule based on mode of reproduction

Guayule (Parthenium argentatum Gray) has been difficult to improve through classical plant breeding because of its facultative apomictic reproductive system and long growth period to harvest (2-5 years). Attempts have been made to increase rubber concentration and yield by selecting for traits correlated with rubber production. The objective of this study was to estimate heritability values in guayule that more accurately account for the contributions of apomictic and sexual reproduction occurring among populations, using a modified parent-progeny regression. Heritability was calculated using a modified version of the formula for single parent-progeny regression (h² = 2b): that is, h² = (2 - p)b, where p = proportion of apomixis and b = regression coefficient. Therefore, heritability was estimated as a range of values between the midparent and single parent regression, which correspond to complete apomixis and complete sexuality, respectively. At 2 years of growth, there was a significant regression between each parent trait and the corresponding progeny trait except rubber and guayulin B concentrations, with heritability estimates ranging from moderate to high for most parameters measured. Because apomixis occurs more frequently than sexual reproduction, most heritability estimates should fall closer to the midparent value, resulting in lower heritability. To increase resin and rubber yield in the progeny, selection should focus on plant height and width because these two traits are highly correlated with resin and rubber yield and have the highest heritability of the traits measured.