Analysis of mode of reproduction of guayule (Parthenium argentatum A. Gray) using flow cytometry and identification of polyhaploids for breeding

Guayule (Parthenium argentatum A. Gray) is a promising domestic source of solid rubber for tire manufacturing in North America. Crop improvement and breeding efforts on guayule have enabled improved germplasm to be developed and deployed on commercial fields. One difficulty in guayule breeding is the identification of the mode of reproduction, particularly in polyploids, which exhibit a mixed mode due to their facultative nature. Guayule polyploid plants range from being triploids (3n = 3x = 54) to octaploids (8n = 8x = 144). Polyploids are known to reproduce by facultative apomixis, while diploids (2n = 2x = 36) follow a sexual mode of reproduction. Flow cytometry is a useful tool for determining ploidy levels of plants. One objective of this study was to estimate the rate of apomixis occurring in different USDA germplasm lines, as well as whether or not there was a meiotic reduction in the megaspore mother cell, and if there was fertilization to form an embryo. We examined the leaf tissue of maternal plants and 100 progeny seeds from these plants, to estimate their rate of apomixis. Combined ploidy information from two different seed tissues (embryo and endosperm) was utilized. Flow cytometric analysis of polyhaploids was also conducted to enable validation of their mode of reproduction, which have not previously been identified in the literature. Results from this study indicated that off-type progeny (seed) would not have been identified if only leaf samples were used, resulting in either, an over-estimation of the rate of apomixis for the accession, or misclassification of the type of reproduction. All identified polyhaploids were analyzed for genetic diversity, and were compared with existing diploid germplasm.