Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/28/2001
Publication Date: N/A
Citation: Interpretive Summary: The slow progress in improving rubber yields in guayule can be attributed to its type of reproduction system. Most of the time, seeds produced without pollination, called apomictic, are the same genetically as the parent plant producing the seed, rather than from sexual reproduction where new recombinations of genes are formed. Plant breeders need these new gene recombinations to improve yield characteristics but are unable to distinguish between the two types of plants. A simple and rapid technique was developed from this study to distinguish between these types, by examining whether embryos were produced from flowers growing in a test tube in the absence of pollen. It was found that seeds from different breeding lines produced different percentages of embryos, reflecting the different rates of apomixis within each line. This technique will benefit both public and private plant breeders who are interested in improving yields and developing guayule into a viable commercial latex crop. It could also permit other scientific studies into the molecular control of the reproductive system.
Technical Abstract: Guayule is a latex-producing perennial desert shrub that may be an economically viable industrial crop for semiarid regions. It exhibits a wide range of reproductive modes, with plants ranging for sexual, self- sterile diploids to predominantly apomictic, self-compatible polyploids. The objectives of this study were to develop and evaluate a rapid and simple technique for characterizing reproductive modes of breeding lines in order to facilitate breeding. Initial field experiments using an auxin test that had been used successfully in grass species showed it to be less feasible for guayule. In vitro flower head culture overcame the obstacles presented by the auxin test, and provided a technique to identify plants that produced embryos by apomixy. Percent embryo production was similar on both Nitsch and Nitsch or Woody Plant liquid nutrient media, and the addition of growth regulators inhibited embryo production. Using Nitsch and Nitsch medium without growth regulators, the reproductive mode of seven breeding lines were found to range from completely sexual to predominantly apomictic. As expected, sexual diploid controls never produced embryos in vitro. Results of this technique were substantiated using RAPD analyses of progeny arrays from controlled crosses.