Characterizing Transgene Flow From GE Papaya Using the GUS Marker

Information about rate and mechanism of gene flow in papaya (Carica papaya L) populations is needed to design containment strategies that can restrict transgene movement in genetically engineered (GE) plants. Specific objectives for this project were to: 1) document and map the adventitious presence of GE papaya plants outside commercial GE plantings, and 2) determine the relative importance of seeds and pollen in dispersing GE papaya plants. Objective 1 was accomplished by surveying feral papaya plants in rural and residential areas of Oahu, mapping their location by GPS, and determining their non-GE vs. GE status by assaying young leaf tissues for transgenic beta-glucuronidase (GUS) activity. In our survey, 71 (25%) of 285 plants were GUS+, and these were widely distributed on Oahu. Gene flow via pollen was monitored by screening for GUS+ embryos in 12-seed samples taken from fruits of non-GE plants. Assays of seeds from 70 non-GE hermaphrodite plants detected no (0%) plants with GUS+ embryos, but 15 (28%) of 54 non-GE female plants produced at least one GUS+ seed. This shows that transgene flow via cross-pollination occurs primarily through female plants, presumably because hermaphrodite plants self-pollinate before anthesis. However, the observed ratio of 1.7 hermaphrodite : 1 female (38 hermaphrodites : 22 females) among GE plants in our survey is a good fit (P>X² = 58%) to the 2 hermaphrodite : 1 female ratio expected for gene flow via seed dispersal from self-pollinated hermaphrodite GE plants, and a poor fit (P>X² = 4%) to a 1 hermaphrodite : 1 female ratio expected if transgene flow were occurring solely via cross-pollination of non-GE female plants by GE pollen. We conclude that seed dispersal is largely responsible for transgene flow among gynodioecious papayas, but we suggest that removing female plants near test sites can improve bio-containment by reducing gene escape from transgenic plants via pollen.