Assessment of Pollen-Mediated Transgene Flow in Citrus under Experimental Field Conditions
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Despite potential benefits granted by genetically modified (GM) citrus trees, their release and commercialization raises concerns about their potential environmental impact. The transfer via pollen of transgenes to cross-compatible cultivars is deemed to be the greatest source for environmental exposure. In this work, three different citrus genotypes carrying the uidA (GUS) tracer marker gene (pollen donors) and a non-GM self-incompatible contiguous citrus genotype (recipient) were used in conditions allowing natural entomophilous pollination to occur. The examination of 603 to 2990 seeds per year showed unexpectedly low frequencies (0.17-2.86%) of transgene flow. Paternity analyses of the progeny of subsets of recipient plants using 10 microsatellite (SSR) loci demonstrated a higher mating competence of trees from another non-GM pollen source population that greatly limited the mating chance of the contiguous cross-compatible and flowering-synchronized transgenic pollen source. This mating superiority could be explained by much higher pollen competition capacity of the non-GM genotypes, as was confirmed through mixed-hand pollinations, indicating that pollen competition strongly contributed to transgene confinement. This is the first study on transgene flow in citrus. It provides crucial information on the safety and field performance of GM citrus that can serve as a basis for further field trials and as a guide for (case-by-case) regulatory policies.