Pollen Competition as a Reproductive Isolation Barrier Represses Transgene Flow between Compatible and Co-Flowering Citrus Genotypes

Abstract

Background/ObjectiveDespite potential benefits granted by genetically modified (GM) fruit trees, their release and commercialization raises concerns about their potential environmental impact, and the transfer via pollen of transgenes to cross-compatible cultivars is deemed to be the greatest source for environmental exposure. Information compiled from field trials on GM trees is essential to propose measures to minimize the transgene dispersal. We have conducted a field trial of seven consecutive years to investigate the maximum frequency of pollen-mediated crop-to-crop transgene flow in a citrus orchard, and its relation to the genetic, phenological and environmental factors involved.Methodology/Principal FindingsThree 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 a much higher pollen competition capacity of the non-GM genotypes, as was confirmed through mixed-hand pollinations.Conclusions/SignificancePollen competition strongly contributed to transgene confinement. Based on this finding, suitable isolation measures are proposed for the first time to prevent transgene outflow between contiguous plantings of citrus types that may be extendible to other entomophilous transgenic fruit tree species.

Evaluating contemporary pollen dispersal in two common grassland species Ranunculus bulbosus L. (Ranunculaceae) and Trifolium montanum L. (Fabaceae) using an experimental approach

“…2006; Hirao & Kudo 2008) and pollen competition (Rognli et al. 2000; Pons et al. 2011), are also critical.…”

“…Pollen dispersal is mainly influenced by distance and pollinator flight range and abundance, but other factors, such as flowering phenology (Schmitt 1983;Kitamoto et al 2006;Hirao & Kudo 2008) and pollen competition (Rognli et al 2000;Pons et al 2011), are also critical. Monitoring the flowering phenology of R. bulbosus and T. montanum demonstrated that the different populations used in our experimental meadow did not show a significant shift in flowering; apart from one population in R. bulbosus, which was originally intended to serve as maternal plants (population ObWa12, see Figure S1).…”

Genetic Structure of Diploid Gametes for the Production of Triploid Citrus Hybrids

“…Its frequency attained 44%, indicating large inverted segment resulting in partial sterility of gametophytes (Iwamasa, 1966). In Valencia (Spain), the pollen viability of the diploid `Mexican´ lime was estimated to be less than 10% (Pons et al, 2011). Interestingly we observed 64% of pollen viability for the DD `Mexican´ lime, higher than the rates ranging from 31% to 41%, reported by Aleza et al (2012a) and Del Bosco et al (1999) for different DD and somatic hybrids.…”

“…Applications of the biotechnological techniques such as genetic engineering are useful for the genetic improvement of many of the citrus cultivars avoiding the barriers of the traditional sexual hybridization (Gmitter et al, 1992;Peña et al, 2001;Pons et al, 2011;. The most used methods are through Agrobacterium tumefaciens or PEG (polyethylene glycol) treatment of protoplasts .…”

“…This methodology can open the way to the introduction of specific traits associated with a known characters into elite genotypes without altering their genetic background. Genetic engineering has been experimentally applied to an increasing number of traits to try inducing resistance to the CTV virus (Soneji et al, 2007;Soler et al, 2012), tolerance to HLB disease (Dutt et al, 2016) and repellency to its psyllid vector (Alquézar et al, 2017), and to enhance tolerance to salinity (Cervera et al, 2000) or for reproductive biology investigation purposes (Pons et al, 2011). In addition, genetic transformation has been used in attempts for the introgression of seedlessness to some elite genotypes as `Mexican´ lime (Koltunow et al, 2000) and `Ponkan´ and `Valencia´ sweet orange (Li et al, 2002;.…”