(Trans)Gene Flow: Mechanisms, Biosafety Concerns and Mitigation for Containment

Abstract

Rapid progress in genetic engineering of plants has opened vistas for manipulation of traits to not only meet the ever-increasing need of food, feed, and fuel but also produce novel compounds in transgenic plants (genetically modified crops or GM crops). The widespread global adoption of transgenic plants has raised doubts and fears about its ill-impact on the health of intended consumers of end-products (humans and animals), on non-target organisms (pollinators, microbes, other biotic factors, etc.), and, environmental concerns (impact on biodiversity through escape of transgene and hybridization causing weediness, erosion of biodiversity). Transgene flow is a multistep process, influenced by several biotic and abiotic factors and pollen-mediated gene flow (PMGF) or gametic transmission is the most common mode. Some of these factors that influence (trans)gene flow include population structure and reproductive strategies of source/donor and sink/recipient species, overlap in geographical distribution, flowering phenology, genome compatibility, pollen-transfer mechanism (pollinators, wind direction and speed, pollen characteristics), pre- and post-fertilization barriers, survival of progeny as feral and volunteer population, etc. Several of these aspects have been investigated using both transgenic and non-transgenic plants and which revealed, for instance, that pollen-mediated flow can occur over large distances. Similarly, seed-mediated gene flow (SMGF) and vegetative propagule-mediated gene flow (VPMGF) of germplasm has also been established as a significant contributor to gene flow. Gene flow events are considered a threat if the resultant hybrids have equal or higher vigour, and are viable and fertile, as has been found in several cases. Indeed, increased vigour is a potential contributor to evolution of aggressive weediness and extinction of wild relatives through processes of genetic swamping, selective sweep, and genetic assimilation, an ecological consequence of (trans)gene escape. Horizontal gene flow, or gene flow between unrelated organisms (e.g. plants to microbes, humans, pollinators, etc.), is the major biosafety concern, although investigations have found such fears largely unfounded. Several containment and mitigation strategies including spatial and temporal barriers, biological barriers grouped under Genetic Use Restriction Technologies (GURTs) such as male sterility, bisexual/total sterility, cleistogamy, apomixis, maternal inheritance (transplastomics), preventing seed dispersal, secondary dormancy, competitive self-thinning, interfering with floral induction or bolting, are widely used or recommended to minimize or prevent (trans)gene escape. Transgenic technology is still a developing area, and next generation of plants generated through Genome Editing while addressing existing biosafety and environmental concerns are also likely to throw up newer challenges, including formulation of appropriate robust regulatory framework for which adequate data based on research needs to be collated and analysed.