Gene flow in the environment Gene flow in the environment – genetic pollution? G.R. Squire, N. Augustin, J. Bown1, J.W. Crawford, G. Dunlop, J. Graham, J.R. Hillman, B. Marshall, D. Marshall, G. Ramsay, D.J. Robinson, J. Russell, C. Thompson & G. Wright iological invasions have had profound effects on affect plants and animals, injure us? Will crops in gen- Bhuman society from the earliest times. The spread eral, and GM ones in particular, reduce even more the of the black death in the Middle Ages, the devasta- biological diversity of arable farmland? Will they con- tions of potato blight, the effects on indigenous taminate other crops, cause more pesticide to be used, species by grey squirrels, dutch elm disease and flat- rather than less as some companies claim? worms have all been seen as detrimental to man or the environment. Others are seen as bringing benefits: An increasing number of people have a stake in the most of our crops evolved elsewhere in the world and debate - pressure groups, farmers, farm advisers, con- many culinary and medicinal herbs were brought to sumers, agrochemical companies and government. Britain by the Romans. Perhaps the greatest invasion Opinions are too often polarised. In this confronta- is the import of vast numbers of exotic plants to gar- tional atmosphere, the need is for clear, independent dens and greenhouses. Ecological invasions are an fact, answers and comment. A part of the debate is intrinsic part of ecology and evolution and we only ethical, but independent research is essential on ques- consider them bad if they impoverish our health, tions that science can legitimately address. This is not livelihood or living conditions. to say that scientists should not be ethical, but that the methods of science can only be applied to answer cer- Recent events brought awareness of a need to be tain types of question through constructing hypothe- knowledgeable and vigilant about where food comes ses and testing them by observation, experiment and from, how it is grown and what it might contain. modelling. It cannot say, for instance, that grey squir- During the 1990s, agriculture and the food chain rels or GM herbicide-tolerant crops are ‘good’ or returned to be daily topics of conversation. In 1998, ‘bad’. The answer depends on your standpoint. Sci- the concern was for genetically modified crops, and ence can, however, define what effect such organisms the term ‘genetic pollution’ came into common usage. are having or might have on wildlife, habitat and There is no place in science for such emotive terms health, and advise government and the public on the but publicly funded science must answer, as far as it risks and hazards. can, the public’s fears with facts and sound interpreta- tion. Invasions are complex biological processes. How do we study them? First there is the movement of plants The questions? Biological science is regularly in the or seeds, and the transfer of genes by pollen. The spotlight. Is habitat being destroyed by land manage- organisms have to find and occupy space and grow in ment practices? Will GM crops escape from fields, their new environment, while the pollen has to find 1University of Abertay, Dundee 45 Gene flow in the environment receptive stigmas and create a new organism. Incom- and interpreting rare events and small numbers. ing plants have to interact for space and resource with Methods and techniques are needed to do three the plants that are already there, and accumulate things: to show that an organism or gene has moved enough mass to reproduce and maintain a stronghold. from one place to another; to measure its effect on the The local environment acts on them and sorts out the existing organisms at that place; and (because these less able. Exchange of genes creates variants that are two can only be done on a small number of the organ- continually re-sorted. The nature of the invaded pop- isms) to build the greater picture from all the available ulation changes from what it was at the beginning. data. The sciences of plant ecology, genetics, zoology, pathology, mathematics and statistics are all put to use The difficulty lies in in this work. Research not the range of interacting only has to break new ground scales. Growth or in each subject but to find death, and exchange of new ways of seaming them genes, occurs through together. direct interactions between individuals. Genetic exchange and detec- The individuals occur tion Advances in knowl- in patches or fragments edge of the genetic code have and the fragments are led to robust techniques for scattered over the land- genetically typing individuals scape. Single fragments and for detecting the move- evolve their own char- ment of genetic material acteristics that fit them from one plant to another. to the local environment. The patches are not inde- Some of the techniques now in use at the Institute1,2 pendent, but potentially connected through the are shown in Figure 1. Each is appropriate for a spe- migration of seed or transfer of pollen. The individu- cific purpose. To confirm geneflow, it is essential to als in distant patches can therefore influence the inter- observe a transfer of genetic material from one indi- actions within any other patch. Clearly, the degree of vidual or group to the progeny of another. Potential connectivity will depend on whether the pollen or receptors and donors need to be genetically typed to seed moves by wind or is carried by insects or other search for distinguishing features in their DNA. Seed animals, and how sexually compatible are the plants in is collected from the potential receptor (mother) the different patches. This system evolves therefore at plants, germinated and the seedlings also genetically scales of both the patch and landscape. This is what typed. The DNA of the mother plant and seedlings makes the study of gene flow and invasions so difficult are then compared. Gene flow has occurred if a partic- and so challenging. Very advanced techniques are ular DNA sequence is found in a seedling but not in being developed and used to examine these exchanges. the mother plant. Further, if the number of potential donors is reasonably small, then some of the tech- New methodologies Movement of seed or genes into niques can trace the plant from which the pollen a fragmented ecosystem usually begins slowly and pro- came. As Figure 1 shows, the approach is being used ceeds gradually. Only when things are well on their to detect and quantify gene flow in plants ranging way are they generally noticed. The crux is detecting from feral brassicas to tropical trees. Microsatellites AFLPs (SSAPs) ISSRs RAPDs Nuclear Chloroplast -Co-dominant Mahogany -Dominant Calycophyllum -Dominant Oilseed rape -Dominant Raspberry Eucalypts Brassicas Grasslands -Single locus Pines -Multi locus Nematodes -Multi locus -Multi locus -Multiallelic Coconut -Biallelic -Biallelic -Biallelic Figure 1 Banding patterns from some genetic technologies, with which it is now possible to assess gene flow and the spatial organisation of genetic variation. The techniques are used at SCRI with the species indicated. 46 Gene flow in the environment The new genetic enabling technologies allow us to do organism enters an ecological assemblage that primar- things that were impossible a few years ago. To answer ily determines its rate of establishment, extinction or most questions, further knowledge is needed of how amplification. Many factors mitigate against the seed or pollen are moved by wind or insects, and ingress of an alien organism, and many invasion whether plants are sexually compatible. This brings in events and many cycles of reproduction are likely to new scales of operation that are influenced by the local be required before it gains a secure place. weather and the flight paths and foraging behaviour of pollinators. Molecular science can contribute to Invasions and population structure In an established knowledge at these scales by helping to define the dis- arable field or wayside patch of ground, perhaps 30 tance over which pollen can move from one plant to weed species cohabit in the soil as dormant seed. another. Among them are likely to be several of the typical, economically damaging, arable weeds: fat hen, Science in this area has recently made major contribu- cleavers, blackgrass, wild oat. Each is at least as capa- tions to the topical and contentious issue of gene ble of exploiting time windows in the weather and transfer from GM crops to wild plants. It has been husbandry as any invading type such as feral oilseed suggested that GM characters will be transferred to rape or feral beet. The existing seedbank species occur weeds giving them resistance to weedkillers and insect in some hierarchy of abundance, and are distributed pests. The flow of genes from some crops to some over space in characteristic ways. Typical abundance wild relatives is possible and will happen if GM crops rankings show a wide range from species that are very are deployed on a large scale. Indeed, crops and weeds dominant (10,000 to 100,000 in a square metre down have exchanged genes since the beginnings of agricul- to plough depth) to those that occur relatively scarcely ture, so there is nothing new in the process itself. So (<1000).Which individuals are dominant depends on far, there are few indications in the UK that crops are the soil, the rotation, the use of herbicide, and other crossing with other species to increase the latter’s abil- factors. The Institute’s unrivalled database on arable ity as a weed. Those crops and wild relatives that are buried seed (seedbanks) is continually probed to 4 most likely to exchange genes in the UK are known in answer questions on arable diversity .