Chapter 5 Genetic Analysis of Apomixis - cimmyt

240 no- P,ossdlaos, Jo. G.'-.... Y.os ScrrioI..plants, and if so, what would be the impact.This issue is especially important in the centersof origin for the crop plants. Furthermore, theissue of how apomixis might affect geneticdiversity, and whether it would increase ordecrease monoculture farming needs to beexplored. Based on field studies on herbicideand/or insecticide resistant plants, we canprobably expect engineered apomixis genes tomove through vertical gene transfer (transferof a gene from plant to plant via sexualreproduction/pollen) (Lutman 1999). The rateof horizontal gene transfer (asexual gene flowbetween organisms) is relatively low and therisk negligible, however, microbiological riskassessment studies in this area could be useful(Syvanen 1994). Given our current knowledge,it appears unlikely that microorganisms couldgain some advantage over wild relatives afteruptake of apomixis genes.Ifapomixis is controlled by multiple genes, theprobability of diffusing this trait to wildrelatives is extremely low. The transfer ofseveral genes to a wild plant should lower itsfitness to a level unacceptable for survival inthe wild (Berthaud, Chap. 2). If apomixis iscontrolled by a single gene, which would resultin obligate apomictic wild races, these raceswould lose their potential to evolve. Ifdominant, an apomixis gene could rapidlybecome fixed in an outcrossing sexualpopulation. Therefore, in theory, apomixistransgenes could possess advantages thatmight result in the uncontrollable spread ofthe transgenes (van Dijk and van Damme2000). Inducible apomictic systems and malesterility might circumvent these problems.Nevertheless, the described possibilitiesindicate that risk assessment studies andresearch to investigate the ecologicalimplications of novel apomictic crops (onceavailable) to the environment are an absolutenecessity. In addition, socioeconomic studieson the positive and negative implications ofthis technology for breeders, seed companies,and farmers-in both developing and developedcountries (see also IPR) will be required, andthe research results should be communicatedto all potential users.SummaryThe extensive introduction of apomixis intosexual crops will undoubtedly rely on geneticengineering, as we anticipate that morecandidate genes (especially regulatory genesand tissue/cell-specific promoters) andenabling techniques will be identified anddeveloped in the near future. Transformationtechnology for all major crops is now availableand inducible systems are currently beingdeveloped and optimized, allowing the controlof transgene expression and activity evenunder field conditions. Adventious apomixisusing already described or novel genes underthe control of ovule-, nucellus- or archesporespecificpromoters is probably the easiest wayto engineer the apomixis trait. Plant breedersand seed producers would like to generateinducible obligate mitotic diplospory incombination with autonomous endospermdevelopment. The latter is probably the mostdifficult aspect of engineering apomixis,espeCially for cereals such as wheat, rice, andmaize, because of dosage and imprintingeffects.Although apomixis is a hot topic in plantresearch, our current understanding of bothapomictic and amphimictic reproductionpathways in higher plants is still extremelylimited. The economic potential of apomixismight provide the impetus to bring apomicticcrops to the marketplace, and in the process itmay well contribute significantly to our futureunderstanding of the molecular regulation ofthe many different sexual and apomictic plantreproduction pathways.International and interdisciplinary approachesand efforts are now needed to study andmanipulate seed reproduction. It will be