Chapter 5 Genetic Analysis of Apomixis - cimmyt

Model Sy.tems to Study the Geneti" ond Developmental Biology .f Apomixi. 11 5an important concern in interpreting datacollected from any system. For example, inmost apomicts demonstrating apospory of thePanicum-type, maternal embryos typicallyarise from four-celled, unreduced embryo sacs,while reduced embryo sacs are usually eightcelled.Apomixis can therefore be readilyscored in these plants by determining therelative abundance of the different structures.The correlation between the form of theembryo sac and nuclear state appears to bevery good, but the technique is also based onthe assumption that the number of unreducedembryo sacs is a direct reflection of the numberof maternal embryos that will reach maturity.As the assessment is conducted before therequisite events involved in apomixis haveoccurred, an overestimate of intact apomixisis possible. In contrast, asexual seed formswithout pollination in autonomous apomicts,therefore, apomixis can be quantified in thesespecies from the seed that sets withoutpollination. This is attractive experimentally,but it presents a conservative bias becauseindividuals with only partial apomixis areeasily scored as either sexual or sterile. It isclear in every case that results should beverified by either using more than one methodfor quantifying apomixis or by establishing thecredibility of the results with either anembryological or a genetic study of the system.Candidate SystemsTwo approaches have been taken indeveloping model systems for studyingapomixis: (i) the modification of an establishedmodel plant species, and (ii) the experimentalmaniplliation of a known apomict.The first approach has the advantage ofimmediate access to extensive records on thegenetics, biochemistry, and developmentalbiology of the system. Experimental methodshave been devised, genotype colle(tion~ art'available, DNAlibraries, probes, map markers,and sequenced genomic fragments arcaccessible, and existing laboratory collaborationsare present for cross-referencingfindings. Conversely, existing model plantsystems have been chosen for reasons otherthan apomixis. This approach, therefore,dictates targeting the mechanism ofapomixis,depending on the nature of the availablemutations. It is also based on the assumptionthat apomixis is a derivative of sexualityand / or tha t it can be derived artificially fromsexuality. While this currently appearsintuitive, it should be noted that thisassumption remains unproven.In contrast, the use of a naturally apomicticspecies enables the selection of both theapomictic mechanism for its applicability andthe plant type for its amenability. The modelplant systems currently used to studydevelopmental biology, however, are obligatesexuals. Consequently, this approach requiresthe establishment of experimental methods forthe selected species, the collection andcharacterization of critical genotypes and otherexperimental tools, the maintenance of thosecollections, and the establishment of newcollaborative networks.Modification of an Existing SystemSeveral genera have been used as models tostudy plant reproductive biology includingAlltirrlzillllnl, Arnbidopsis, Capsella, Dauctts,Hordellm, Lentna, Lo/illlll, Lycopersicum,Nicotwna, Perilla, Petllnia, Pharbitis, Sinapis,Xantlzilllll, and Zea. Recently, with an increasedemphasis on molecular genetics, the mostimportant model system has become thecruciferous species Arabidopsis tlwliana, withless emphasis on the solanaceous generaLycopersiclInI, Nicotiana, and Petllllia, and themonocotyledonous genus Lea. These plantsreprod uce through seed by obligate sexuality.Two approdches have been taken to useobligate sexual species as models for apomixisresearch.