Chapter 5 Genetic Analysis of Apomixis - cimmyt

Chapter 13Induction of Apomixis inSexual Plants by MutagenesisUTA PRAEKELT AND ROD SCOTTIntroductionThe ability to manipulate reproduction in cropplants from the sexual to the apomictic mode,and vice versa, is highly desirable (Hanna1995; Jefferson and Bicknell 1996). Severalchapters in this volume deal with the variousapproaches that have been taken toward thisgoal. The most promising sh'ategy so far hasbeen the transfer of natural apomixis genesfrom wild species into related sexual cropplants by introgression (Hanna et al. 1992;Leblanc et al. 1995; Savidan, Chap. 11).Unfortunately, this is likely to remain limitedto those crops that have apomictic relatives,and therefore will not be widely applicable.In light of this situation, efforts are being madeto identify the gene(s) that confer apomixis,both to gain a better understanding of thegenetic regulation of the trait and to eventuallyfacilitate transfer to a wider range of speciesby genetic engineering methods. As describedby Grimanelli et a!. (Chap. 6), one key steptoward gene isolation is the genetic mappingof apomixis genes, and considerable progresshas been made with some species (Kindigeret al. 1996). However, because of the intrinsicdifficulties in mapping apomicts, and becauserecombination around the apomixis locusappears reduced (Grimanelli et a!., Chap. 6),map-based cloning of the apomixis gene(s) islikely to proceed slowly.Recently, an alternative approach, usingmutagenesis, has been considered by severalgroups both for the identification of naturalapomixis genes and for the de novo inductionof apomixis in sexual plants (Koltunow et al.1995). Mutagenesis has been widely andsuccessfully applied to the study of manyaspects of plant growth and development.Thanks to rapidly advancing methods in allilTeas of DNA technology, improvements inplant transformation methods, and theaccumulation of mapping and sequencingdata, the isolation of genes via their mutantalleles has become a feasible approach in manyareas of plant research. Bicknell (Chap. 8)describes the development of Hieracium as amodel apomict (see also Bicknell 1994a,b,c;Bicknell and Borst 1994). The aim is to inducereversion tosexual reproduction by insertionalmutation of the responsible gene(s) and toisolate them via the inserted sequence.In this chapter we describe the reciprocalapproach, the mutagenesis of a model sexualplant in an attempt to induce apomixisde novo.Mutated alleles conferring apomixis orindivid ua1 components of apomicticdevelopment can be identified with relativeease in a model plant, and the cloned geneswould then be available for transfer to otherspecies. The possibility of a mutagenicapproach resulting in the isolation of thedesired mutants greatly depends on themethodology employed. Therefore, in order tomaximize the opportunities for the inductionand detection of these mutants, severalconsiderations must be taken into account.Details of the mechanisms of apomixis have