Chapter 5 Genetic Analysis of Apomixis - cimmyt

230 1\omas Dresselhaus, Joh. G. (IJrmall, aid YVeJ SavidaftAll these advantages taken togetherundoubtedly would lead to large increases inagricultural production and prompted VielIe­Calzadaet al. (1996a) to coin the term"AsexualRevolution" to describe the potential impactof the technology.The possible economic benefits of thetechnology are also considerable. In rice,added productivity would total more thanUS$2.5 billion per year (McMeniman andLubulwa 1997). It is prOjected that the heterosiseffect alone would result in yield increases ofmore than 30% (Yuan 1993; Toennissen,Chap. 1). Of today's US$15 billion globalmarket in commercial seed, hybrid seedaccounts for 40% of sales (Rabobank 1994), afurther indication of the enormous economicpotential of apomixis for agriculturalenterprises.Unfortunately, scientific and economicpotential shed little light on the actualintricacies of how the genes involved inapomictic reproduction work. Many haveconcluded that the genes that control apomixisare also crucial for sexual development,indicating that apomixis is a short-circuitedsexual pathway (Koltunow et al. 1995;Grossniklaus, Chap. 12). The geneticengineering of apomixis, therefore, requires abetter understanding of both apomictic andsexual pathways of reproduction.In general, apomixis is thought to occur inpolyploid species (Asker and Jerling 1992),especially in the Rosaceae, Asteraceae, and inthe Poaceae (for review see Berthaud, Chap. 2).For most species in which apomixis has beendescribed, diplOids reproduce sexually, whilepolyploids of the same species are apomictic.Most natural apomicts reproduce throughfacultative apomixis (Asker and Jerling 1992;Berthaud, Chap. 2). The degree of apomicticreproduction is influenced by the geneticbackground, ploidy level, modifier genes, andthe environment. There is also a great diversityof apomictic behavior: nine types ofgametophytic apomixis have been describedin addition to sporophytic apomixis(adventitious embryony) (Crane, Chap. 3).Unfortunately, apomixis is not found in themost important cultivated crops, which couldbe a resultofcrop domestication, selection, andsegregation analysis (Grossniklaus, Chap. 12).There are three main options for theengineering of apomixis into sexual crops:(i) transfer the trait into crops from wild,naturally apomictic relatives throughnumerous backcrossings, (ii) screen sexualcrops for apomictic mutants, and (iii) de novosynthesize the apomictic trait directly intocrops. These approaches will be discussed inthe following pages.Transfer of the Apomixis Traitto Sexual CropsBreeding and Introgressionfrom Wild RelativesGenerally, breeding apomictic species is verydifficult, consequently, there have been only afew breeding programs, and these focused ona very limited number of tropical grass species.The basic structure of such breeding programsis described in this book, using Brachiaria asan example, an important forage grass in SouthAmerica, (do Valle and Miles, Chap. 10).Obligate apomicts cannot serve as maternalplants and breeding of such species is thereforeimpossible. The polyploid and highlyheterozygous nature of most apomictic plantsfurther complicates genetic analysis. Inaddition, controlled pollination is needed toanalyze reproductive behavior (methods aredescribed by Sherwood, Chap. 5). Additionaltechniques are needed to monitor reproductionbehavior in progeny plants of new varieties.Such techniques are described in this book byBerthaud (Chap. 2), Crane (Chap. 3), andLeblanc and Mazzucato (Chap. 9). Thetechniques described include chromosomecounting, flow cytometry, clearing and