Chapter 5 Genetic Analysis of Apomixis - cimmyt

Chapter 12From Sexuality to Apomixis:Molecular and Genetic ApproachesUELI GROSSNIKlAUSIntroductionSexual reproduction usually producesgenetically diverse progeny, a feature that hasbeen exploited in the selection andimprovement of agricultural crops over thecenturies. In contrast, apomixis results in theproduction of genetically uniform progeny.Apomictic embryos are derived from anunreduced cell lineage and developindependent of fertilization (Gustafsson 1947;Nogler 1984a, b; Asker and Jerling 1992;Koltunow 1993). Thus, apomictic seeds aregenotypically identical to the mother plantand usually form a genetically stable clone.The clonal nature of apomictic offspring bearstremendous potential for seed production andcrop improvement. It offers the possibility forthe immediate fixation of any desiredgenotype and its indefinite clonalpropagation. The transfer of apomixis tosexual crops will completely transformcurrent breeding strategies and seedproduction (Hanna and Bashaw 1987; Savidan1992; Savidan and Dujardin 1992; Dickinson1992; Jefferson 1993; Hanna 1995; Koltunowet al. 1996; VielJe-Calzada et al. 1996; Jeffersonand Bicknell 1996; Grossnik.laus et al. 1998a;Savidan 2000). The resulting agricultural,commercial, and social benefits for bothindustrialized and developing countrieswould be enormous (Jefferson 1994;Koltunow et al. 1995; Grossniklaus et al. 1998c;and Toenniessen, Chap. 1).Apomixis is an asexual form of reproductionthrough seeds and occurs in more than 400species (Bashaw and Hanna 1990; Asker andJerling 1992;Carman 1995, 1997). Having beendescribed in plants belonging to almost 40different families, it is thought to have evolvedindependently in several taxa from sexualancestors. Apomixis can be viewed as adevelopmental variation of the sexualreproductive pathway in which certain stepsare short-circuited (Koltunow 1993; VielleCalzada et al. 1996; Grossniklaus et al. 1998a).Thus, apomictic and sexual reproduction areclosely related to one another and share manyregulatory components. It is very likely thatthe genes controlling apomixis also play crucialregulatory roles during sexual development.Therefore, the engineering of apomixis willrequire a better understanding of the geneticbasis and molecular mechanisms that controlsexual plant reproduction. Whereasmegasporogenesis and megagametogenesishave been studied extensively at themorphological and ultrastructural levels (e.g.,Cass and Jensen 1970; Willemse and van Went1984; Russell 1985; Mogensen 1988; Huang andRussell 1992; Schneitz et al. 1995; Christensenet al. 1997), the molecular and genetic basiscontrolling these key steps in sexualreproduction are almost entirely unknown.The engineering of apomixis in sexual cropswill only be possible through aninterdisciplinary and multifaceted approach tostudying the regulation of reproduction at thegenetic and molecular levels in both sexual and