Chapter 5 Genetic Analysis of Apomixis - cimmyt

112 a... A. 8icUeI1993). Before the trait can be successfullycommercialized, more information isrequired about the impact of environmentalvariables on the expression of apomixis andtheir interaction with sexuality, particularlywith regard to fertility and resourceallocation. Ultrastructural studies are alsorequired to more clearly elucidate thecytological and histological events involvedin apomixis, such as the role of differentialcallose deposition (Carman et al. 1991;Leblanc et al. 1995a; Peel et al. 1997).Attributes of A Model SystemThe innate advantages and limitations of anysystem will dictate the available researchopportunities and impact on the rate ofpossible progress. Therefore, beforediscussing candidates, it is helpful to considerthe features that would facilitate their use,specifically in a study of the developmentalbiology and molecular genetics of apomixis.Biological AttributesTo facilitate experimental progress, there area number of cultural characteristics toconsider in choosing a model system. Forsimplicity, the plant should be easy tocultivate, both in vivo and in vitro. Ideally itshould also be a perennial that can be easilypropagated vegetatively to permit themaintenance of sterile or self-incompatiblesexual biotypes. A small, compact plantstature, which does not require training, willfacilitate the manipulation of largepopulations, such as those used duringmutant screens and inheritance studies. Forthe rapid turnover of experimentalpopulations it is convenient to use a specieswith a short generation time, abundant seedset, and adequate seed fertility. This isparticularly important for studying apomixisbecause plants are assessed at the time offlowering or seed formation. For theevaluation of apomixis, it is advantageous touse a species in which apomixis can be easilyassessed, preferably in a format that can bequantified, to facilitate the study of allelicdifferences and epistasis.The inheritance of apomixis is typicallyassessed by crossing sexual and apomicticbiotypes, often using the sexual plant as thematernal parent. Although it is usually alsopossible, and sometimes necessary, to performthe reciprocal cross, it requires the separationof recombinant (B IIand Bmhybrids) from non­recombinant (maternal and polyhaploid)progeny. Inheritance studies, therefore, requirethat cross-compatible sexual and apomicticbiotypes are available, and if a sexual recipientis used, that microsporogenesis andmicrogametogenesis are functional in theapomictic biotypes used. Sexual biotypes arealso useful in molecular studies for assessingthe developmental importance of reintroduced,putative control sequences.Types of ApomixisTwo prinCipal mechanisms of apomixis havebeen reported: (i) adventitious embryony, inwhich the maternal embryo arises directlyfrom a somatic cell, and (ii) gametophyticapomixis, in which the maternal embryo isderived from an egg cell within an unreducedembryo sac. Gametophytic apomixis is furtherdivided into diplosporous and aposporousmechanisms, depending on whether theunred~ced embryo sac arises from amodification of the meiotic apparatus or froma separate cell, respectively. Intermediatesbetween the two forms of gametophyticapomixis have been reported (Gustafsson1946), indicating that they possibly representmodifications of a single developmentalmechanism. For more detailed descriptions ofmechanisms, see Nogler (1984a) andKol tunow (1993). Most current research effortson native apomixis focus on gametophyticmechanisms, including studies of bothaposporous and diplosporous species.