Chapter 5 Genetic Analysis of Apomixis - cimmyt

G...Ii, Elgil..';"!!o/ Apomlxl. i. 5....1Cropo: ACr~iuII A.....me.t of the Apanixl. w ..1ogy 231squashing techniques, sectioning, molecularmarkers, and the "auxin test." Ultrastructuralstudies using electron microscopy (Naumovaand Vielle-Calzada, Chap. 4) reveal even moreinformation, but are very laborious, timeconsuming,and poorly suited to large-scaleprogeny analysis. Flow cytometry analysis ofseeds is a fast and easy tool and thus probablythe method of choice for first progeny testings.This is because large numbers of progenypopulations have to be produced andinvestigated at each generation in order toanalyze reproductive behavior (Matzk et al.2000; Savidan, Chap. 11).Several sexual crop plants are closely relatedto wild apomicts, and introgression of theapomixis trait through wide crosses hassuccessfully been performed with wheat,maize, and pearl millet (reviewed by Bicknell,Chap. 8; Savidan, Chap. 11). Nevertheless,there are some limitations: total male sterilitywas observed frequently in F jhybrids of widecrosses, representing a dead end once theapomixis trait is obligate. In wide crossesbetween Tripsacum and maize, fertile apomicticBC 4with less than 11 Tripsacum chromosomescould not be identified (Savidan, Chap. 11),resulting in maize lines devoid of agronomicvalue. Another disadvantage of this approachis that transfer of natural apomixis genes fromwild species into related sexual crops byintrogression is likely to remain limited to thosecrops that have apomictic relatives and so willnot be applicable to other species.Mutagenesis ApproachesMutagenesis approaches have been describedin great detail earlier in this book byGrossniklaus (Chap. 12) and Praekelt and Scott(Chap. 13). Therefore, we will discuss only themain conclusions here.The basis for all mutagenesis approaches is theassumption that apomictic reproductionpathways are developmental variations of thesexual pathway, thus a short-circuited sexualpathway. Mutant screens have therefore beendesigned to induce sexuality in apomicts andapomictic mutants in sexual plants by theinactivation of genes. Many mutants wereidentified as being defective in meiosis,megasporogenesis, and gametogenesis (forreview, see Yang and Sundaresan 2000;Grossnikiaus, Chap. 12). Mutant analysis ofmegagametogenesis, for example, suggeststhat a large number of loci are essential forembryo-sac development. Other mutants aredescribed as displaying autonomous embryoand/ or endosperm development. Thecorresponding genes have been recentlycloned. Mealfisl (medea/jertilization independentseed 1) is a gametophyte maternal effect geneprobably involved in regulating cellproliferation in the endosperm and alsopartially in the embryo (Grossniklaus et al.1998b; Luo et al. 1999). Fis2 shows a similarmutant phenotype and encodes a putativezinc-finger transcription factor (Luo et al. 1999).Autonomous endosperm development wasobserved in the fie (fertilization independentendospermlfis3) mutant. Mealfisl and fie/fis3display homology to Polycomb proteins(Grossnikiaus et al. 1998b; Ohad et al. 1999),which are involved in long-term repression ofhomeotic genes in Drosophila and mammalianembryo development (pirrotta 1998).The most important conclusion derived fromthe des~ription of these mutants is that all theelements of apomixis can indeed be inducedby mutations in sexual plants. In addition, it isobvious that more than one mutation will benecessary to obtain vital apomictic seeds insexual crops. Nevertheless, a combination ofsuch isolated genes could be used for knowngene approaches, but additional genes will beneeded to obtain fully developed seeds. Untilnow, most mutagenesis screens haveconcentrated on the partial or completeinactivation of the genes that are needed for