Chapter 5 Genetic Analysis of Apomixis - cimmyt

Fro.. Sexoalny to Aponixls: MoIeatIo, .d G...tl< A,p,....s 199have been conducted in angiosperms and themutants described so far have been identifiedfortuitously. Well-defined sexual modelsystems are best suited for such screens. Ratherthan performing large-scale genetic screens forreproductive mutants in many different cropplants, it may be easier to isolate the relevantgenes from Arabidapsis (or maize) and mimicthe mutant phenotype in crops through geneticengineering.De Novo Engineering ofApomixis through BiotechnologyAs outlined above, the introduction ofapomixis into a wide variety of sexual cropswill be most efficiently achieved throughgenetic engineering. To maximize itsusefulness and versatility in a bioengineeredform apomixis will have to be dominant;otherwise the fixation of hybrid genotypes willbe very slow. The engineering of apomixisthrough biotechnology will require a concertedeffort in three main areas: (i) the identificationand characterization of candidate genes thatcan be used to manipulate the reproductivesystem; (ii) the isolation of promoters that allowa precise control of gene expression at thespatial and temporal levels; and (iii) thedevelopment of efficient technologies tointroduce and control transgenes and/or toperform targeted mutagenesis of endogenousgenes.The identification of regulatory genes that canbe used to control apomixis is being pursuedby a variety of approaches using bothapomictic and sexual systems. Insertionalmutagenesis in Hieracium and Tripsacum(Bicknell, Chap. 8; Grimanelli et aI., Chap. 6)or positional cloning based on mappingapproaches and comparative genomics willlead ~9 .the identification of the componentscontrolling apomixis in natural apomicts. Insexual model systems, the characterization andmolecular isolation of existing reproductivemutants that show individual components ofapomixis is underway and will provide us withnovel tools to manipulate sexuality towardsapomIXIS. These efforts are beingcomplemented by new screens that speCificallytarget relevant aspects of reproduction insexual species. Several laboratories are alsotrying to isolate plant homologs of yeastmutants that could play crucial roles indetermining the meiotic lineage andnonreduction. It must be emphasized that ourunderstanding of the molecular mechanismsthat control plant reproduction are stillextremely limited.In addition to regulators of the sexual pathway,many other genes may be useful for theengineering of apomixis. Such genes includeglucanases that degrade callose, the absenceof which serves as a consistent indicator ofcellsinitiating megagametogenesis. Genespromoting cell wall formation could also beuseful to prevent fertilization of the egg celland promote parthenogenesis. Anotherimportant class is made up ofgenes that controlthe cell cycle. Heterochronic or heterotopicexpression ofsuch regulators could potentiallybe used to trigger cell division and initiatedevelopmental events such asmegagametogenesis and embryogenesis.Recent studies in Arabidopsis have shown thatmisexpression of eye/inlAt in root cells cantrigger extra rounds of cell division (Doerneret a!. 1996) and that eye/inD controls the growthrate irftobacco (Cockcroft et a!. 2000). It will beinteresting to see whether similar experimentscan induce cell proliferation in the egg cell.Other growth regulators, such as planthormones, have not been studied in detailduring sexual reproduction, but may playcrucial roles in initiating developmentalprograms relevant to apomixis.Targeted misexpression ofcandidate genes willrequire well-defined regulatory sequences thatcan be used to drive transgene expression. Todate, only a few promoters have been