Chapter 5 Genetic Analysis of Apomixis - cimmyt

Su...I.. P........ to ldeollly.od ""'.liIy Apollllxls133required to indicate apomixis and determinethe nature of the hybrids detected. RFLPs andRAPDs can also be used in the same way, butat greater expense.2. Degree of apomixis expression. Manyoffspring are needed to obtain a good estimateof the degree of apomixis. Both auxin tests andflow cytometric analyses of pollinated ovariesor seeds provide good estimates of sexualpotential, though distinguishing 2n + 0 fromn + n offspring might be difficult in certaincases. In contrast, systematic chromosomecounting within progenies is useful fordetecting 2n + nand n + 0 off-types, but it doesnot separate 2n + 0 from n + n offspring, andwithout flow cytometry it becomestremendously time consuming. Progeny testscombining cytology and marker analysesrepresent the best option for identifying thedifferent classes of offspring within apomicticprogenies. To limit cytology work (when flowcytometry is not available), markers can beapplied first to separate maternal offspringfrom (poly)haploids or hybrids. The origin ofthe latter may be determined according to thepatterns they produce (i.e., 2n + n off-typesmust carry all bands from the mother plant,plus extra bands from the pollen), and thencytologically confirmed.Choosing a ProcedureThere are four main areas of apomixis research,each with distinct constraints and objectives:(i) the search for apomixis or elements ofapomixis in new taxa, coupled with geneticstudies in wild populations, (ii) germplasmcharacterization of apomictic species,(iii) genetic and biological studies for furthermanipulation of apomixis, and (iv) breedingof apomicts and introduction ofapomixis intosexual crops.Since gametophytic apomixis is formidablylimited to perennial, polyploid, andoutcrossing species, the search for apomixis inadditional species should begin with taxapresenting these traits. The very first screeningcan be based on the expression of the alreadymentioned "indicators of apomixis," whilemore discriminative procedures may beapplied to promising specimens. Forgermplasmevaluation, a representative sampleof the collection must be chosen on the basis ofmorphological and cytological data, and traitsof agronomic value such as disease resistance.Chromosome number, repro-d uctivedevelopment, and degree of apomixis are theprimary [actors for which basic data must becollected to develop strategies for furtherresearch. Genetic studies also may beattempted to genetically dissect apomicticmechanisms (number of genes involved andtheir effects). Following this preliminary work,appropriate tools for larger-scale screeningshould be developed or chosen according tothe apomixis characteristics of the collection(e.g., callose patterns for diplospory, ES clearingfor apospory of the Panicum-type, etc.).Sexual parents involved in crosses for apomixisinheritance studies must be carefully chosenusing cytoembryology. Highly facultativeapomicts are easily misclassified as sexualsusing progeny tests. This causes distortions ofsegregation ratios for mode of reproductionamong progeny. In the same way, looking fordifferences between sexual and apomicticdevelopment at the molecular level requires theanalysis of genotypes that are wellcharacterized for mode of reproduction. Thismay allow the development of near isogeniclines, an important step in identifying thegene(s) controlling apomixis.Before apomixis can be transferred into cropsor used in breeding programs, researchers needprocedures to identify apomictic genotypes(see do Valle and Miles, Chap. 10; Savidan,Chap. 11) and to quantify apomixis ingenotypesselected for varietal release. Progeny