Chapter 5 Genetic Analysis of Apomixis - cimmyt

Chapter 5Genetic Analysis of ApomixisROBERT T. SHERWOODIntroductionGeneticists interested in analyzing theinheritance of apomixis face challengingproblems. Apomixis overrides certainprocesses essential to the analysis ofinheritance, i.e., it usurps meiotic megasporogenesisand megagametogenesis. Obligatelyapomictic plants cannot serve as maternalparents in hybridization. However, sinceapomixis normally does not prevent meioticpollen formation, apomicts can be used asmale parents in crosses with sexual orfacultatively apomictic female parents. Whenfacultative apomicts function as maternalparents, three types ofprogeny may be formed(see Berthaud, Chap.2), and each type mustbe distinguished when testing genetic models,i.e., normal B IIhybrids from fertilization ofreduced embryo sacs, apomictic progeny fromparthenogenetic embryogenesis of umed ucedeggs, and B m progeny from fertilization ofunreduced eggs by reduced pollen yieldingnonmaternal types at an increased ploidylevel. Sexual reproduction is unknown insome apomictic species, accordingly, sexualplants must be found or created before geneticmanipulation is possible. Sexual members ofagamic complexes usually are at a differentploidy level than the apomicts. The polyploid,highly heterozygous nature of most apomicticplants complicates genetic analysis (Stebbins1950; Nogler 1984a).Early reports indicated that apomixis isheritable, but did not point to specific genesor genetic systems (Gustaffson 1946--47; Nogler1984a). Successful hybridization was difficult,and methods for classifying progeny weretedious and unreliable. Discovery of thecytologically distinctive Panicum-type ofapospory in the 1950s fostered the beginningofcreditable inheritance studies. Pistil clearingtechniques were introduced in the 1970s (seeCrane, Chap. 3) that permitted the classificationof large numbers of progeny. More recently, theapplication of molecular technology tocharacterizing, locating, and isolating apomixissequences has augmented our understandingof the regulation of apomixis (see Grimanelliet aI., Chap. 6). Presently, it appears that theexpression of aposporous apomixis requires adominantly acting master gene or linkage unit;roles have been indicated for dosage, additivity,recessive lethality, and modifying genes. Thelimited data available for diplosporous taxaindicate that diplospory also may be regulatedby a dominant linkat with modifiers. Applieddiligently, the methods suited to hybridizationand classification of apomictic plants describedbelow can lead eventually to a resolution ofthese difficult problems.MethodsThis section discusses methods for selectingparents, characterizing parents and progeny,and making crosses of apomictic species.Chapters 2, 6, 9, and 10 should also beconsulted. Chromosomal constitution,reproductive behavior, and phenotype ofbothparents must be completely known.