Chapter 5 Genetic Analysis of Apomixis - cimmyt

Breodiog .f Apomidk 5,...., 139facultative apomicts are found (Harlan et al.1964; Voigt and Bashaw 1972; Bashaw 1980;Savidan 1983; Hanna and Bashaw 1987).Differences in ploidy level are common amongsexual and apomictic species of tropicalgrasses (Burton and Forbes 1960; Carnahanand Hill 1961; Dujardin and Hanna 1983;Norrmann et al. 1989). However, in groups inwhich apomixis is found, diploid accessionsare generally obligatory sexual whilepolyploids display different degrees ofapomixis ranging from essentially sexual toobligate apomicts (de Wet and Harlan 1970;Quarin and Norrmann 1987; Valle et al. 1989;Valle 1990; Asker and Jerling 1992). In specieswith higher ploidy levels (6x or 7x), such as B.humidicola, sexuality may be found at thetetraploid level (Valle and Glienke 1991).Sexually reproducing genotypes in the tropicalforage grasses outcross and are highlyheterozygous (Bashaw and Funk 1987). Somedegree of self-incompatibility or stronginbreeding depression is common (Bashawand Funk 1987). Rates of self-fertility in sexualB. ntziziensis were not affected by chromosomedoubling and ranged from 7.2 to 8.4%,according to Lutts et a!. (1991). Whenhybridization with apomicts has beenpossible, resulting progenies are highlyvariable owing to segregation in theheterozygous parents.Since hybridization and production of fertileprogeny are more effective when progenitorsare at the same ploidy level, basic studiesleading to the determination of chromosomenumber should be undertaken early in theprogram to enhance the chances of successfulrecombination of attributes by conventionalcrossing.A third prerequisite for efficient breeding ofapomicts, as in any plant improvementprogram, is the establishment of clear,achievable objectives, and the identification ofsources of the desired attributes in the existinggermplasm. This presupposes intimateknowledge of the species of interest, in orderto identify limiting factors not readilyovercome by simple selection of superiorgenotypes from the available germplasm oramenable to improved cultural practices. Oncea constraint has been identified (e.g., diseasesusceptibility or low forage quality), thenatural germplasm needs to be screened toidentify candidates for hybridization. Ideally,the desired attribute(s) can be found inapomictic or cross-compatible sexualaccessions with a superior agronomicbackground.General Structure of aBreeding ProgramA general selection and breeding scheme forapomictic forage species is presented in Figure10.1. Note that Brachiaria serves as the examplefor the topics under discussion.Brachiaria is native to the tropical savannas ofAfrica (IBPGR 1984), encompassing about 90species with wide morphological andphenological differences (Clayton andRenvoize 1982; Ren voize et a!. 1996).Apomictic cultivars of some of these species,cylogenelicsMode of reproduction1-----+_ Genetic markers[Morphological characterizationetc.•I.. ...SEXAPO.. ...APO SEX 5 APOIIRelease of ...new cultivarsFigure 10.1 Selection and breeding scheme forapomictic forage species.