Chapter 5 Genetic Analysis of Apomixis - cimmyt

144 (..iIda 80rges cit Va" DOd Ja~. W. Mieskept in vases of water. On the day ofpollination, inflorescences were shaken overpetri dishes to collect pollen, which was usedon flowers from which stigmas had justextruded. The inflorescence from the sexualplant was prepared by removing unopenedand old flowers. After brushing the turgidstigmata with pollen from the apomicticparent, the racemes of the sexual plant wereindividually bagged and labeled. Bags werecollected when seed shattering started.Scarified seeds were individually germinated4-6 months later in styrofoam trays with asand:perlite mixture (2:1) or in petri dishes,and then transferred to plastic bags with soil,from which they were later transferred to thefield (Valle et al. 1991).Mode of reproduction was determined byembryo-sac analysis on 30--40 ovaries of 376individual first-generation hybrids fromgreenhouse crosses in Brazil. No reliablegenetic marker yet exists to determine hybridnature of the progeny, therefore attempts todiscriminate among individuals were madeusing morphological characteristics and/orelectrophoresis. Whenever parental materialsdisplay wide differences in morphology or inband patterns, hybrids showing intermediatecharacteristics can be readily identified.2n sexuali" 't4n apomictic 4n sexual 5 4n rsame orFl~ictiC1olhersf------4n apomictic 4n sexual ...F2.--L----.agronomicevaluationsFigure 10.3 Hybridization scheme for breedingBrachiaria (adapted from Gobbe et 01. 1983).Studies conducted at CIAT identified an alphabetaesterase system capable of discriminatingamong putative hybrids of carefully selectedprogenitors (Cruz et al. 1989a, 1989b; Calderonand Agudelo 1990).Second generation crosses in Brazil includedsexual x apomictic backcrosses, crossesbetween half sibs, full sibs, selfing of sexualF\s, 3-way hybrids, and facultative apomicticx apomictic crosses. Results from thisexperiment (Table 10.3) point to a singledominant gene determining apomixis, asproposed for Panicum maximum (Savidan1983), for Brachiaria (Ndikumana 1985), andCenchrus (Sherwood et al. 1994). The excessnumber of sexual plants observed in somecrosses may be due to crossing procedures;sexual maternal plants were not emasculated,and no special precautions were taken to avoidpollen circulation in the greenhouse, exceptafter pollination when flowers were bagged.Table 10.3 Segregation for mode of reproduction inBrachiaria hybridsType uoss SEX APO STER AbnF 1B.ruliliensis x B. decumbens 79 49B.ruliliensis x B. brilon/ho \25 123F 2B.ruliliensis x B.decumbens 2 0 2 0B.rulilief/Sis x B. brilon/ho 7 0 0 0BCB.ruliliensis x B.decumbens 10 9 0 0B.ruliliensis x B. brilan/ha 9 7 0 03-WB.ruliliensis x B.decumbens 24 21 9 1B.ruliliensis x B. brilon/ho 31 6 0 0FSB.ruliliensis x B.decumbens 38 27 4 0B.ruliliensis x B. brilantha 24 32 3 1HSB.ruliliensis x B.decumbens 5\ 53 6 8B.ruliliensis x B. brilon/ha 61 23 6 0BC: oockcrass, 3-W: three-way hybrids, FS: full-sibs, HS: holf·sibs.