Chapter 5 Genetic Analysis of Apomixis - cimmyt

Iroodlog .f Apoonictk Specie. 147species rely on large-scale hybridizationbetween sexuals (facultative apomicts) andobligate apomicts to produce largepopulations, from which superior apomictichybrids are isolated (e.g., Burton and Forbes1960; Taliaferro and Bashaw 1966; Gobbe etal. 1983; Bashaw and Funk 1987; Savidan etal. 1989). Three-way or double crossesinvolving more than one apomictic male maybe required, depending on the distribu'tion ofdesired attributes among available apomicticgenotypes. Theseschemes will require carefulselection of parents and the evaluation of largepopulations to find the desired combinationof characteristics in a true breeding apomicticgenotype, Such approaches offer theopportunity to generate novel apomicticgenotypes, however, they are essentiallyconservative in the longer term because theopportunity for genetic gain is eventuallyexhausted,To continue genetic advances, a systematicscheme for recycling selected hybridgenotypes will be required (i.e., populationimprovement by recurrent selection). AnyBrachiaria breeding population must obviouslyinclude sexual genotypes to ensure geneticrecombination. An importantconsideration inthe development of populations is whether toattempt to include and maintain apomicticgenotypes in the populations. Several authorshave suggested improvement of a sexualpopulation (e.g., Pernes et al. 1975; Miles andEscandon 1997). The scheme for sexualpopulation improvement proposed by Pemeset al. involves recurrent crossing to eliteapomicts. Therefore, the superior sexualhybrid genotypes in each crossing cycle n~edto be identified to resynthesize a fully sexualpool.Miles and Escandon (1997) proposed recurrentintrapopulation improvement of aheterogeneous sexual population developedfrom sexual segregants selected from progenyof an initial series of crosses of a sexualtetraploid biotype with apomictic genotypes.In the case of Brachiaria (and otherspecies witha similar genetic control of reproductivemode), the second scheme would have theimportant advantage ofobviating the need todetermine reproductive mode in eachgeneration. As the frequency of favorablealleles is increased in the sexual population,hybridization with elite apomictic genotypeswill generate an array of improved apomicticand sexual segregants while the sexualpopulation remains fully sexual, i.e., advancedfrom purelysexualclones selected from withinthe population. Superior apomicts in thehybrid populations would be candidates forcultivar release. They could also be used in thesubsequentcycle ofsexual x apomictic crosses,although it would not be expected thatcrossing back to the parental sexual poolwould lead to maximum expression ofheterosis. A sexual population based onselected first cycle sexual x apomictic hybridsis being developed at CIAT. Thirty-two suchhybrids, involving a total of ten apomicticpaternal parents crossed to the same sexualtetraploid B. ruziziensis, were initially selected.The selection was subsequently reduced to 30,when it was determined that one clone wasvery susceptible to a virus and anotherexhibited a low, but consistent, percentage ofapomictic embryo sacs. Each clone wasvegetatively propagated ten times andgenotypes were planted in random spacing inan isolated field plot. This population will bemanaged bystandard halfsib or mass selection(Figure 10.4).A second population containing both sexualand apomictic genotypes has been formedfrom sexual x apomictic hybrids (Figure 10.5).In each generation, two types of progenies areplanted in alternating positions in a squaregrid: (i) apomictic progenies of selectedapomicts (reproductive mode determined by