Chapter 5 Genetic Analysis of Apomixis - cimmyt

Chapter 10Breeding of Apomictic SpeciesCACILDA BORGES DO VALLE AND JOHN W. MILESIntroductionFrom a plant breeding perspective, apomixismay restrict genetic recombination, but it alsoprovides a unique mechanism for developingsuperior cultivars and preserving thesegenotypes indefinitely. Apomictic plants, likesexual plants, develop seed in the ovule of theflower, but egg and sperm nuclei do not fuseto form an embryo. Therefore, the embryo ofan apomictic plant receives all of itschromosomes from the mother plant. Unlikemost asexually propagated plants-such asbanana, potato, or horticultural crops that arepropagated from vegetative parts of the motherplant-an apomictic plant is propagatedthrough the very convenient vehicle of seed.Early investigators (e.g., Darlington 1939) wereled to believe that apomixis was anevolutionary "blind alley" due to the apparentlack of variation in natural apomicticpopulations. Indeed, obligate apomixis posesa formidable barrier to plant breeding: withoutthe new gene combinations that result fromsexual cross breeding, genetic improvementcannot occur, except by rare, random, andgenerally deleterious mutations. In truth,sexual or partially sexual plants have beenfound in native populations of most apomicticspecies, generating sufficient genetic variationto maintain the species under changingenvironments and providing germplasm forplant improvement.Aside from citrus fruits (which exhibitapomictic reproduction through seed, but aregenerally propagated vegetatively), only a fewforage and turf grasses have active apomicticbreeding programs; these include species ofEragrostis, Paspalum, Poa, Panicum, Pel1niselum,Cel1chrus, and Brachiaria. These species (oragamic complexes) have many commonattributes, which will be addressed in a generalmanner la ter in this chapter. Bashawand Funk(1987) reviewed many aspects of breedingapomictic forage grasses, and recent papershave specifically considered plant breeding inthe genus Paspah,m (Savidan 1987; Burton1992); in Cenchrus ciHaris (Bashaw and Funk1987); in Panicum maximum (Savidan et al.1989); in Pennisetum (Hanna et a1. 1992); andmost recently in Brachiaria (Miles and Valle1996). In this chapter we focus on the Brachiariabreeding programs in Brazil and Colombia toillustrate pertinent aspects of apomixis vis avis breeding programs.Prerequisites for an EffectiveForage Breeding ProgramBeef production, especially in the tropics,largely depends on pastures, either native orplanted to superior introduced species.Scientific research on forages is relativelyrecent compared to field crops. Experiencewith tropical forages is even more limited; thefew commercially available cultivars are littlemore than "side of the road" collections, whichwere often accidentally introduced, mostlyfrom Africa, evaluated in small plots, and thenmultiplied for release.