Chapter 5 Genetic Analysis of Apomixis - cimmyt

156 rn. s.mdaosearch for an opitimum apomictic donor forpearl millet has been conducted. Three speciesstudied by Dujardin and Hanna (1989) thatshow crossability with the crop are Pennisetumorientale, a tetraploid with 211 = 36; P. setaceum,a triploid with 211 = 27; and P. squamulatum, ahexaploid with 211 = 54. They all reproduceapomictically and their apomixis wasdescribed as obligate, which means that 100%of the observed progeny appeared to bematernal in field tests (Dujardin and Hanna1984a, b).Advantages of P. squamulatum as a donorspecies for apomixis include good pollenfertility, 4-nucleate embryo sacs, and a uniqueIssue # 1. Obligate vs. Facultative Apomixis: An Artifact?The facultativeness of apomixis has beenconsidered to be a disadvantage (Bashaw etal. 1970; Bashaw 1975) because (I) it may resultin W1controlled variation in the progeny whilefarmers require homogeneous varieties and(ii) it i apparently quantitatively inherited,i.e., under a c mplex, yet unknown geneticDiffere~ces in timing of developmentbetween meiotic and apomeioticembryosacsshould be considered in order to provide anaccurate estimate of the degree offacultativene s. This difference has beenfound in 'everal aposporous species asidefr m Pallicl/m, e.g., RanllnclIllIs al/riCOlllllscontrol. evertheless, acultativeness may be (Nogler 1984), Brachiaria spp (Ndikumananeed d in attempts to transfer apomi is tocrops through wide hybridization. Widecro ses gen rally produce highl terilehybrids that canonly be backcrossed by Llsingthem as female. If these hybrids are obligateap micts, the wide cross approach fortransferring ap mixis is a dead end. But isobligate apomiXis ever totally obligate?Asker (1979) assigned a que ti n mark toobligat apomi ·s. The developmentalproces has been described at th vule level,where meiosis. ucceed - r fails. At the plantlevel, obligate ap mixi is alreadyqu stionable. At the level of the popuJationor specie, obligate ap mi i is likely anartifact ofthe screening toot ( ee Leblan andMazzucato, Chap. 9).large number of ovules in th case of Psqllill/I//fall/III (DuJardin and Hanna 1984a)have b n examined and 8% were classifiedas abort db' d )n the absence of a normallyd el ped Jl;lbryo sac. In PlIl1icl/nt IIII1X;IIlWI/,an th r ap por us tropical forage rrass,ovul with n sac could be either abortive,in hich case they show enlarged nucellarcell' with little or no cytoplasm in an overallshrivelled ovary, or in early meioticd velopm nt stages. Sexual embryo sacs (ES)were ignificantly late as compared to thenucellar unreduced ES (Savidan 1982a).1985),P~-paltllllnotatum(MartinezetaI.1994),and dipl( p rQus Tripsacllm species (Leblancand avidan 1994), among others. Dujardin(personal camm.) confirmed that the nu eUifrom vules he classified as aborted wereperfectly normal, hence apomixis in the P.sqllall/llmlllm introduction was perhaps not asobligate as originally thought.Recent data (Hanna el al. 1993) showing highdegrees offacultativeness in later generationhybrid derivatives can possibly bereinterpreted in the light of this hypoth is.Modification of the genetic or epigenicbackground is known to affect facultativeapomixise pres ion, with an extremely highrate fs xuahty p ibly being observed. Thiswa" n in guin agrass (PauiCilm maximum),In ne natural interspecific hybrid with P.iufes/llm (see al Berthaud, Chap. 2).Though apllmi i is probably alwaysfacultative in the wild to some extent, thefacultativeness f the donor specie in atransfer attempt should be limited and/orcontrollable for apomixis to be properlymanageable in agriculture. Therefore, acompromise must be found between thefacultativeness required for male sterilehybrids to be backcrossed, and the finalobjective of relative homogeneity in thefarmers' fields.