Chapter 5 Genetic Analysis of Apomixis - cimmyt

Yraosfer of Apomilis 1~ ...gIt WIde Cro.... 161Hybrids that are totally male sterile andobligately apomictic are indeed dead ends:pollinating such hybrids with the crop pollenwill produce only maternal offspring, i.e.,perfect copies of the sterile Fl' However, ifapomixis is slightly facultative, off-types canbe produced, some of which maybe n + nandstill apomictic, representing progress towarda return to the chromosome number of thecrop. The rate of facultativeness has to be low,however, if one expects the backcrossprocedure to eventually produce an apomicticcrop germplasm with a high degree ofapomixis. Analyses made on Panicummaximum (Savidan 1982a,b)show that the rateof facultativeness, and more precisely of n + noff-types, may remain relatively conservedthrough generations of hybridization. It wastherefore suggested that a limited range ofvariation could possibly allow selection backto obligate apomixis. In the intergenericbackground of maize x Tripsacum hybridderivatives, the variation observed (Table 11.3)appeared less stable, possibly because theapomictic Tripsacum progenitor was alreadymuch more facultative than the guineagrassaccessions used by Savidan (1982). By selectingamong Tripsacum accessions for their ability toproduce hybrid derivatives in backcrossing Flswith maize, the team possibly selected one ofthe most facultative of the apomictictripsacums.Table 11.4 shows the cumulative result of theanalysis of approximately 6,000 progeniesproduced from maize x Tripsacum BetS withIssue # 3. Can apomixis be expressed at the diploid level?In the wild, apomixis is found only amongpolyploids (although a few, questionableexceptions have been cited, see Asker andJerling 1992). Population geneticists havesuggested thatsexuality wouldbeeliminatedif apomixis could be expressed at the diploidlevel (Pemes1912; Marshall and Brown 1981).Nagler (1984) claimed, with little evidence tosupport it, that apomixis is probably linkedto a lethal factor expressed at the haploid(gamete) level only. After obtaining 23­chromosome pearl millet x P. orientale Be,plants, Hanna et aI. (1993) stated thatpolyploidy is probably not needed for theexpression of genets) controlling apomixis,because these 23-chromosome plants hadonly one (simplex) set of nine P. orientalechromosomes. The genomic structure oftheseplants is likely 14 Pm + 9 Or however,suggesting that the locus involved couldpossiblybe present in triplicate. Another suchcase of apomictic expression in anonpolyploid form was previously reported(Dujardin and Hanna 1986), which related toa polyhaploid plant from a pearl millet x P.squamulatum p) hybrid which had 2n =41 =14Pm + 27Sq. This haploid had 2n = 21chromosomes. Again, as the 2n = 21­chromosome plant likely had sevenchromosomes from pearl millet and 14 fromthewild species that had a basicchromosomenumber of nine, the locus involved waspossibly in triplicate and not in duplicate.In the Tripsacum project, a few polyhaploidswere obtained in the progeny of 2n =56 =20m + 36tr BC)s (Leblanc et al. 19%). Theseplants have ne set of maize and one set ofTripsacll/ll chromosomes, as confirmed by insitu hybridization (Leblanc et al. 1996), andsome .of them could express apomixis.Whether they represent exceptional cases ofrecombination between apomixis and a lethalsystem linked to it is open to speculation (seeGrimanelli et al. 1998b). Grimanelli et al.(1998b) suggest, however, that apomixis canbeexpressed evenwhen the allele(s) involvedare in a duplex situation, a position that rej«tsthe hypotheses of dosage effect presentedearlierbyMogie (1988) and Noirot (1993), andsuggests that the transmission barrier,whatever its nature, may be overcomethrough haploidization to produce functionaldiploid apomicts.