Chapter 5 Genetic Analysis of Apomixis - cimmyt

stabilization of apomixis, i.e., the ability ofapomicts to t" .:ltatively produce progenysexually without such progeny being sexualrevertants (Carman, in preparation). Themapping of genes or linkage groups withstrong effects on apomixis penetrance isimportant, and, as in many other breedingstudies, segregation ratios and levels offacultativeness should be verified by testingputative sexual segregants for apomixis(cytology of 10D-200 ovules and/or progenytests involving molecular markers) overmultiple years and multiple locations.Putative segregants should be consideredunreliable for mapping studies until such testsare performed. Finally, because levels ofapomixis expression (facultativeness) oftenare highly variable in Mendelian analyses(reviewed by Carman 2000), QTL analysesshould be conducted to estimate the numberof genes involved.Making Crops ApomicticAccording to HFA theory, "apomixis genes"that confer apomixis when transferred tosexual species do not exist. In contrast,heterozygosity at many loci is required; thisheterozygosity involves traits such as floralinduction stimuli and stages of ovuledevelopment in which meiosis, embryo-sacdevelopment, and embryony occur.Transferring such linkage groups to sexualspecies will not confer apomixis unless thetiming of female reproductive developmentencoded by the recipient plant isappropriately asynchronous with thatencoded by the alien segment. Hence, HFAtheory predicts that programs designed tointrogress apomixis into crops will experiencedifficulties producing apomictic lines thatpossess anything less than one to a few alienchromosomes.It should be possible to produce apomicts inat least some crops by (i) selection forappropriate parental phenotypes (divergencein female developmental schedules), (ii)crossing the appropriately-divergentphenotypes, and (iii) stabilizing apomixisthrough cytogenetic modifications that isolatethe responsible heterozygous loci fromrecombination (polyplOidy or inversion ortranslocation heterozygosity) or throughtransgenic modifications that obligately abortfemale meiosis. The success of this approachwill depend on the existence in primary genepools ofsufficient genetic variability for femaledevelopmental schedules and on correctlyidentifying the appropriate cytological parentalphenotypes. It may be pOSSible to enhanceinsufficient variabili ty in some crops byoutcrossing to the secondary gene pool.According to HFA theory, the required genesare not apomixis genes per se, but consist ofnormal genes with multiple ecotype-specificalleles, which when found in specificcombinations confer temporally-divergentschedules of sexual female development tonatural ecotypes.AcknowledgmentsThis study was supported by a Centers ofExcellence grant from the State of Utah and bythe Utah Agricultural Experiment Station,Logan, UT 84322-4845; approved as journalpaper no. 4891.ReferencesAsker, 51, and LJerling. 1992. Apomixis in Plants. Boro Rolon, Florida:0( Pr~s,Bo"aglia, E. 1989, The evolulion of the female gametophyte ofongiosperms: on interpretative key, Annali di Batanica 47: 7-144,Bennen, M.D. 1977. The time and duration of meiosis. Phi/osophiwlTranslations of the Royol Society of london, Series B. 277: 201-26.Bennen, M.D., and H. Stern. 1975. The time and duration of preleptotenechromosome condensation stage in U1ium hybrid cv. block beauty.Proceedings of the Royol Society of london, Series B. 188: 477-91Blakey, LA., C.L Dewold, and S.L Goldman. 1997. Cl>-segregation of DNAmarkers with Tripsocumferlilily. Moydiw42: 363--69.Brummill, R.K. 1992. Vaswlor PIont Families and Genera. Whintable, Kent,U,K.: Royal Botanic Gardens, Kew, Whitstable Litho, IJd.Carmon, J.G. 1986. Genetic engineering: potentials for underslanding andusing apomixis. Presentation at the workshop "The Polenliol Use ofApomiXis in Crap Improvement," April 21-26, 1986, RockefellerFoundation, Bellogio Study and Conference Center, Belogio, Ita~.