Chapter 5 Genetic Analysis of Apomixis - cimmyt

Transfer of Apomixis throughWide CrossesChapter 1 1YVES SAVIDANIntroductionInterspecific hybridization has been usedextensively to transfer agronomicallyimportant genes that control resistances todiseases and insect pests. Recent advances intissue culture, especially in molecular biology,have further widened the scope of alien genetransfer-and the outlook for widehybridization in crop improvement seemsmore promising than ever. But does thatoutlook also apply to the transfer of sequencesinvolved in plant reproduction, especiallythose involved in apomixis?The interface between conventionalcytogenetical approaches and new moleculartechniques makes the "conventional" widecross approach very competitive when the traitis simply controlled and the gene(s) to betransferred is (are) available in a species thatbelongs to the secondary gene pool. Thegenetic analyses reviewed by Savidan (2000)and Sherwood (Chap. 5) suggest that apomixisa good candidate and offer support for theongoing wide cross projects. Such projects haveencountered unexpected difficulties, andseveral papers have questioned the ultimatelikelihood of transferring apomixis to any crop.Nevertheless, knowledge gathered through thequest for wide crossing apomixis into usefulcrop species, which relates to the geneticcontrol, transmission, and expression of thetrait (Grimanelli et aI., Chap. 6), has provenextremely valuable for those investigatingother approaches. Accordingly, three paths arenow being pursued in the effort to introduceapomixis into major crops: (i) the widehybridization, (ii) the identification, isolation,and manipulation of sequences from wildapomicts, and (iii) the creation of an apomicticreproduction de novo, from individualmutations (Grossniklaus, Chap. 12; Praekeltand Scott, Chap.13). In this chapter, I reviewprogress to date and the problems or questionsthat have emerged from work aimed at widecrossing of the apomictic trait.Scientists have tried for decades to use widecrosses to transfer the apomixis trait intovaluable food crops, including wheat, maize,and pearl millet. The first attempt involvedmaize and was initiated approximately fortyyears ago (Petrov et al. 1979, 1984). Crossing atetraploid maize (2n =4x =40) with a tetraploidTripsacum dacty/aides (2n =4x =72), the Russianscientists successfully produced maize­Tripsacum Fls and BC lhybrid derivatives that,according to progeny tests, reproducedapomictically. The BC lplants combined 20maize chromosomes with one complete set (18)of Tripsacum dacty/aides chromosomes. Efficienttechniques for evaluating chromosomenumbers, embryo-sac analysis, etc., were notavailable, making screening of large numbersof progenies for apomixis difficult.Consequently, little progress was made in thistransfer effort. Recently, the Russian materialswere transferred to the United States, andintrogression efforts were reinitiated. Animportant piece of information generated by