Chapter 5 Genetic Analysis of Apomixis - cimmyt

T,..,fe, .f Apomixis ,.,... WIde Cro"e, 155cytogenetics, e.g., in Tripsacum, Galinat (Galinatet al. 1970; Galinat 1971) described four maizechromosomes that are capable of pairing withTripsamm chromosomes. Meanwhile, mappinganalyses (Grimanelli et aI., Chap. 6) suggest amore widespread colinearity between themaize and Tripsacum genomes.4. Pollen fertility. Except for the highlyfacultative apomicts, first generationhybridizations between the crop and donorspecies must use the latter as male. Severalapomictic species have been described withgreatly reduced male fertility (e.g., Elymusrectisetus, the apomictic wild relative of wheat);in such cases, a preliminary selection is needed.5. Type of apomixis. Apospory has alwaysbeen presented as an easier type of apomixisto work with, being associated with 4-nucleateembryo sacs in tropical and subtropical grassesand transmitted as a single dominant gene(Savidan 1982a; Nogler 1984; Asker and Jerling1992; Savidan 2000). Recent studies ondiplospory in Tripsacum strongly challenge thisview, bolstered by flow cytometry, which canbe used to analyze modes of reproduction(Grimanelli et al. 1997), and screens that usedifferent types of molecular markers.Nevertheless, the type of apomixis must stillbe considered, as different types ofscreens maybe applied to different types of apomixis.Whether one type of apomixis than another ismore likely to be expressed in a particular cropbackground is still largely speculative.6. Degree of apomixis (or degree of facultativeness).The degree of apomixis appears tobe a major factor related to the feasibility ofwide cross transfer of apomixis. An obligateapomixis cannot be used unless some degreeof male fertility is recovered in the F1s, whichis seldom the case in interspecific hybrids; butto produce near obligate apomictic crops,facultativeness must be low and wellcontrolled. This factor is addressed in moredetail later in this chapter.7. Agronomic characteristics. A species withpoor agronomic traits will produce hybridsand hybrid derivatives that may conserveundesirable traits for several generations,slowing the progress of the transfer.8. Previous knowledge. Previous knowledgeconcerning the interspecific or intergenerichybridization under consideration is a definiteadvantage. For example, knowing the numberof backcrosses needed to go from the maize­Tripsacum Fls to a 20-chromosome recoveredmaize (Harlan and de Wet 1977) was importantin developing the first work plan for the lRD­CIMMYT apomixis team and in maintainingits confidence about the feasibility of itsapproach.Case History: PennisetumPrnl1isetum glaucum, a cultivated pearl millet,has a basic chromosome number of x = 7. Theonly known and widespread tetraploid wildspecies with the same basic chromosomenumber is P. purpureum (211 = 4x = 28). Thoughdescribed as aposporic by Brown and Emery(1958), this species appears to be entirelysexual, as confirmed by a cyto-embryologicalsurvey made in morphologically uniform wildpopulations from West Africa (Y. Savidan,unpublished). Apomixis has been described inseveral other Pel1l1isetum species, all of whichbelong to the secondary or tertiary gene poolsand share a basic chromosome number of x =9. Dui.ardin and Hanna (1989) demonstratedthat three out of the seven apomictic speciestested were capable of producing F Ihybridswith pearl millet. The genus Pel1l1isetum,however, is one of the most complex in thegrass family. In addition, the number ofspeciesvaries greatly according to the taxonomist, themost conservative estimates beingapproximately 100 different species(Purseglove 1972), most of which are perennial,polyploid, and likely apomictic. Because of alack of available germplasm, no extensive