Chapter 5 Genetic Analysis of Apomixis - cimmyt

Tra.sf.r .f Apomixis tllroogll WIde Cross.. 157potential, among the few species tested, forgiving some female and male fertility to the FIS.Disadvantages include the requirement of abridge species, P. purpureum, the different basicchromosome number (x = 9, as compared withx = 7 in pearl millet), and the hexaploid levelof ploidy. Progress made on mappingapomixis in Pennisetum and its implications forour understanding of the genetic control arepresented in Grimanelli et al. (Chap. 6).Case History: TripsacumNumerous maize x Tripsacum hybrids havebeen produced since the pioneering researchof Mangelsdorfand Reeves more than 70 yearsago (Mangelsdorfand Reeves 1931). Extensivehybridization studies have been carried outby Galinat (1971), Harlan and de Wet (1977),James (1979), and Bernard and Jewell (1985),among others. The main objective of thesestudies was to evaluate the potential role ofTripsacum in maize evolution and / or thefeasibili ty of gene transfer, though notnecessarily for apomixis. Claims ofintrogression have been made (Simone andHooker 1976; de Wet 1979; and Bergquist1981), but the Tripsacum progenitors involvedwere not tested beforehand for the target traits;consequently, the same traits couldpresumably have been present in neighboringmaize collections. However, all these studiesshowed that from a maize-Tripsacum F Ihybridit was possible, in a few generations, to recovera 20-chromosome maize with somemorphological features that were not presentin the original maize progenitor. Most of thesestudies were based on using a diploid sexualTripsacum, and most concentrated on a singlespecies, T. dilctylaides. Between 1990 and 1992,maize was successfully crossed with 66apomictic populations, representing eightdifferent species and intermediate formsbetween species (Table 11.1); 895 F Ihybridswith 2n =46 =10M + 36Tr were obtained fromthese crosses. Most of these (598, or 66.8%)involved T. dactylaides subspecies orinterspecific-like accessions involving someform of T. dactylaides. This confirmed highcrossability for T. dactylaides. The number ofPI plants per number of pollinated ears,however, showed a higher crossabilitybetween maize and T. wpilatense, which hasthe smallest area of distribution in Mexico(being found only in the Canon de Zopilote,between Mexico City and Acapulco).Advantages of using T. dilctylaides as the donorspecies include good pollen fertility and anapomixis characterized by an absence ofcallose around the megasporocyte andsubsequent cells, which is easily detected influorescence microscopy (Leblanc et al. 1995b;Leblanc and Mazzucato, Chap. 9). Diplosporyis further characterized by endosperms with aploidy level different from that ofsexual seeds,resulting from the fertilization of twoTable 11.1 Crossabilities between maize and wildTripsacum species and presumed naturalinterspecific hybridscode nb.pop ears emb. cult. Fls Fls/earZP 2 41 860 573 118 2.88DT 2 92 324 169 97 1.05iMZ 2 23 1119 140 20 0.87ilT 6 103 1143 427 83 0.81iDH 7 132 1527 452 84 0.64iPL 4 65 2169 257 33 0.51DH 30 776 10816 2892 386 0.50PL 1 4 10 1 0.25IT 5 132 779 123 32 0.24iDM 3 75 2513 444 14 0.19DM 7 121 3655 813 17 0.14LC 1 10 38 5 1 0.10BY 5 96 2091 390 7 0.07iBY 2 62 1847 352 2 0.03PR 1 20average 1732 895 0.52nb.pop.= number of populotions studied; eors= number of moize earspollinated with the TripsDcum spedes; emb.= number of countedembryos, three weeks aher pollination; cult.= number of embryoscultured; F1s= number of F 1 hybrids grown to maturity. Speciescodes: ZP= lzopilotense; DT=ldactyloides dactyloides (US types);MZ= lmaizar; IT=lintermedium; DH= ldacty\oides hirsutum; PL=lpilosum; DM= ldactyloides mexicanum; LC= llancealatum; BY;lbravum; PR= lperuvianum; i= intermediate forms (presumesnatural interspecific hybrids).