Chapter 5 Genetic Analysis of Apomixis - cimmyt

162 hOI Scrvidao211 = 56 chromosomes, i.e., 20 maize + 36 to be apomictic, while a 211 = 38 maize xTripsacum chromosomes. Note that the Tripsacum BC hybrid that produces progenyaverage rate of facultativeness at that level ranging from 211 =22 to 2n = 32 is sexual. Anwas very close to that of the Tripsacum alternative can be offered by using markersprogenitor, although variation was important. linked with apomixis, provided that apomixisis indeed controlled by one gene or smallA few dihaploids have been obtained fromsegment of DNA, and that such markers arethe progeny of 211 = 56 BCls, as 11 + 0 off-typesclosely linked.(Table 4). They grew well, flowered, andproduced a good seed set. Their progeny were A 1:1 segregation for apomixis and sexuality80% maternal and 20% 211 + 11 hybrids with was observed among maize x Tripsacum Fls,211 = 38 chromosomes. as 31 hybrids were classified as apomictic and30 as sexual, based on embryological analyses.The backcross series was continued in anThese plants were used for a bulk segregantattempt to recover apomictic maize plantsanalysis (see Grimanelli et aI., Chap. 6) aimedwith only a few Tripsacum chromosomes. Atat identifying molecular markers thateach generation, plants were screened forcosegregate with apomixis. Three RFLPapomixis and chromosome number. Embryomarkerswere first identified as linked withsac analyses, which have been usedapomixis; these belong to the same linkageextensively in several genetic analysesgroup in maize and are located on maize(Sherwood, Chap. 5), cannot be applied tochromosome-610ng arm (Leblanc et al. 1995).intergeneric hybrids or hybrid derivatives inOther markers were subsequently addedwhich inflorescences are too precious to be(Grimanelli et al. 1998a, and Chap. 6).destroyed. Modes of reproduction aretherefore estimated using progeny tests, e.g., Using both flow cytometry and markera211 = 38 maize x Tripsacum BC hybrid that assisted screening for apomixis, rare but usefulproduces mostly 211 = 38 progenies is likely apomictic plants can be selected among manyat each generation. A sourcepopulation must be grown toTable 11.3 Facultativeness of apomixis and. diplospory rate inthe Tripsacum accession used in the backcross transfer of constantly prod uce new progenyapomixis into maize and three BC progenies, showing until the next generation population1variation for this rate. D: diplospory rate.is large enough to enable progress tobe achieved in the backcrossNo. of 211+0 211+n II+nprogenies maternal off-types off-types others D% ,program. With a rate of only 3%-T.d-,-o-cty-l,-oi-:-de-s------------------ useful plants, we decided to raise the#65-1234 98 69 26 3 0 96.9 BC I population to 3,500 plants. After8(1-6-82 55 40 15 0 0 100 about 6,000 progeny had been8C I -6-52 98 73 22 1 2 99.0 analyzed, we substituted this BC I8C]"5-45 78 63 6 8 1 89.7 nursery with a BC nursery obtained3from in vitro multiplication of the 2n=Table 11.4 Chromosome numbers of BC38 apomictic off-types produced by1 (2n = 56) progenies asestimated by flow cytometrythe BC 2polyhaploids (2n = 28). Morethan 2,500 BC 3s were established inProgenies maternal off-types off-types off-typestotal no. 211+0=56 211+11=66 11+11=38 11+0 the field. The analysis of a 125,000­6259 5006 1024 218 11% 80.0 16.4 3.5 0.2