Chapter 5 Genetic Analysis of Apomixis - cimmyt

16 M......thodhexaploidy through 2n + n hybridization, and(ii) a change from hexaploidy to triploidy bymeiosis and parthenogenetic development ofthe embryo. From triploidy to tetraploidy thepathway is as previously described (2/1 + /1hybridization) and involves diploid plants aspollinators. Complete cycles of tri-tetra-hexahaploidplants linked to diploid plants arepossible. During these cycles, recombinationand fertilization events occur, helped by theparthenogenetic development of reducedembryo sacs and by fertilization of unreducedembryo sacs. Apomixis, in this case, enhancesthe functioning of sexuality that is distributedover several generations.Cycles and SexualityIn all agamic complexes, two different ploidypools are found: a lower ploidy pool (usuallydiploid) with sexual forms and a higher ploidypool (usually several ploidy levels, the mostfrequent being the tetraploid level) withapomictic forms. Absence of apomixis at thediploid level is thought to be due to either alack of expression of this trait at this ploidylevel or to an absence of transmission throughhaploid gametes (Nogler 1984; Grimanelli etal. 1998). The sexual pool is where most of thegenetic recombination occurs and is thereforethe pool where most of the selection on newcombinations is acting.n+n (6x)2n+n (4x) 2n+n (4xl 2n+n (4x) 2n+n (4xlFigure 2.2 Evolution of ploidy levels in Tripsacumfrom fertilization of female gamete (n or 2n) by amale gamete (n) from 2x, 4x or 6x plants orparthenogenetic development of egg cell (n+O).Gene flow from the diploid to the polyploidpool is realized in several ways. Diploidsexual plants, in some cases, can produce 2/1female gametes (Harlan and de Wet 1975). Ifthese gametes are pollinated by pollen fromtetraploid plants, tetraploid progeny will beproduced that will be sexual to a certainextent, providing an opportunity for a newburst of diversity to be tested at the tetraploidlevel. Another flow, as discussed earlier,comes from the pollination of unreduced eggsfrom triploid plants by normal pollen grainsfrom diploid plants. The triploid plants canresult from crosses between diploid andtetraploid plants. As can be seen, manyopportunities exist for the diploid pool tocontribute to the genetic diversity of theapomictic tetraploid pool. In the A/1tl?l1/1ariacomplex, several genomes from diploidspecies can be accumulated in polyploidspecies (Bayer 1987).In the polyploid apomictic pool, new geneticcombinations may also arise through residualsexuality (n + /1 progeny). We have also seenevidence that sexuality is distributed overseveral generations by creation of 2n + nprogeny in one generation, followed by n + 0progeny in the next generation. By permittingsome perenniality for each stage of the sexualcycle, this wealth of genetic recombination isfavored by apomixis, and it may becharacteristic of the apomictic mode. Moreexperimental data and modeling are requiredto isolate all of the factors involved in thegenetic recombination of apomicts.Management ofApomictic VarietiesTwo types of apomictic varieties can bedistinguished: forage varieties, which arealready released as apomictic varieties, andapomictic varieties ofcrops such as maize andpearl millet, which may be released in thenear future.