Chapter 5 Genetic Analysis of Apomixis - cimmyt

20 JM Iertltaodmarkers to retain the a allele, (ii) the productionof near isogenic lines through backcrossingwith the landrace, and (iii) the selfing ofisogenic, heterozygous (Aa) lines to produceaa apomicts.2n + n ProgenyIn Tripsacum, we saw an average of 10% ofprogeny come from 2n + n hybridization; insome samples, this rate rises to 35%. Crossesbetween apomictic species of Pennisetum alsoproduced this type of progeny (Bashaw et al.1992). These forms are less frequent in otherspecies, such as Panicum maximum. If this traitis inherited during the transfer of apomixis,what behavior can be expected from cultivatedapomictic forms?The transfer projects now underway considera type of apomixis linked to pseudogamy.Once apomictic varieties are produced, mostprobably they will be also pseudogamous. Inthis case, we are concerned with the ratiobetween embryo ploidy and endospermploidy, as it has been often reported that a ratiodifferent from 2:3 (or 2:5) would introducesome developmental incompatibility at theseed level and a loss in productivity(endosperm development also depends onmaternal:paternal genome ratio; see Chap. 6,11, 12, and 13). However, for the Tripsacum, weobserved that triploid plants produce seedseven when their pollen environment comesmostly from tetraploid plants. In this case, theploidy ratio between embryo and endospermis 3:8. The 2n + n progeny we detected werefrom normal seeds with normally developedendosperm. In Tripsacl/m, the 2:3 ratio (or 2:5)between embryo ploidy and endospermploidy does not appear to be necessary for seedfilling. In general terms, we have twohypotheses to consider:1. Endosperm development is deficientwhen the ratio of embryo ploidy toendosperm ploidy is different from 2:3(or 2:5). In this case, ears display poorlyfilled kernels (with 2n + n embryo) atharvest time. There is a potential loss ofproduction due to the presence of these2n + n embryos, but these kernels wouldnot be selected as seed for the nextgeneration.2. Endosperm development is not affectedby a ratio of embryo ploidy to endospermploidy different from 2:3 (or 2:5). In thiscase, kernels with 2n + n embryos wouldgo undetected and could be used as seedfor the next generation. Apomictic plantsobtained from such embryos are triploid;they may produce normal seeds but thepollen could be sterile, which could limitfield production. If the pollen is stillfertile, as noted with triploid Tripsac!lm,no loss in production should be detected.However, ploidy buildup will occur, andmany different ploidy levels will bestored in the same variety. This ploidybuildup could raise chromosomenumbers to levels far above the optimumfor productivity, potentially resulting inlower production.In nature, 211 + n progeny production is astrategy that takes advantage of geneticrecombination, as these plants would give rise,after meiosis, to some haploid progeny byparthenogenetic development of reducedembryo sacs. In the case of an apomictic crop,it is a trai t tha t should be reduced oreliminated.Relationship between Wild Relatives andApomictic VarietiesFor the purpose of discussing the relationshipbetween wild relatives and apomictic varieties,we will use the maize-teosinte model,however, it is our belief that it can beextrapolated to pearl millet in instances wherewild relatives are still in contact with cultivatedplants. Teosinte is only found in Mexico andGuatemala. Relationships between wildrelatives and maize are not identical over thedistribution area of teosinte. The variety