Chapter 5 Genetic Analysis of Apomixis - cimmyt

Clanllkatlo. of Apamictlc ~is .. 33one or a few key loci to affect this pattern.Ehlenfeldt and Hanneman (1988) analyzed thecrossing behavior of diploid hybrids betweentwo diploid, sexual Solanum species that differin endosperm balance number, and theyconcluded that three unlinked loci controlledit. Obviously, comparable evidence is neededfor the immediate relatives of autonomouslyparthenogenetic apomicts.OutlookThe speculative nature of the precedingdiscussion emphasizes how little isexperimentally known about meiosis,megasporogenesis, gametogenesis, andfertilization in sexual plants, and how little ofthe diversity of apomictic behaviors has beenstudied with modern methods ofgenetics, cellbiology, and molecular biology. Most apomictsare polyploid, perennial herbs with longgeneration times, and the effort to maintainsuitably large, uncontaminated populations ofadvanced generation hybrids has repeatedlydefeated even rudimentary attempts tounderstand the inheritance of apomixis. Thepattern of genomic affinity is not known formost pseudogamous or semigamousapomicts; recombination between thecentromere and the causative loci can furtherchange the expected segregation. The generalproblem is that more than one equally bestfittingmodel can be found for the samesegregation data, especially when multipleallelism and complex dosage requirements areconsidered and advanced generation selfs andbackcrosses are not available. Since thenumber of developmental inductions isconstrained by the number and interaction ofcausative loci, the difficulty in completelyunderstanding the genetics of apomixis hashamstrung the more direct developmentalapproaches. This is particularly true for therelationship among imprintable loci,endosperm function, and embryo inductionin autonomous apomicts. Endospermimprinting relationships might well becomethe biggest obstacle to the utilization ofapomixis in sexual crop species.At the level of cell and molecular biology,critical questions remain regarding thesequential and parallel nature of clocks thatgovern the relative and absolute duration ofevents in the Polygonum-type, and thus thenumber ofcontrol points and interconnectionsthat suffice to push the whole program along.Diagnostic mRNA or protein suites have notyet been recognized for the steps in thePolygonum-type and applied to recognizeparallel steps in apomictic developments. Thepromoters, reading frame sequences, andaction of genes at control points remain amystery. Thus the basis of one of the mainassumptions in the preceding developmentalinterpretations, namely that the induction oflate steps cancels intervening steps within thePolygonum-type, has yet to be justified.Natural apomicts are difficult experimentalsubjects, but they provide embryologicalbehaviors that might be even more difficult toscreen from mutagenized populations ofArabidopsis thaliana or annual crop species,especially if the appropriate mutations requiregains in function. Apomictic studies mightcontribute significantly to our futureunderstanding of sexual plant reproductionand to the successful utilization of apomixisin currently sexual crops.ReferencesAsker, S. E., and LJerling. 1992. Apomixis in Planls. , Baca Ratan,Rorida: CRC Press.BoMaglia, E. 1946. Ricerche cariolagiche ed embriologiche sui genereRudbeckio (Asleroceoe). I-V. 11 gametaflto femminile e mos