Chapter 5 Genetic Analysis of Apomixis - cimmyt

220 Ura P"",k.h .d Rod ScottStubby head could indeed be a very usefulmutant, as it produces viable seed by apospory.Although the nature of the mutation is notknown, the mutation affects several elementsof apomixis in a single step. Whilst this couldbe due to a single gene mutation, thepleiotropic nature of the mutant phenotypecould well indicate that it is caused by adeletion encompassing a number of genes.4. Conclusions. The above examples ofm u tants with apomictic characteristicsillustrate a number of points relevant tomutagenic approaches in model plants.The most important conclusion is that all theelements of apomixis can be induced bymutation in sexual plants. In one case, stubbyhead, several elements were inducedsimultaneously to produce a viable form offacultative apospory. It would be veryinteresting to analyze these mutants at theDNA level, but the isolation of genes fromthese crop species would not be a Simple task.Unlike Arabidopsis, the cloning of genes viatheir mutant alleles is not routine.None of the mutants described herein wereoriginally isolated in screens for apomicticcharacteristics. The majority were isolated asreduced fertility mutants and some aspleiotropic mutants. Therefore, it seems worthconsidering which kinds of mutantphenotypes, other than apomixis itself, couldbe screened for in Arabidopsis. First, it mightbe worthwhile to screen for reduced fertilitymutants, a simple screen that would involvetesting for seed set in the M 2. A large numberof male-sterile mutants have been isolatedfrom Arabidopsis. Some are present in thecollection of T-DNA tagged lines (availablefrom the Arabidopsis Stock Centers atNottingham, U.K. and in Ohio, U.s.A.); thesecould all be screened for maternal progenyafter pollination with a dominantly markedpaternal line. Second, the tri and el mutantsboth give rise to shrunken seeds, and since ithas been shown that an unbalanced ratioaffects seed size in Arabidopsis too, thisphenomenon could be used as a screen for theisolation of Arabidopsis mutants that produceunreduced egg cells.Current Approaches to theIsolation of Apomictic Mutantsin Model Sexual PlantsThe most important precondition for the largescalescreening of apomictic mutants is theavailability of male sterile lines that do not setseed in the absence of pollination. Arabidopsisis ideally suited for such an undertakingbecause it offers several ways of establishingmale-sterile lines. The most useful of thesecauses conditional male sterility, which allowsthe propagation of homozygous seed byselfing under permissive conditions. Inaddition, Arabidopsis provides easily scoreddominant and recessive markers forsubsequent screening of mutagenizedprogeny.The most obvious screen for apomicticmutants of a male-sterile sexual plant is forseed set in the absence of pollination. However,there have been no reports of mutants (in anymodel plant) that produce viable seed in theabsence of pollination. This may well bebecause the number of progeny screened inthis way was not large enough to detect suchmutants, as these could indeed be very rare.For the reasons already outlined, however, italso seems likely that more than one genemutation is required to induce a viable formof apomixis.Current efforts in several laboratories aredirected toward the induction of partialapomictic development. Basically two maintypes of screens are being conducted. The firstidentifies mutants that show partialdevelopment of fruits in the absence of