Chapter 5 Genetic Analysis of Apomixis - cimmyt

and meiotic or developmental mutants that arewell characterized in sexual plants. Numerousmutants are known in grasses, especially inmaize (Neuffer et a!. 1997), for various aspectsof sexual reproduction. Furthermore, largenumbers of such mutants can be generatedthrough classical (e.g., chemical) or transposonmutagenesis. Recent results of comparativemapping among grasses (Bennetzen andFreeling 1993; Ahn and Tanksley 1993; Mooreel a!. 1995) demonstrate that most grassesprobably share the same basic set of genes, andthat the obvious differences separating thespecies are based on allelic variations and noton their relative gene combinations..Therefore, we suggest that the genes whoseactions produce an apomictic phenotype insome grasses almost certainly can be found insexual species. In this instance, comparativemapping could be used to identify genes inmaize or some other sexual grass that areorthologous to the apomixis genes, and thenuse them to isolate their counterparts in theapomictic species.The process of identifying maize orthologs ofgenes responsible for apomixis involves threesuccessive steps: (i) candidate genes areidentified through phenotypic characterizationand genetic mapping; (iJ) promisingcandidates are then isolated in maize; oncecloned, lhe isolated genes are sequenced, andlhe sequence information is used to cloneorthologous genes in the apomicts; (iii) therelationship between the alleles isolated in theprevious steps and the expression of apomixisis confirmed using a reverse genetic strategyin apomictic plants. For step iii, theconstruction of transposon tagging populationsin apomicls are of great interest to R.Bicknell and the CIMMYT apomixis team.Three criteria can be employed to selectcandidate genes: (i) because apomixis oftenaffects only the female function, we proposethat the gene(s) responsible for the failure ofmeiosis have a megasporogenesis-specificphenotype, meaning that mutants of interestshould affect only the female function; (ii) asin diplosporous plants, the candidates shouldaffect early stages of meiosis, ideally, theinduction of meiosis; meiotic mutations actingat later stages in meiosis are probably notdirectly related to apomixis; and (iii)interesting candidates should be able toproduce unreduced gametes, (thus, as inapomictic plants, the completion of unreducedgamete formation implies that the checkpoints(Hartwell and Weinert 1989), which usuallyact during the meiotic cell-cycle to ensure theproduction of normally reduced haploidgametes, failed to override abnormal behavior.With aposporous-Iike mutants, obviousphenotypes relate to the induction ofmegagametogenesis in somatic cell. Sheridanet al. (1996) describe a remarkable example ofthis type of mutant.Manipulation of gene expression in modelspecies: To date, this is probably the mostwidely used approach for developingapomictic cultivars, (details are discussedelsewhere in this volume). Current workcenters on large-scale mutant screening inArabidopsis and Petunia Gefferson and Bicknell1996; Ohad et a!. 1996; Chaudurhy et al. 1997;Grossniklaus et al 1999; Luo el al. 2000}. Thebest prospecl from these approaches would bethe engineering of a mode of apomixis thatbetterCmeets the requirements of agriculturalproduction than the apomixis mechanismsfound in the wild (see Jefferson and Bicknell1996, and Chap. 8). The remarkable resultsobtained recently with a sel of mutations inPolycomb-related genes in Arabidopsis(Grossniklaus et a!. 1998; Luo et a!. 2000) arevery encouraging. They demonstrate thatphenotypes related to apomixis mayeventually be obtained by manipulating theexpression of genes involved in sexualreproduction, without reference to apomixisas seen in the wild.