Chapter 5 Genetic Analysis of Apomixis - cimmyt

178 IJeIGr.........development without entering meiosis or afterpremature meiotic abortion and nuclearrestitution (Crane, Chap. 3). Likewise,parthenogenetic embryogenesis is usuallyinitiated prior to anthesis and often beforefertilization of the central cell in pseudogamousspecies. Thus, it appears as if specificdevelopmental events are initiated prior tocompletion of the previous ones. Heterochronicdevelopment is a hallmark of apomixis, withwhich specific developmental events arereplaced asynchronously or coexist andcompete with events occurring in normalsequence.In addition to a change in the temporal orderof developmental processes there is also arelaxed constraint on cell fate decisions.Whereas in sexual species a single nucellar cel1is usually committed to the meiotic pathway,several nucellar cells in apomictic species havethe potential to form unreduced gametophytes.The regulation of individual developmentalevents appears to be conserved betweenapomictic and sexual pathways. Therefore, itis likely that key regulatory genes playingessential roles in sexual development aremisregulated in either space and/or timeleading to the developmental alterationsobserved in apomicts. Precocious initiation ofmegagametogenesis and the prematureactivation of the egg cell could be caused bymisexpressed regulatory genes that performthe same functions during sexual reproduction.Thus, the gene(s) control1ing apomixis does notnecessarily encode altered gene products, butrather could be under relaxed or aberranttemporal and/or spatial control.Several models accounting for the precociousinduction of developmental events and theinterrelationship with the sexual pathway havebeen proposed (Peacock 1993; Koltunow 1993).Developmental checkpoints similar to the onesproposed to control proper progressionthrough the cell cycle (Hartwell and Weinert1989; Murray 1992) may ensure a strictsequential order of developmental stepsduring sexual reproduction. In apomicts,developmental checkpOints and feedbackmechanisms may be ignored or altered,leading to the initiation of a developmentalevent before the completion of an earlier one(Koltunow 1993).Alternatively, rather than misexpression ofregulatory genes in the nucellus, more generalchanges in the cellular machinery could causeapomixis. For instance, an increase in theduration of the cell cycle may allow genes tobe expressed at an earlier time in developmentthan usual. Such a situation has been observedfor genes with large introns in Drosophila. Thegenes knirps (kni) and knirps-related (knrl)encode highly similar proteins, but knrlcontains a large 19 kb intron (Nauber et al.1988; Oro et al. 1988; Rothe et al. 1989). Thus,knrl is only functional at nuclear division cycle13 during cleavage, when the cell cycle hasbecome long enough to allow RNApolymeraseto transcribe the entire knrltranscription unit before the initiation of M­phase (Rothe et al. 1992). In contrast, kni isexpressed already at nuclear division cycle 9.An intron-Iess knrl gene can fully rescue thekni embryo lethal phenotype (Rothe et al.1992). Two mutants have been isolated thatallow for the functional substitution of kni byknrl. Both act by lengthening the cell cycle and,thus, al'1ow knrl to be transcribed at an earlierstage of development than usual (Ruden andJackie 1995). A similar situation may beencountered in apomictic species, withduplicated genes being activatedheterochronically. The duplicated genes maybe paralogs present in the same genome ororthologs from two genomes brought togetherthrough hybridization.The models discussed above do not take intoaccount the tight association of apomixis withpolyploidy, although they are certainly