Chapter 5 Genetic Analysis of Apomixis - cimmyt

Th. Go•• [H.,,: Go..... Colli.io,,, ..dApomixi. 103heterokaryons to skip Sand G2 and proceedprecociously to mitosis (Lewin 1994).Many examples of checkpoint phenomena canbe hypothesized from an examination of Figure7.1. For example, if embryo-sac developmentsignals from one genome are superimposed onmegasporogenesis signals from anothergenome, meiosis may be skipped (diplospory),similar to the heterokaryon examplesdescribed above, or embryo-sac developmentmay be ectopic (apospory). Accordingly,apomictic-like tendencies would occur inpolyploids only if major differences in timingof megasporogenesis and embryo-sacdevelopment (relative to other ovule and ovarytissues) exist among the ancestral ecotypes orspecies (Figure 7.2). Such natural variation maybe infrequently found in highly cosmopolitangenera, i.e., genera with broad latitudinal andecological distributions, but possibly absent inless cosmopolitan genera.The HFA theory was refined using criteriatabulated in Table 7.1. Hence, it explains thesecriteria as well as many inconsistencies in theapomixis literature. For example, apomixis,polyspory, and polyembryony are rare buttend to occur together in cosmopolitanfamilies, such as Poaceae, Asteraceae, andRosaceae, because sufficient ecotypic variationin reproductive start-times, etc., is rare in mostfamilies but high in these cosmopolitanfamilies. Sexual reproduction of themonosporic Polygonum-type occursfacultatively in apomictic and polysporicspecies because, barring deletions ormutations, each parental genome containsgenes required for normal reproduction, andgrowing conditions may occasionally favor theexpression of one genome over the other,causing sexual development to occur.Facultativeness may be influenced by (i)differential silencing of genomes (epigeneticFigure 7.2 Model of how asynchronously-expressed duplicate genes cause diplospory and apospory inpolyploids containing two genomes divergent in the temporal expression of female developmental schedules(floral induction, megaspore formation, gametophyte development, and embryony). Bolded developmentalphases are skipped as described below.Developmentally-critical stages'Genome 2 3 4Genome I Archespore Meiosis Embryo sac Double fertilization/(unmodified)early embryonyGenome I Embryo sac Double fertilization/ Fertilization of(modified) : early embryony central cell onlyGenome II Meiosis Embryo sac Double fertilization! Fertilization 01early embryony