Chapter 5 Genetic Analysis of Apomixis - cimmyt

124 Olivier lt~1ao< aod Aock.. Mau"atothe ovule, unreduced egg cells may not bereceptive. This loss of receptivity is not yet wellunderstood, but several hypotheses have beenproposed, including chemical or mechanicalbarriers (e.g., a complete cell wall around theegg) and a temporal window of receptivity,among others.Consequences of Apomictic SeedFormationIn sexual reproduction, the two gametes thatfuse are produced through meiosis. Sexualdevelopment allows genetic recombinationand segregation, thereby enhancing geneticdiversity. Aside from strict autogamy and fromthe very specific case of permanenttranslocation heterozygosity (El1strand andLevin 1982), offspring from sexual plants arenew genotypes. Apomictic pathways arecharacterized by unreduced egg cel1parthenogenesis, resulting in offspring that areexact genotypic replicas of the mother plant.However, genetic recombination may occurduring apomictic reproduction in plants thatshow partially synaptic and restitutionalmeiosis or somatic DNA rearrangements(Richards 1997).Complete (100%) maternal progenies arerecovered when the mother plant reproducesthrough obligate apomixis. But generally,apomixis is facultative and progenies comprisevarious types, each resulting from a differentTable 9.1 The four theoretical offspring dasses in progeniesfrom facultative pseudogamous apomicts. Note thatapomeiotic mechanisms can induce chromosome losses andresult in unbalanced unreduced female gametes.Egg cell originReduced megasporealter meiosis: ngameteApomeiosis (aposporyor diplospory):2n gameteEgg cell development afterFusion with asperm cell (+n)Sexualityn+n offspringNew genotypes"Genomic accumulation"2n+n offspringNew genotypesParthenogenesis(+0)(Poly)haploid productionn+O offspringNew genotypesApomixis2n+0 offspringMaternal genotypescombination of failure or success in meiosisand fertilization (Table 9.1). A fairly strictgenetic control for both the formation ofunreduced ES (reviewed by Sherwood, Chap.5) and the degree of apomixis (Asker 1980) hasbeen reported in most taxa studied.Levels of Screening andRelated ToolsThere are several indicators of apomixis,including high frequency of multipleseedlings, high seed fertility in plants expectedto be sterile (e.g., wide hybrids, triploids,autopolyploids, and aneuploids), homogeneousprogenies, etc. (Bashaw 1980; Hannaand Bashaw 1987; den Nijsand van Dijk 1993).They are sometimes difficult to use in the caseof wild materials and, in all cases, furtherinvestigation is required to assess apomixistype and level of expression. The relativeadvantages or disadvantages of the screeningprocedures presented here are discussedfurther in "Choosing Suitable Procedures."Analyses at the Plant Level1. Molecular markers cosegregating withapomixis. To date, the identification ofisozymic or molecular markers strongly linkedwi th apomixis is the only procedure fordetecting apomixis prior to flowering.Molecular marker-based analyses inapomicts were conducted either to.investigate the molecular basis ofapomixis, to assist in transferringapomixis into crops, or to ultimatelyisolate the gene(s) responsible for itsregulation. Segregating progenies orbulk segregant analyses were usedafter determining the reproductivebehavior on the basis ofcytoembryological observations orprogeny testing. Because ofconflicting results, debate continuesover whether apomixis is controlledby a single locus or by multiple loci.