Chapter 5 Genetic Analysis of Apomixis - cimmyt

If apomixis is caused by hybridization, theparental phenotypes (divergent floralinduction stimuli and meiotic start times inovules, etc.) may be identified by acombination of phenological and cytologicalstudies. Because many developmentalpathways occur asynchronously in apomicts(Figure 7.1), it may be possible to use molecularprobes to determine if the asynchronoussignals originate from the same genomes ordifferent genomes. For example, do bothgenomes in a tetraploid apomict produce bothmeiotic and embryo-sac development signalsat the same time, or does one genome producemeiotic signals (and not embryo-sacdevelopment signals) at the same time theother genome is producing embryo-sacdevelopment signals (see Figure 7.2)?Many criteria should be considered in testingfor such asynchrony. First, it would bedesirable for the apomict to (i) be allotetraploidwith known sexual diploid progenitors, (ii)contain well-mapped genomes, (iii) beamenable to transposon tagging, (iv) be easilygrown with a short vegetative phase, and (v)have ovules readily accessible to cytologicalanalyses. At present, no apomict meets all ofthese criteria. Second, molecular probes thatrecognize mRNAs specific to differentdevelopmental stages would need to beproduced, and the genes from which they aredeveloped would need to contain adequateintergenomic sequence divergence such thatprobes unique to each genome could beproduced. Such probes may currently be underdevelopment.Portions of meiotic prophase and earlyembryo-sac development occur simultaneouslyin most aposporous apomicts and allTaraxacum- and Ixeris-type diplosporousapomicts (Carman 1997; Peel et al. 1997a). TheHFA theory would be confirmed if thefollowing two conditions are observed: (i) aprobe unique to meiotic prophase of genomeA plus a probe unique to early embryo-sacdevelopment from genome B detect theirrespective genome-specific mRNAs in ovulesfixed and sectioned during meiotic prophase,and (ii) the probe for meiotic prophase fromgenome B does not detect its respectivegenome-specific mRNA product (or viceversa). This would confirm that one genomecodes for meiosis (but not embryo-sacdevelopment) at the same time the othergenome is coding for embryo-sac development.This test would not be valid if the mRNAsynthesis identified by the respective probesis produced in response to transactingregulatory genes.Implications of the HFA TheoryIf the HFA theory is correct, much of theapomixis literature will need to bereinterpreted. This includes interpretations ofhow apomixis evolved, the role of apomixisin evolution, the genetic control of apomixis,and how apomixis might be transferred to (orinduced to occur in) sexual species.Evolution of Apomixis andRelated AnomaliesAccording to HFA theory, many secondarycontacts must have occurred between ecotypes(or closely related species) that had beenisolated from each other for many years alonglatitudinal or other ecological gradients. Majorclimatic shifts could account for suchseconQary contacts (Carman 2000). Thedistribution patterns of most apomicts indicatea Pleistocene origin (Stebbins 1971; Asker andJerling 1992), i.e., the geographic distributionsand centers of diversity of many apomicts arecentered near the margins of the Pleistoceneglaciations, but their ranges often encompassthe ecological ranges of the putative sexualprogenitors, which lie north and south of theglacial margins. Eight major glaciations, whichcovered as much as 20% of the earth's surface,occurred during the Pleistocene. These wereseparated by warm interglacial periods lasting