Chapter 5 Genetic Analysis of Apomixis - cimmyt

98 Jolo.G.(_explain the existence of apomixis must alsoaddress these highly significant peculiaritiesin genome composition, phylogeneticrelatedness, and developmental (ormechanistic) affinities.Other genome-related factors may abnormallyaffect female development in polyploids orpaleopolyploids. For example, meiotic rates arelinearly correlated with DNA content, but theregression line is much steeper in polyploids.And, meiosis in tetraploids usually requires thesame period of time as in related diploidscontaining half the DNA. This occurs becausegenes for meiosis in polyploids are duplicated(Bennett 1977). That the meiotic rate to DNAcontent regression slopes in paleopolyploidsreflect either a diploid or a polyploid conditionmay be critical to the evolutionofapomixis andrelated anomalies, Examples include Scillanonscriptus, 2/1 = 2x = 16, which belongs to ananeuploid series with x = 6 to 9, 15, and 17 asstabilized base numbers, and Convallariamaja/is, 2/1 = 2x = 38, with a sole base numberofx = 19. Both arepaleopolyploid diploids withlarge quantities of DNA, but their meiosesoccur in only50% of the time predicted for nonpaleopolyploiddiploids with similar amountsof DNA, i.e., they behave as polyploids. Incontrast, Omit/roga/um virens (2/1 = 6), which isat the bottom of a descending aneuploid seriesin which x = 3 to 5 and 7, Allium cepa (2/1 = 16),which is at the middle of an aneuploid serieswith x = 7 to 9, and Fritillaria me/eagris (2n =24), which is at the top of an ascendinganeuploid series with x = 7, 9, and 12, arepaleopolyploid diploids with slow meiosesthat is indicative ofdiploids with considerableDNA. Duplicate genes for meiosis in thesespecies have either been lost throughaneuploidy or genetically silenced. Hexaploidnulli 5B tetra 5D wheat is another example.Meiotic rates in this line reflect a tetraploid, nota hexaploid, probably because of imbalancedsets of meiotic genes (Bennett 1977).Several questions relevant to the evolution offemale developmental anomalies can beformulated from this information. Forexample, how is the synchrony of femaledevelopment affected when some of theduplicated genes responsible for megasporogenesis,embryo-sac development, andembryony from one of two genomes aresilenced or lost during diploidization(paleopolyploid formation)? What happenswhen there are duplicate doses of genes forcertain stages of meiosis or embryo sacdevelopment and not other stages, as isanticipated in highly aneuploid paleopolyploidpolysporic species? Could suchimbalances cause some of the anomalies ofembryo sac development observed inpolysporic species, such as a precociousgametophytization of the MMC or theformation of 4 to 32 nucleate embryo sacs?Total quantities of DNA also influence thetypes of life cycles angiosperms assume. Forexample, species of Fritillaria (many of whichare polysporic paleopolyploids) have largeamounts of nuclear DNA and their meiosesmay require 3-4 weeks to complete. Incontrast, annuals have little DNA and veryshort meioses (Bennett 1977), and apomixisand related anomalies are rare among them(Asker and Jerling 1992). Hence, a minimumthreshold in duration of meiosis may be aprerequisite for the evolution of certainreprod~ctiveanomalies.Reproductive anomalies in angiosperms mightalso be influenced by differences in meioticdurations between genders. In cereals, femaleand male meioses are generally synchronousand similar in duration. However, in speciesin which female meiosis occurs later than malemeiosis, differences in duration may be asgreat as 50 % (Bennett 1977). Such differencesmight encourage anomalous development inone gender but not the other.