Chapter 5 Genetic Analysis of Apomixis - cimmyt

Chapter 7The Gene Effect:Genome Collisions and ApomixisJOHN G. CARMANIntroductionIn the vast majority of angiosperms, femalemeiosis results in the formation of a tetrad ofmononucleate megaspores, of which threedegenerate and one forms the geneticallyreduced8-nucleate female gametophyte(Polygonum-type embryo sac). Consistentlyexpressedcytological deviations from thisnorm occur in certain species in 506 of the13,479 genera of angiosperms recognized bythe Kew Botanical Gardens (Carman 1997).However, most species in most of these 506genera reproduce normally. Thus, thepercentage of species consistently expressingreproductive anomalies (probably < 0.3 %) isfar less than the percentage of genera (3.8 %)currently known to contain anomalous species.Reproductively-anomalous species occur in atleast 184 families, which is 53 % of those inwhich some embryological analyses have beenreported, and are much more abundant insome families than in others (Carman 1997).The reproductive anomalies considered in thischapter generally belong to three categories:gametophytic apomixis, polyspory, andpolyembryony. Gametophytic apomictsproduce unreduced embryo sacs that containparthenogenetic eggs, are generally polyploid,and occur in at least 33 of 460 families ofangiosperms. Diplospory and apospory aretwo major subdivisions of gametophyticapomixis (referred to as apomixis hereafter)and occur when unreduced embryo sacs formprecociously from ameiotic megaspore mothercells (MMCs) or nearby somatic cells,respectively (Asker and Jerling 1992; Carman1997; Peel et al. 1997a, b; Crane, Chap. 3).Polysporic species (bisporic or tetrasporic) aresexual and occur in at least 88 families. As indiplospory, embryo sacs of polysporic speciesform precociously from MMCs, but onlyportions of meiosis not critical to geneticreduction are affected (Battaglia 1989; Johri etal. 1992; Carman 1997). Polyembryonyinvolves the formation of embryos from cellsof other embryos, synergids, antipodals,nucelli, integuments, and even leaves (Tisseratet al. 1979; Johri et al. 1992). Like parthenogenesisin apomicts, polyembryony oftenbegins before pollination (Naumova 1993).This chapter summarizes these anomalies interms of developmental similarities,phylogenetic associations, and gene effecthypotheses, and it discusses implications of thegene effect hypotheses for future research andplant improvement. It concludes that a majorpurging of some widely accepted dogmaconcerning the evol ution and geneticregulation of apomixis will probably occur asthe mechanisms involved give way to moreaccurate evolutionary, developmental, andmolecular characterizations.Developmental Biology andPhylogeny of Reproductively­Anomalous SpeciesAt the developmental level, some apomicticmechanisms resemble sexual polysporicmechanisms more than other apomictic