Chapter 5 Genetic Analysis of Apomixis - cimmyt

30 (\aries f. Crao.differentiating the egg and micropylar-endpolar nucleus. The egg cannot form if the thirdmitotic division does not occur; its precursordefaults to being a polar nucleus. Thus meiosisII and the first embryo-sac mitosis can beskipped without affecting female fertility oroffspring ploidy, as in the Adoxa type of sexualembryo sac, however the last divisions arecritical for female fertility. The details of thenumerical abnormalities observed in Cooperia,and their implications for the evolution ofsexual embryo sacs, are reported elsewhere.Following are the author's interpretations ofthe nine apomictic types of embryo sacs.Ameiotic Developments ofMgagametophytesEndomitosis in the Allium odorum-typewould result from relaxation of thechromosomal double-strandedness checkpointpostulated above for DNA synthesis.Endomitosis can occur in a wide variety ofplant cells, including, most relevantly, theanther tapetum (Crane et al. 1993), which ishomologous to cells in the nucellus.Megasporocyte endomitosis does not affect themorphological course of meiosis; it canprecede any of the sexual types of embryo-sacdevelopment, and it is probablyunderreported. One consideration is how thechromosomes behave after chromatidseparation and secondary replication in thepremeiotic G z nucleus. If they scarcely move,the former sister chromatids would lie in closeproximity as pairing begins in leptotene, and--mttltiva!en!? rarely, if ever, would form.The Taraxacum-type results from induction ofthe meiotic interphase during the prophase ormetaphase of meiosis 1. The contractedchromosomes are thus induced to decondenseand await the second meiotic division.Another possible explanation for theTaraxacum-type is induction of an Elymusstyle"waiting state" (see under Elymus below)during prophase I, with recovery in time toundertake meiosis II, although this inductionwould not necessarily result in a restitutionnucleus. Direct induction of meioticinterphase would likely acceleratemegasporogenesis relative to sexual ovules,whereas entering a "waiting state" wouldcause a significant delay.Induction of meiotic interphase also explainsthe Ixeris-type, but in this case the inductionis superimposed.on a bisporic or tetrasporictype of reduced embryo sac, e.g., the Fritillqriatypein Rudbeckia (Ba ttaglia 1946, 1963).Assuming a checkpoint for chromosomaldouble-strandedness, bisporic and tetrasporictypes lack the second meiotic division, andthus have their own induction of megasporegermination after the first meiotic division(bisporic types) or during the first meioticdivision (tetrasporic types). Accordingly, therestitution nucleus awaits the first embryo-sacmitosis rather than meiosis II in the Ixeris-type,and there is no cross-wall to separate 2n"megaspores."In the Blumea-type, meiotic interphase isinduced in the premeiotic megasporocyte. Itsgenetic basis could be the same as in theTaraxacum-type, and the two types couldcoexist in the same species or individual if thetiming of induction varied. PrOVing theexistence of the Blumea-type is intrinsicallydifficult because it must be shown that therestitutional stage of the Taraxacum-type isabsent. Therefore, measuring the frequenciesof these two types in a polymorphic individualmight not be feasible with microscopictechniques that kill the ovule before it isexamined. The outcome of superimposing theBlumea-type on a bisporic or tetrasporicsexual type remains unknown. It mightsuperficially conform to the Antennaria-type.The Elymus-type appears to result fromincomplete differentiation of themegasporocyte from the nucellus. Not only is