Chapter 5 Genetic Analysis of Apomixis - cimmyt

00,,11k.1Io1 of ApIIIlktl< M.d'.II,.., 31its callosic wall deficient, but the pattern ofvacuolation and the occasional presence of anoxalate crystal in the megasporocyte conformto typical behaviors of the adjacent nucellarcells. Theex ten t to which this similari ty causesthe wall to be deficient is not clear, nor is thedegree to which the deficiency prevents anecessary physiological isolation for fulldifferentiation of the megasporocyte. Thecytoskeleton of the megasporocyte isdisturbed, as evidenced by frequently obliquedivision planes and the deformation of thenucleus. Perhaps the meiotic spindleapparatus is not completely suppressed, andso interacts with the preapomeiotic nucleus.But this does not explain the seconddeformation in the chalazal 211 megaspore.Another feature of the E1ymus-type is the needfor parallel control of development. Themegasporocyte enters a "waiting" state andvariably emerges in time to undertake meiosisII, the first embryo-sac mitosis, or anintermediate condition, respectivelyaccounting for dyads, directly binucleate sacs,or hemidyads. During the "wait," a master"clock" continues to run, albeit abnormallyslowly in comparison to the Polygonum-typein related sexual genotypes; this clock allowsrecovery from the wait.The Antennaria-type can be explained in twoways: by induction of megaspore germinationin the megasporocyte or as a much belatedrecovery from an Elymus-style waitingcq'"ndition. The frequently abnormalehlargement and vacuolation of themegasporocyte are consistent with the latterexplanation, but intraspecific polymorphismfor the Antennaria- and Hieracium-types inPatentilla and Paa is consistent with the formerexplanation. Details of gene expression ingerminating Polygonum-type megasporesshould provide clues as to which examples ofthe Antennaria-type result from megasporegermination. It should be noted that thetetrasporic sexual types also involve inductionof megaspore germination during meiosis I,and that having tetrasporic sacs does not seemto predispose individual taxa to evolve theAntennaria-type or vice versa. Thus different,separately inducible aspects of megasporegermination might impact meiosis in differentways, or the event that calls up megasporegermination might fun