Chapter 5 Genetic Analysis of Apomixis - cimmyt

216 Uta Praektlt.. Rod Scana minority of apomicts, endosperm productionis autonomous, i.e., it does not requirefertilization, and here the endosperm is diploidwith no paternal contribution. However, themajority of apomicts are pseudogamous andrequire fertilization of the central cell. Becausemost apomicts produce normal reduced malegametes, the expected m:p ratio in theseendosperms is 4m:1p, given that the polarnuclei are diplOid. This ratio, however, has notbeen observed in many of the examined cases,and it appears that the correct (ancestral) ratiois obtained in a variety of ways. In Diclumthiumannulatum, the unreduced polar nuclei remainunfused at fertilization and either (i) a reducedsperm fuses with only one polar nucleus andthe other polar nucleus degenerates, or (ii) eachpolar nucleus is fertilized with a reducedsperm (Reddy and d'Cruz 1969). Similarly, inRanuneulus auricomus, two fused unreducedpolar nuclei are fertilized by two reducedsperm (Rutishauser 1954). Acommon strategyamong apomictic grasses is to produceunreduced embryo sacs with four rather thaneight nuclei; the Single polar nucleus isfertilized by a reduced sperm (Warmke 1954).These observations are probably the mostconvincing evidence that the evolution ofbothautonomous and pseudogamous apomictsmust have occurred in a genetic environmentcontaining either pre-adaptations oradditional mutations. Autonomous apomictsmay have evolved in a background where thestringent requirement for a balancedendosperm had been relaxed.It was recently shown that in Arabidopsis thepresence in endosperm of maternal orpaternalexcess, resulting from reciprocal crossesbetween diploid and tetraploid parents,greatly affects seed size (Scottet aI.1998). Seedswith maternal excess (4m:1p) are smaller, butseeds with paternal excess (2m:2p) are largerthan normal. However, these seeds havenormal viability. As indicated earlier, haploidseeds can be induced in Arabidopsis andBrassiea juneea by the application ofbrassinolide to the stigmas of emasculatedflowers. Endosperm formation in these seedsoccurs in the absence of any paternalcontribution, and as expected, the seeds in bothcases are smaller than normal diploid seeds;however, they give rise to stable haplOid plants.These are important observations with respectto the screens for apomixis in Arabidopsis,because they suggest that, should a mutationlead to parthenogenetic embryo development,an imbalance in the m:p ratio of the endospermshould not hinder viable seed production,provided of course that the endospermproliferates and supports seed production. Theeffect on seed size by an imbalancedendosperm could, furthermore, be exploitedfor the screening of mutants. However, amutation resulting in parthenogenetic embryodevelopment does not necessarily induceendosperm proliferation. This may require anadditional stimulus, such as that normallyprovided by pollination or a second mutation.The problem of the endosperm is one of themost important aspects to consider in amutagenesis program, and because of it, wedo not expect to be able to induce viable formsof apomixis by a single round of mutagenesisof sexual plants. The aim of severalmutagenesis and screening programscurrently in progress is to facilitate theidentificlltion of mutants that have only somecharacteristics of apomixis, such asparthenogenesis or autonomous endospermdevelopment, and which do not necessarilyproduce fertile seed in the absence ofpollination.Which Mutagen?The choice of mutagen is an importantconsideration because it determines the typesof mutations obtained; here we need onlydistinguish between the two categories of"change of function" and "loss of function"