Chapter 5 Genetic Analysis of Apomixis - cimmyt

lod"lIo. of ApomIxis 10 SelIai PIoo,. ~y ..'......Is 213been described elsewhere in this volume,therefore only a brief summary is given hereof the individual components of apomicticversus sexual development that could beseparately affected by mutations. Wesummarize some of the earlier work withmutagenesis and describe some of the mostinteresting mutants with elements of apomixisthat have been isolated in various plants. Thefact that none of these potentially usefulmutations has been characterized at the DNAlevel underlines the importance of using wellcharacterizedmodel plants for this work. Thefeasibility of obtaining mutants of Arabidopsiswith apomictic characteristics has beenconfirmed recently by the identification ofseveral mutants with partial seed development.We describe the various approachescurrently underway in several laboratories,which are aimed at the induction of apomicticcharacteristics in model sexual plants.ConsiderationsComponents of ApomixisThe two major types of gametophyticapomixis that can be distinguished, diplosporyand apospory, differ from sexual developmentin more than one aspect (for reviews see Asker1980; Asker and Jerling 1992; Nogler 1984; andCrane, Chap. 3). Since mutants with individualcomponents of apomixis have been identifiedpreviously, and can be expected in futuremutagenesis experiments, the main differencesare summarized briefly. Also, as pointed outbelow, apomictic tendencies are occasionallyobserved in sexual plants and can beinfluenced by environmental stimuli.1. Avoidance of meiosis. In diplosporousapomicts, meiosis is avoided by twoprincipally different mechanisms: in the first,meiosis of the archesporial cell is directlyreplaced by a mitotic division; in the second,failure of chromosome association or synapsisis associated with nuclear restitution, andfollowed by equational division of the entirechromosome complement in the secondmeiotic division. Subsequent developmentleads to the formation of a functionally normalembryo sac with an unreduced egg and centralcell. Sexual plants occasionally produceunreduced gametes, and through fertilizationthese give rise to the polyploid series that canbe observed in many plant species (Asker1980). The production of unreduced embryosacs in some plants, including Brassica species,is revealed after distant or "prickle" pollination(which stimulates parthenogenesis; see below)by the appearance of matromorphous progenyat considerable frequencies (Eenink 1974a).2. Fonnation of aposporous embryo sacs. Inaposporous apomicts, unreduced embryo sacsarise directly by mitosis from nucellar cells,usually in addition to a sexual, reducedembryo sac with normal meiosis. In facultativeapomicts, sexual and apomictic embryo sacsdevelop side-by-side or in separate ovules andgive rise to a mixture of sexual and maternalprogeny. Although female meiosis is mostlynormal, in many cases the aposporous embryosacs, which are not delayed by meiosis,develop at a faster rate than the products ofmeiosis, which frequently degenerate at themegaspore stage.3. Parthenogenesis. The unreduced egg cellin both diplosporous and aposporous embryosacs d~velops directly into an embryo, withoutfertilization by a sperm nucleus. It is not clearwhether fertilization is prevented actively oras a consequence of precocious parthenogenesis.Again, parthenogenesis occasionallyoccurs in sexual plants, and can be induced inmany plants by certain stimuli such as pricklepollination. Of particular interest to the presentinvestigation is the frequent occurrence ofmatromorphy, or diploid parthenogenesis, inBrassica oleracea, a species that belongs to thesame family as Arabidopsis, the Brassicaceae.