Chapter 5 Genetic Analysis of Apomixis - cimmyt

Model 5'11." to Stody til. Gett.,kl _ Dn........tal Biology of ApotIIIII.117inheritance of that trait can be monitoredthrough each generation, providinginformation on its genetic basis in the systemunder study. Finally, as indicated above, whenthe recipient species is a crop, the product maybe of immediate commercial value. Theprincipal disadvantage of transfer is that theavailability of apomictic relatives typicallydictates the mechanism used. It has alsoproven difficult to incorporate the trait intoobligate sexual crops with a significant levelof expression. The reason for this difficulty isunclear. It may be associated with theinheritance of modifiers or an importantassociation with polyploidy, which is lostduring the backcrossing program. Alternatively,the difficulties encountered may bemore a problem of experimental scale. Mostcrop species do not have a close apomicticrelative so introgression requires widecrossing. This often results in poor fertilityamong the progeny and little, if any, crossingover during meiosis. Large populations needto be formed and assessed for ploidy andapomixis, but traditional methods ofchromosome counting and thin sectioning aretoo labor-intensive to be practical for mostresearch teams. Recent advances in DNAquantification through flow cytometry andanalytical cytology using clearing techniquesare overcoming these difficulties. Over the pasttwo years, researchers in the maizeI Tripsacumprogram have screened more than 200,000plants for chromosome number using flowcytometry (Savidan, personal comm.) and theexperimental populations have beenadvanced to the BC s generation.Development of a Model System from anExisting ApomictApomixis occurs throughout the plantkingdom. Species utilizing various forms ofasexual reproduction that involvegametopnytic structures have been recordedamong the algae, pteridophtyes, and in morethan 400 species of flowering plants from morethan 35 families (Asker and Jerling 1992;Carman 1997).Which is the best to study? Different speciesclearly have different advantages. The uniquebiology of ferns, for example, presents someunusual opportunities to study apomixis(Sheffield and Bell 1981). The events ofmegasporogenesis and megagametogenesisare physically separated in these plants,permitting the study of the individualcomponent processes of apomixis in isolation.Unlike the angiosperm embryo sac, the freeliving fern gametophyte is isolated fromparental influence, and, in some systems,parthenogenetic development of thesporophyte can be induced in vitro (Sheffieldand Bell 1987). Furthermore, the sporogenictissue is relatively exposed in ferns,simplifying the study ofevents associated withthe avoidance of meiosis. It is interesting tonote that Manton (1950) reported thatunreduced spores of Crytomium arose fromtissue immediately adjacent to meiotic tissuein the sporangium, a situation closelyanalogous to the development of aposporousinitials in the angiosperm ovule. Finally, as thefern sporophyte develops without the need foran endosperm, difficulties associated with thedevelopment of that tissue do not arise. Thereare, ofcourse, several limitations in using fernsas model systems, particularly for a molecularstudy of development. They are often largeslow-growing plants that can be difficult tocultivate, and they present some realchallenges when conducting controlledfertilizations with motile sperm cells. Finally,very little is known of the molecular biologyof this group, which would impede progressin any molecular study of apomixis.Apomixis occurs throughout the angiospermsincluding representatives of bothmonocotyledonous and dicotyledonousgenera. Many of the most comprehensive