Chapter 5 Genetic Analysis of Apomixis - cimmyt

214 Uta Praebll aod Rod ScottCrosses with pollen from different speciesresult in various proportions of matromorphsarising from the parthenogenetic developmentof unreduced egg cells (Eenink 1974b). Thusin Brassica, two of the most importantingredients ofapomixis are revealed by distantpollination: (i) the presence of unreducedembryo sacs, and (ii) the inherent capacity forparthenogenesis. Haploid parthenogenesishas recently been induced both in Arabidopsisand in Brassica juncea by the application ofbrassinolide, a steroid hormone first isolatedfrom Brassica pollen (Kitani 1994).4. Endospenn development. In autonomousapomicts, endosperm development occursspontaneously, but in the case ofpseudogamous apomicts, it depends onfertilization of the central cell nucleus by asperm nucleus. In some cases, pollinationwithout fertilization has been suspected oftriggering endosperm development.Endosperm plays a crucial role in theformation of viable seed, and requires specialconsideration in the design of a mutagenesisscreen. The problem of the endosperm isdiscussed later in more detail.Genetic Control of ApomixisWith perhaps one exception (Carman 1997;Carman, Chap. 7), it is now generally acceptedthat apomixis has evolved from sexualancestors by mutation rather than being aconsequence of polyploidization andheterozygoci ty. This is an importantassumption for mutagenic approaches to thestudy of apomixis. Much discussion focuseson whether apomixis is regulated by a singlegene and could therefore be induced by asingle mutation in a sexual plant. Given thedifferent components of apomixis, it seemsmore likely that a number of mutations wereneeded in the evolution of a viable apomictfrom a sexual ancestor. The apparent singlelocus inheritance that has been reported inseveral cases could be explained by the tightlinkage of several genes, a possibility that isconsistent with the lack of recombinationobserved between molecular markersassociated with apomixis (Grimanelli et a!.,Chap. 6). Even if a Single mutation, perhapsresulting in the inhibition of meiosis, wasresponsible for the evolution of apomixis, it islikely to have occurred in a background thatpermitted its expression (Mogie 1988) and hastherefore evolved only in a subsection of generaand families. For these reasons, we do notexpect that a single mutation in a sexual plantcould result in the production of viable seed inthe absence of fertilization. However, given thevariety of apomictic forms that can bedistinguished, such as in diplospory andapospory, apomixis has probably arisenindependently in different species and perhapsinvolves different genes in each case.Consequently, there may be ampleopportunities for the induction of someelement of apomixis in a sexual plant bymutations in a number of different genes.An important aspect to consider is the apparentdominance, in many cases, of apomixis oversexuality (Nogler 1984; Mogie 1988; Leblancet al. 1995). Mutations that completely abolishgene function, such as deletions, can bedominant only if expression of that gene issubject to gene dosage. One of the hypothesesthat have been put forward on the control ofapomixis is that the responsible gene(s) encoderegulatory functions that initiate or represscertain developmental programs (Koltunow etal. 1995). Ifapomictic development is repressedin sexual plants by a negative regulatoryprotein whose concentration is crucial, then areduction in the level of this protein could besufficient to induce a developmental pathwaythat is suppressed only when two copies of thegene are present. The fact that many apomictsare facultative, and that the proportion ofapomictic progeny can be influenced byenvironmental factors, supports the hypothesis