Chapter 5 Genetic Analysis of Apomixis - cimmyt

IId.cllo••f ApOllllxl. 10 Sex..1Pia.,. by MII......i. 215of a finely tuned regula tory function forapomixis genes.An alternative route to a dominant allele is amutation leading to a change in function of theprotein, e.g., an altered specificity. Mutationsof this type could be very rare indeed.However, many regulatory proteins are activeonly after forming a complex with otherproteins, and changes in many amino acidresidues, particularly those exposed on thesurface, could alter their binding affinity,resulting in reduced activity of the entirecomplex. These possibilities have to be takeninto account in the design of any mutagenesisexperiments, as they may influence the choiceof mutagen (see below).How Important is Polyploidy?The majority of apomicts are polyploid. Ifpolyploidy is a prerequisite for apomixis, ashas been suggested by some researchers,screening for apomictic mutants in a diploidspecies is not a promising proposition.Fortunately, there are a few examples of diploidapomicts, the most relevant to this work beingArabis, a genus comprising species with a rangeof base numbers and ploidy levels (Bacher1951; Carman 1997). Some of the diploids aresexual, while others are pseudogamousdiplosporous apomicts. The observed variationin ploidy of pollen nuclei of sexual andapomictic species, and the occasionaloccurrence of unreduced embryo sacs in thesexual species, form the basis for the evolutionof this agamouscomplex, and support the viewthat polyploidy is a result, rather than aprerequisite, of apomixis. However, accordingto Carman (personal comm.), care should beexercised in assigning a particular ploidy tosexual or apomictic plants because anapparently diploid species may represent adiploidized polyploid (paleopolyploid).Notwithstanding this cautionary note, one ofthe most encouraging examples of apomixisat the diploid or near diploid level is the recentdevelopment of a polyhaploid hybrid betweendiploid (sexual) maize and tetraploid(diplosporous) Tripsacum, containing onecomplete set of chromosomes from each of theparent species. Although male-sterile, thisautoallotriploid hybrid (three genomes)reproduces apomictically after pollinationwith maize, demonstrating that a genetic locusconferring apomixis normally in a tetraploidcan also function in a diploid(-like)background (Leblanc et al. 1996).In addition to being a consequence ofapomixis, polyploidy has an importantfunction in apomicts. As discussed by Nogler,the apospory "factor" or allele in Ranunculusauricomus can only be transmitted by diploidor polyploid gametes, and acts as a recessivelethal factor in haploid gametes. Apparently,the normal (sexual) allele is required for someaspect of gamete formation or function. Thissuggests that, while not essential in anindividual plant that produces unreducedgametes heterozygous for the apomixis allele,polyploidy may be important for themaintenance and spread of apomicticpopulations.The Problem of the EndospermThere is overwhelming evidence for theimportance of a balanced ratio between thematernally and paternally inherited genomesin the endosperm of many species. Endospermis usuplly triploid, resulting from the fusionof two maternal polar nuclei with one pollennucleus, producing a ratio of two maternal toone paternal genome (2m:1 p). Deviation fromthis ratio often leads to embryo abortion or tosubfertile, abnormally shaped seed. The mostplausible explanation for these observationsis the parental imprinting of genes, which,unlike in animals, does not affect the embryobut does affect the endosperm of thedeveloping seed. Various strategies seem tohave evolved during the evolution ofapomixisto ensure normal endosperm development. In