Chapter 5 Genetic Analysis of Apomixis - cimmyt

Ge••ti< Alaly.;'.f Apamil;' 67genes been located. No conventionalmorphological, agronomic, or physiologicaltraits are specifically associated with apomixis.The lack of linkage information is hardlyunexpected given the obstacles to traditionalmapping in species that have the barrier tocrossing imposed by apomixis and for the mostpart are alloploids with indistinguishablechromosomes, irregular chromosomeduplication, and secondary economic status.Conventional, unlinked, monogenicallyinherited traits have been used as markers todistinguish maternal from hybrid progeny. Ifthe maternal parent is homozygous for arecessive trait, and the pollen parent ishomozygous dominant, uniformity of progenyfor the maternal marker suggests maternalinheritance. Homozygous or heterozygousdominant markers in the pollen parent havebeen employed to reveal hybrid Fls (Hanna etal. 1970; Hanna and Powell 1973; Dujardin andHanna 1989; Hignight et al. 1991).Isozyme polymorphism has been used tocharacterize variability in apomictic parentsand progeny (Marshall and Downes 1977;Hacker 1988; Cruz et al. 1989; Roy andRieseberg 1989; Bayer et al. 1990; Kojima et al.1991; Poverene and Voigt 1995; Gustine et al.1996; Berthaud, Chap. 2; Leblanc andMazzucato, Chap. 9).Several molecular markers that apparently arelinked with apomixis genes have been found(Leblanc and Mazzucato, Chap. 9; Grimanelliet aI., Chap. 6). Ozias-Akins et al. (1993, 1998)and Lubbers et al. (1994) described a randomamplified polymorphic DNA (RAPD) markerand a sequence-tagged site (STS) markertightly linked with apomixis in Pellniseillmspecies. Gustine et al. (1997) described twoadditional linked RAPD markers in P cilinreand derived a preliminary linkage map of threemarkers with the apospory locus. Leblanc etal. (1995b) prepared three restriction fragmentlength probes (RFLP) from a maize-TripsnCllmdnctylaides F\ population that cosegregatedwith diplospory. They also were linked on thelong arm of chromosome 6 of maize.Biological Tests for ParthenogenesisMatzk (1991a) devised an auxin test fordetecting parthenogenetic capacity.Unpollinated plants are treated with DIC;2,4-0; 2,4,5-T; or CPAA. Parthenogeneticindividuals form grains with a mature embryobut no endosperm. Results are positive forparthenogenetic mutants of nonapomicticspecies (barley, wheat) and for apomictic plantsof apomictic species. The test can be used toscreen for parthenogenetic plants in sexualspecies and to detect sexual plants in apomicticpopulations. It has proven useful incharacterizing Pan pmlensis lines that vary indegree of facultative apomixis (Matzk 1991b;Mazzucato et al. 1996).Naumova et al. (1993) described a cytologicaltest for quantitative analysis ofparthenogenesis in Pan pmlensis. Embryo sacswere isolated mechanically and examined forspontaneous embryogenesis.An ovule culture medium facilitatedidentification of apomixis in diplosporousAllillm II/beros/lm (Kojima and Kawaguchi1989). Up to 80% of apomictic embryos, butno sexual embryos, showed development onthe medium.•Combined Cytological, Progeny,Biological, and Marker TestingWhen used alone, none of the progeny testingmethods discussed above can unequivocallyestablish the reproductive status of every plant.Two or more approaches applied together aremore informative (Naumova et al. 1993;Mazzucato et al. 1996).Whole plant progeny testing views the endproduct of seed formation and is the ultimatetest of whether apomixis is functional.Cytological examination of ovules during