Chapter 5 Genetic Analysis of Apomixis - cimmyt

Ge••ti< loal"is 01 Apomixi. 75having genotype Annnn. However, theobserved ra tio 36:annn:8Annn:5Annaa:2nannn didnot fit the expected ratio 3:2:3:2.Bicknell and Borst (1996) observedsegregation for sexuality and apomixis amongtetraploid regenerants of H. pilosel/n derivedfrom reduced calli of an apomictic biotype.They considered this evidence for dominantinheritance of apomixis. Bicknell (1994, andChap. 8); Koltunow et al. (1995) believeHierncilllli can be a model system for studyingmolecular genetics of apomixis.Mitotic DiplosporyMost reports on diplospory, summarizedbelow, were handicapped by lack of suitablesexual parents and by unavailability ofconvenient classification techniques.Eragrostis curvlIla (weeping Iovegrass,Poaceae). Crosses of naturally occurringtetraploid sexual plants with tetraploid andhexaploid apomicts gave F Iprogeny testsegrega tions ind ica ti ng tha t a pom ix is ismonogenic and dominant (Voigt and Burson1983). Results of a cross with an aneuploidindicated possible dosage effects, but mayhave been confounded by Bill hybridizationor chromosome elimination.Tripsacllm dactyloides (Eastern gamagrass,Poaceae). Sherman et al. (1991) crossed asexual diploid female parent with a triploidapomict. Forty-six hyperdiplOid progeny wereidentified as hybrids. All but two of theseshowed cytological indiCations of apomixis;the degree of diplospory ranged frompredominantly sexual to highly apomictic.The authors believed this indicated thatapomixis is incompletely dominant, or thatminor additive genes on variouschromosomes affect penetrance of apomixis.Recent production of sexual tetraploids bycolchicine doubling should facilitate futurestudy of inheritilnce. The tetraplOid T.dnctyloides parent genotype is simplex for thediplospory allele. Maize-TripsnclIlIl Fjssegregated 1:1 for mode of reproduction(Leblanc et al. 1995b).Partllcllillln argelltatllln (guayule,Asteracae). Gerstel and Mishanec (1950)reciprocilily crossed a sexual diploid (2/1 = 36)with il facultiltively diplospofOushyperdiploid (2/1 = 37). With the sexual plant,~s the milternal parent, all F] diploid hybridprogeny (21 plants) were sexual. In thereciprocal, most progeny, as expected, werematernal or polyploid apomicts, but fourdiploid sexual F1s were formed (Gerstel et al.1953). They concluded that apomixis genesacted recessively but additively andpostulated that polyploids with two ilpomixisgenomes and one sexual genome wereapomictic.There is an alternate interpretation. Thestarting materials and results with Pnrti/(:/Iilllllresemble those of NogleI' (1984b) foraneuploid plant T of Rail I IIICII Ills (Figure 5.1).The Parilic/lilllll results may indicate the samecontrol as in Ralill/lwllls, i.e., a single factordominant for apomixis, that acts as a recessivelethal, with the polyhaploid parent beinggenotype Ann and the diploids being nn.Restitutional DiplosporyTaraxacum (dandelion, Asteracae).Eutriploids (211 = 24) and many hypotriploids(2/1 = 23) have facultative diplospory. Certain2/1 = "23 aberrants are primarily sexual(Sorensen 1958). Mogie (1988) offered thefollowing interpretation of earlier studies.Expression of apomictic phenotype inTI1rt1XnCfllll depends on one or more geneslocated on one chromosome and on dos,lge.At least two copies of the mutant apomixisallele are required to obtain apomixis; theallele prevents meiosis in diplosporousapomicts. The dominance relationshipbetween the wild type and mut,mt allele isdetermined by balance and environmenl.