Chapter 5 Genetic Analysis of Apomixis - cimmyt

G...tk holy,1s OfApoonllls 77Savidan (1991a, 1992), Peacock (1993), andothers suggest that a single master gene isresponsible for induction of embryo-sacformation. They view induction as triggeringa cascade of events that requires direction bymany genes with a potential for modifying theend result. Savidan (1989) suggests that severalgenes controlling apomixis may be linked onone small chromosome segment or Iinkat.Jefferson (1993) suggests tha t apomixisinvolves phenotypic mutations at several lociacting together as a non-recombining unit andhaving the appearance of a single gene.Accordingly, classical genetic observations ofsegregations might be less informative thanexpected (Grimanelli et al. 1995, and Chap. 6).Some early studies on Panicoideae interpretedthe data as indicating not only a locus forapospory, but also a second, independent locusthat affected sexuality (Burton and Forbes1960, reinterpreted by Burton 1992; Taliaferroand Bashaw 1966). In the Taliaferro andBashaw (1966) report, the gene was postulatedto be epistatic to the apomixis allele. Hanna etal. (1973) proposed two loci with genes actingadditively to confer sexuality. Theseinterpretations followed observations thatselfing of apparently sexual parents gaveprogeny segregating for mode ofreproduction. The data sets from these threestudies could not be fitted to the singletetraploid apomixis gene model or to any othercited model (Sherwood et al. 1994). Savidan(1982b) noted some rare facultative genotypesof Pal1icum maximum with nearly 90%sexuality. Later, Savidan (1991a, b) proposeda technical explanation for the observation ofapomictic progeny when naturally occurringtetraploid sexual parents were selfed in theearlier studies. He believes the tetraploidparents were facultative apomicts ofgenotypeAaaa with a high frequency of sexualreproduction and that the parents weremistakenly classified sexual because of thelimitations of the sectioning and progenytesting techniques used at the time. However,selfing or crossing Aaaa parents should give51 progeny of 1:3, S:A (Figure 5.1), not thereported ratios of 2.5:1 or 13:3, so the matter isnot yet resolved.Alternatively, the data of Hanna et a!.,Taliaferro and Bashaw, and those of all otherstudies on Panicoideae can be accommodatedin one genetic model that postulates twotetrasomically inherited loci-the A locus witha gene dominant for apomixis (and recessivelylethal), and a Blocus with the dominant alleleepistatic to A (Sherwood et al. 1994).Regulation of DiplosporyRecent studies indicate that segregation fordiplospory in maize-Tripsacum progenyinvolves a single Mendelian factor, withperhaps some modifying factors (Leblanc etal. 1995b; Grimanelli et al. 1995; Savidan et al.1995). The factor appears to be an apomixislinkat. Savidan et at. (1995) stated they "maynow have a series of concrete reasons to believethat apomixis is indeed controlled bysomething more complex than this dominantgene, including at least one recessive factorwhich prevents apomixis expression indiploids." See also Grimanelli et al. (Chap. 6).Regulation of Facultative ExpressionAll apomictic species for which inheritancedata are available show facultative expression.DegreE!'of apomixis could be due to dosage orpenetrance effects of a major gene and / or tomodifying genes. Data for Ramll1culus (Nogler1984b) and Paspalum (Quarin 1986, 1992)indicate that penetrance of the A allele isincomplete; degree of apomixis may increasewith increased number ofA alleles. Quarin andHanna (1980) suggested that a certain geneticthreshold must be reached for apomixis to beexpressed in some Paspalttm. A single dose ofthe A allele is sufficient to support a high levelof apomixis in Pel111isetum (Sherwood et al.1994) and Pal1icum (Savidan 1981).