Chapter 5 Genetic Analysis of Apomixis - cimmyt

70 Robert T. SherwoodIdentification of Genomes andChromosomes with Apomixis GenesInterspecific or intergeneric crosses haveprovided evidence for apomixis genes in agenome or chromosome unique to one of theparents. Sexual tetraploid vaseygrass(Paspalllnl lIrvillei) with the genomic formulaIIJJ was hybridized with hexaploid P.dilatatllnl biotypes with genomic formulas ofIIJJXX and IlJJXX 2(Burson 1992). Thepentaploid hybrids were facultativeapomicts, indicating a gene(s) controllingapomixis was located in the X genome.Examinations of aneuploids indicate thatgenetic control for apomixis may reside inone chromosome (Sorenson 1958; Nogler1984b; Mogie 1988). This also appeared toexplain introgressive transfer of aposporyinto a sexual Pennisetllnl glallCllnl background(Dujardin and Hanna 1989). It appeared thatthe apomictic 29 chromosome BC) plantreceived 28 chromosomes from pearl milletand one, bearing an apomixis gene, fromPe/wisetllnJ sqllanJlIlatllnl; however, moleculardata of Ozias-Akins et al. (1993) did notsupport this assumption.RFLP markers were used successfully tolocate a portion of a sexual maizechromosome homologous to the TripsaCllnlchromosome bearing the diplospory gene(Leblanc et al. 1995b; Grimanelli et aI.,Chap.6). Three RFLP probes cosegregatedwith diplospory in a maize-TripsaCllnJ F)population that was segregating for mode ofreproduction. The markers also wereassociated with the long arm of maizechromosome 6.Testing InheritanceStarting PointGenetic relatedness, chromosomal balance,and sexuality of the maternal parent are theprimary considerations in selecting parents(Savidan 1990a). Generally speaking, the bestsituation is to use intraspecific crosses;interspecific crosses are a second choice, andintergeneric crosses are a poor third alternative.Interspecific and intergeneric crosses of distantparents usually create abnormalities thatinterfere with reproduction. Selection ofgenetically close parents, however, must betempered with consideration of naturalcrossability rather than arbitrary taxonomicdistinctions. Interspecific crossing has beenrelatively successful among some species of theDichanthillnl / Bothrioclrloa / CapillipedillnJcomplex (de Wet and Harlan 1970) and withinBrachiaria (Valle and Miles, Chap. 10). On theother hand, a composite species such asPotentilla argentea may constitute a complexaggregation of sexual and apomictic biotypesthat do not readily interbreed (Asker 1970a, b).Crosses between plants at the same ploidy levelare preferred in order to reduce meioticdisturbances and sterility. Hybridization acrossploidy levels leads to intermediate, and oftenunstable, chromosome numbers. Triploidhybrids resulting from crossing diploid andtetraploid parents can express apomixis, butthey will not be desirable for further crossingor selfing except for developing aneuploids.Sexual parents should be devoid of apomixisalleles. Savidan (1990a) warns of hiddenapomixis within naturally occurringtetraploids that appear to be sexual. Hesugges,ts that sexuality in natural tetraploidscould be facultative, even if it looks obligate ina progeny test, because of modifying factorsthat influence expression of the apomixis gene.Savidan goes on to recommend creation oftotally sexual tetraploids by colchicinedoubling of sexual diploids.Crossing SchemesAll apomixis inheritance studies published todate have included crosses between sexualmaternal parents and apomictic pollen parentsfollowed by characterization of the Fl' Usually,severaI crosses were made, and fi rst selfed