Rice Genetics IV - IRRI books - International Rice Research Institute

Tripsacum, a close relative of maize, carries a locus conferring diplospory. Kindigeret al (1996) evaluated several apomictic and sexual maize-Tripsacum hybrids for thepresence or absence of Tripsacum chromosomes. Using cytological and molecularanalysis, they associated the control of diplospory with the long arm of chromosome16 of Tripsacum, corresponding to the long arm of chromosome 6 in maize. Blakey etal (2001) exploited a relationship between the expression of apomixis in natural polyploidsof Tripsacum dactyloides and fertility as measured by percent seed set. Theyfound fertility linked to six DNA markers on Tripsacum genetic linkage groups A, D,F, I, H, and L. Linkage group D has a syntenic relationship to the short arm of maizechromosome 5, where markers for apospory in Brachiaria and in Pennisetum map(Pessino et al 1997). Also included in this marked interval is the gene Ameiotic1(Am1), whose function is required for the initiation of meiosis in both micro- andmegaspore mother cells. The absence of expression of Am1 in the female is, in alllikelihood, a prerequisite for the expression of apomixis (Blakey et al 2001).Restricted recombination around apomixis lociDNA marker analysis has shown that recombination around some apomixis loci isrestricted (Ozias-Akins et al 1998, Roche et al 1999, Grimanelli et al 1998a). Whenpearl millet (Pennisetum glaucum), which reproduces sexually, was pollinated withan aposporous wild relative (P. squamulatum), 12 marker loci segregated with aposporyand showed no recombination (Ozias-Akins et al 1998). As recombination is usuallyrestricted in interspecific crosses compared with intraspecific crosses, the restrictedrecombination seen for the apomixis locus and the 12 markers could be coincidental.Alternatively, restricted recombination may have evolved as a mechanism to allowseveral genes needed for apomixis to function together as a single genetic unit. WhenRoche et al (1999) examined the behavior of the same markers in two intraspecificcrosses between aposporous and sexual forms of Cenchrus ciliaris (= P. ciliare[L.]Link), they found that 9 out of the 12 markers were linked to apospory and 8 stillshowed no recombination. These data indicate that the restriction of recombination isunlikely to be coincidental but they do not yet constitute proof that the region ofrestricted recombination contains more than one gene for apomixis.Diplospory in Tripsacum involves both the failure of meiosis and the parthenogeneticdevelopment of the unreduced gametes. One single dominant allele is believedto be responsible for the whole developmental process. Construction of a linkage mapfor the chromosome controlling diplosporous apomixis in T. dactyloides was carriedout in both tetraploid-apomictic and diploid-sexual Tripsacum species using maizerestriction fragment length polymorphism (RFLP) probes (Grimanelli et al 1998a). Ahigh level of colinearity was observed between the Tripsacum chromosome carryingthe control of apomixis and a duplicated segment in the maize genome (chromosomes6L and 8L). In the apomictic tetraploid, a marked restriction of recombinationoccurred compared with the corresponding genomic segment in sexual Tripsacumand maize. This result is again consistent with the idea that apomixis, although inheritedas a single Mendelian allele, might really be controlled by a cluster of linked loci,but the idea needs further substantiation.Molecular tools for achieving synthetic apomixis in hybrid rice 381