Apomixis in Tripsacum: comparative mapping of a multigene phenomenon.

A relationship has been established between the expression of apomixis in natural polyploids of Tripsacum dactyloides and fertility as measured by percent seed set. Thus, fertility may be reliably used as a defining phenotype for apomixis when scoring the progeny from diploid (2n = 2x = 36) x tetraploid (2n = 4x = 72) crosses in Tripsacum. By exploiting the relationship between apomixis and fertility, as defined by seed set, analyses were performed on a set of related second-generation triploid populations segregating for apomixis. These populations were derived from sexual (diploid) x apomictic (tetraploid) crosses. Six out of 25 genome-dispersed restriction fragment length polymorphism (RFLP) markers co-segregate with fertility. Five of these markers were previously reported and include: php20855, tda48, tda53, umc62, and umc83, and are linked to Tripsacum genetic linkage groups F, I, H, L, and A, respectively. Significantly, we report here the syntenic relationships of the maize chromosome intervals to Tripsacum that segregate for numerous meiosis-specific and fertility-associated genes. Utilizing RFLP locus comparative mapping based on conservation of chromosome (genic) regions between related species, it may be concluded that the genes controlling fertility have been preserved in both Tripsacum and maize. A sixth marker, umc166, has also been shown to co-segregate with fertility and is conserved in both grass species. Specifically, umc166 is linked to Tripsacum linkage group D and, by syntenic comparison, to the short arm of maize chromosome 5. Encoded within this marked interval is the gene Ameiotic1 (Am1) whose function is required for the initiation of meiosis in both micro- and megaspore mother cells and whose absence of expression in the female is, in all likelihood, a prerequisite for the expression of apomixis.

Evidence for a common sex determination mechanism for pistil abortion in maize and in its wild relative Tripsacum.

Cultivated maize (Zea mays) and several other members of the Tribe Andropogoneae produce unisexual florets. In maize, the formation of two staminate florets in each spikelet on the tassel and a single pistillate floret in each spikelet on the ear includes a pistil abortion process that requires the action of the TASSELSEED2 gene. In Eastern gamagrass (Tripsacum dactyloides) the GYNOMONOECIOUS SEX FORM1 gene appears to perform a similar role in pistil abortion. These genes were shown to be homeologs by restriction fragment length polymorphism mapping and by the failure of the gsf1 and ts2 alleles to complement one another in intergeneric hybrids. Molecular analysis of the gsf1 allele shows that it is caused by a 1.4-kb deletion mutation. Both TASSELSEED2 and GYNOMONOECIOUS SEX FORM1 show similar expression patterns in subepidermal cells of pistils just before abortion. These results suggest that the formation of staminate florets in the Andropogoneae represents a monophyletic trait.

Co-segregation of the gynomonoecious sex form1 gene (gsf1) of Tripsacum dactyloides (Poaceae) with molecular markers.

The only monogenic trait in Tripsacum to date was first identified in the prolific sex form variant Tripsacum dactyloides (L.) L. forma prolificum Dayton et Dewald. The expression of this trait is controlled by the presence of a single-gene, recessive pistillate mutation hereby designated the gynomonoecious sex form1 gene (gsf1), after the registered plant germplasm accession GSF-I (PI483447) from which it was first identified. This trait confers a high degree of feminization to the primarily male floral structure of the Tripsacum rachis. Two molecular markers were found to co-segregate with the gsf1 gene in a diploid (2n = 36) F2 population of Tripsacum dactyloides, where the female parent (GSF-I) had been previously determined to be homozygous recessive for the gene. Phenotypic scoring data were compared with restriction fragment length polymorphism data and linkage relationships were determined. The gsf1 gene is located ~7 cM from tda48, a Tripsacum-derived molecular marker, and ~9 cM from npi286, a maize-derived molecular marker. The marker npi286 also maps within ~5 cM of the tassel seed2 locus (ts2) of maize, which confers a similar change in the inflorescence of the maize tassel.