The gene designated gynomonoecious sex form1 (gsf1) (Blakey et al., submitted) in the homozygous recessive form, in a diploid, confers a high degree of feminization to the otherwise primarily male floral structure of the Tripsacum rachis. It was named after the phenotype of a variant of Tripsacum dactyloides var. dactyloides identified by C. L. Dewald and R. S. Dayton (Crop Sci. 25:715, 1985; Phytologia 57:156, 1985), germplasm accession WW1582 (GSF-I), Woodward, OK. This gene had been previously characterized as a monogenic recessive with the proposed gene name tstr for tassel seed-Tripsacum (Dewald et al., Am. J. Bot. 74:1055-1059, 1987).
The normal (Gsf1) phenotype, typical of most Tripsacum species, has solitary pistillate spikelets in the lower one-eighth to one-third of the raceme and paired staminate spikelets throughout the remaining portion of the raceme. The recessive mutant phenotype (gsf1) has paired pistillate spikelets in the midsection and bisexual spikelets in the terminal portion of the raceme, in addition to the "normal" solitary pistillate spikelets in the lower portion of the raceme.
A diploid (2n=36) F2 population of 113 individuals from the cross WW1582 (gsf1/gsf1) X WW1218 (Gsf1/Gsf1) was scored for segregation of the mutant phenotype. Phenotypic scoring data were combined with the restriction fragment length polymorphism (RFLP) map data set and linkage relationships were determined using the program MAPMAKER. Two-point analysis revealed linkage of gsf1 to two genomic DNA probes. The RFLP loci were approximately 7 cM (tda48) and 9 cM (tnpi286) on either side of the gsf1 locus on linkage "Group I" of the Tripsacum genetic map (Blakey et al., in preparation), where the molecular marker DNA probes were derived from Tripsacum (probe: tda48) and maize (probe: npi286), respectively.
The alteration in floral structure of gsf1 mutant is morphologically similar to the feminizing effect of the tassel seed mutants of maize (Emerson, J. Hered. 11: 65 - 76, 1920; Nickerson and Dale, Ann. Mo. Bot. Gard. 42: 195-212, 1955; Irish and Nelson, Am. J. Bot. 80: 292-299, 1993). Based on this similarity and molecular evidence of probable synteny between the linkage map region of Tripsacum linkage group I and maize chromosome 1 (Blakey et al., submitted; Blakey et al., in preparation), gsf1 may be homoeologous to the tassel seed2 (ts2) gene of maize.
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