Molecular and biological analyses of a homeotic mutant and the regulation of floral organ formation in asparagus

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The gender of asparagus is known to be determined by a male active factor M. Some lines of asparagus cultivars carry a homozygous genotype for M. These lines are practically called supermale, since all of the progeny express male phenotype, when pollens from these lines have been used for pollinaton. Therefore, the establishment of supermale lines of asparagus has been tried, in order to produce seed populations only with male genotype. To isolate the homozygous genotype on M male asparagus plants with andromonoecous phenotype, which are able to seed by self-fertilization, have been used. However, lines with flowers capable of self-fertilization are extremely infrequent, and the establishment of supermale lines strongly depends on cultivars. Asparagus differentiate flowers initially as hermaphroditic morphology, and then mature into female and male flowers by degeneration of the inappropriate floral organ. The identity of floral organs is regulated under the expression of MADS-box genes. Flowers set on a line of 'Gold Schatz' (GS#2) form carpel-like organs at whorl 3, and the reduced expression of class-B genes was found to relate the homeotic change. In addition, floral organs at whorl 3 of 'GS#2' escape the possible degeneration and continue to develop until maturation. The progeny obtained by mating with wild-type asparagus showed only normal phenotype, suggesting that the mutation must be recessive. In future, the establishment of methodology to produce progeny showing the homeotic change may be required. On the other hand, for readily applicable research, we examined diagnostics of female and male asparagus. As a result, the utilization of DNA markers may be efficient for identifying gender of asparagus plants at the juvenile stage. Furthermore, supermale individuals were distinguishable from male individuals by quantitative PCR using the DNA markers, suggesting that supemale carries homozygous genotype on male-activating factor M.