To elucidate the mechanism(s) underlying dioecious flower development, the present study analyzed a SUPERMAN (SUP) homolog, SlSUP, which was identified in Silene latifolia. The sex of this plant is determined by heteromorphic X and Y sex chromosomes. It was revealed that SlSUP is a single-copy autosomal gene expressed exclusively in female flowers. Introduction of a genomic copy of SlSUP into the Arabidopsis thaliana sup (sup-2) mutant complemented the excess-stamen and infertile phenotypes of sup-2, and the overexpression of SlSUP in transgenic Arabidopsis plants resulted in reduced stamen numbers as well as the suppression of petal elongation. During the development of the female flower in S. latifolia, the expression of SlSUP is first detectable in whorls 2 and 3 when the normal expression pattern of the B-class flowering genes was already established and persisted in the stamen primordia until the ovule had matured completely. In addition, significant expression of SlSUP was detected in the ovules, suggestive of the involvement of this gene in ovule development. Furthermore, it was revealed that the de-suppression of stamen development by infection of the S. latifolia female flower with Microbotryum violaceum was accompanied by a significant reduction in SlSUP transcript levels in the induced organs. Taken together, these results demonstrate that SlSUP is a female flower-specific gene and suggest that SlSUP has a positive role in the female flower developmental pathways of S. latifolia.

Summary Sex-linked genetic markers of Silene latifolia were mapped to the sex chromosomes with the aim of analyzing their evolutionary origins. SlAP3X/Y has been particularly uncertain for years. Its X-counterpart has not been identified, and very few Y-deletion mutants delete SlAP3Y. In this study, we verified the sex chromosome-linkage of SlAP3X and SlAP3Y through cloning, sequencing, and fluorescent in situ hybridization (FISH). SlAP3X, spanning a region of 1.6 kb, consists of 6 exons and 5 introns. SlAP3Y has 7 exons and 6 introns, the second intron being particularly large (24.4 kb). SlAP3Y spans 31 kb in total. Their Harr-plot analysis and the chromosome positions indicated by FISH using their own probes support the hypothesis that the translocation of SlAP3X/Y has occurred on either chromosome.