Sex-Related Gene Network Revealed by Transcriptome Differentiation of Bisexual and Unisexual Flowers of Orchid Cymbidium tortisepalum

Despite extensive research on orchid reproductive strategies, the genetic studies of sex differentiation in the orchid family are still lacking. In this study, we compared three sexual phenotypes of Cymbidium tortisepalum bisexual flowers as well as female and male unisexual mutants. Through comparative transcriptomes, we analyzed the sex-biased differentially expressed genes (DEGs) and gene co-expression networks of sex organs (gynostemium and ovary) among them, identified the candidate genes of sex differentiation, and validated their expression by qRT-PCR. The C. tortisepalum unisexual mutants with degenerated phenotypes were compared to the bisexual plants with respect to both the flower organs and plant morphologies. Totally, 12,145, 10,789, and 14,447 genes were uniquely expressed in the female, male, and hermaphrodite sex organs, respectively. A total of 4291 sex-biased DEGs were detected among them, with 871, 2867, and 1937 DEGs in the comparisons of bisexual vs. female, bisexual vs. male, and male vs. female flowers, respectively. Two co-expressed network modules, with 81 and 419 genes were tightly correlated with female sexual traits, while two others with 265 and 135 genes were highly correlated with male sexual traits. Two female-biased hub genes (CtSDR3b and CtSDR3b-like) nested in the female modules, the homologs of maize sex determinant tasselseed2, may control the feminization of C. tortisepalum. At the same time, two male-biased hub genes (CtYAB2 and CtYAB5) nested in the male modules, the homologs of grape sex determinant VviYABBY3, may control the androphany of C. tortisepalum. This study discovered the molecular regulation networks and proposed a model for orchid sex differentiation, therefore providing for the first time the genetic basis for the sex separation in the orchid family.

Cryptic dioecy in Mussaenda pubescens (Rubiaceae): a species with stigma-height dimorphism

BACKGROUND AND AIMS

Evolutionary transitions from heterostyly to dioecy have been proposed in several angiosperm families, particularly in Rubiaceae. These transitions involve the spread of male and female sterility mutations resulting in modifications to the gender of ancestral hermaphrodites. Despite sustained interest in the gender strategies of plants, the structural and developmental bases for transitions in sexual systems are poorly understood.

METHODS

Here, floral morphology, patterns of fertility, pollen-tube growth and floral development are investigated in two populations of the scandent shrub Mussaenda pubescens (Rubiaceae), native to southern China, by means of experimental and open-pollinations, light microscopy, fluorescence microscopy and scanning electron microscopy combined with paraffin sectioning.

KEY RESULTS

Mussaenda pubescens has perfect (hermaphroditic) flowers and populations with two style-length morphs but only weak differentiation in anther position (stigma-height dimorphism). Experimental pollinations demonstrated that despite morphological hermaphroditism, the species is functionally dioecious. The long-styled (L) morph possesses sterile pollen and functions as a female, whereas the short-styled (S) morph is female sterile and functions as a male. Self- and intra-morph pollinations of the S-morph were consistent with those expected from dimorphic incompatibility. The two populations investigated were both S-morph (male) biased. Investigations of early stages of floral development indicated patterns typical of hermaphroditic flowers, with no significant differences in organ growth between the floral morphs. Meiosis of microspore mother cells was of the simultaneous type with tetrads isobilateral in shape. The tapetal cells in anther walls of the L-morph became vacuolized during meiosis I, ahead of the uninucleate microspore stage in the S-morph. In the L-morph, the microspore nucleus degenerated at the tetrad stage resulting in male sterility. Microsporogenesis and male gametophyte development was normal in the S-morph. Failure in the formation of megaspore mother cells and/or the development of megagametophytes resulted in female sterility in the S-morph, compared with normal megasporogenesis in the L-morph.

CONCLUSIONS

In M. pubescens, cryptic dioecy has evolved from stigma-height dimorphism as a result of morph-specific sterility mutations.