Abstract
BackgroundEvolution of unisexual flowers entails one of the most extreme changes in plant development. Cultivated spinach, Spinacia oleracea L., is uniquely suited for the study of unisexual flower development as it is dioecious and it achieves unisexually by the absence of organ development, rather than by organ abortion or suppression. Male staminate flowers lack fourth whorl primordia and female pistillate flowers lack third whorl primordia. Based on theoretical considerations, early inflorescence or floral organ identity genes would likely be directly involved in sex-determination in those species in which organ initiation rather than organ maturation is regulated. In this study, we tested the hypothesis that sexual dimorphism occurs through the regulation of B class floral organ gene expression by experimentally knocking down gene expression by viral induced gene silencing.ResultsSuppression of B class genes in spinach resulted in the expected homeotic transformation of stamens into carpels but also affected the number of perianth parts and the presence of fourth whorl. Phenotypically normal female flowers developed on SpPI-silenced male plants. Suppression of the spinach C class floral organ identity gene, SpAG, resulted in loss of reproductive organ identity, and indeterminate flowers, but did not result in additional sex-specific characteristics or structures. Analysis of the genomic sequences of both SpAP3 and SpPI did not reveal any allelic differences between males and females.ConclusionSexual dimorphism in spinach is not the result of homeotic transformation of established organs, but rather is the result of differential initiation and development of the third and fourth whorl primordia. SpAG is inferred to have organ identity and meristem termination functions similar to other angiosperm C class genes. In contrast, while SpPI and SpAP3 resemble other angiosperms in their essential functions in establishing stamen identity, they also appear to have an additional function in regulating organ number and identity outside of the third whorl. We present a model for the evolution of dioecy in spinach based on the regulation of B class expression.
Highlights
Evolution of unisexual flowers entails one of the most extreme changes in plant development
Most flowers at the shoot apex developed as normal females, indicating a complete transformation of floral identity from male to female (Figure 2c). These results indicate that SpPI and SpAP3 have prominent roles in the regulation of sexual dimorphism beyond homeotic transformation of single organs
Rumex acetosa, and Silene latifolia all begin floral development with both stamens and carpels. In all of these cases, B and C class floral organ identity genes are expressed early in male and female flowers and are not involved in triggering sexual differentiation [20,22,36,37]. In both Thalictrum dioicum [25] and Spinacia oleracea [26], in which sexual differentiation occurs at the organ inception stage, B and C class floral organ identity are differentially expressed at floral initiation
Summary
Evolution of unisexual flowers entails one of the most extreme changes in plant development. The ABC model for floral development has been extensively tested and applied to a wide variety of angiosperm species and has been found to be broadly conserved on sequence, expression, and functional levels Those few exceptions to the canonical Arabidopsis/Antirrhinum model have been illuminating in understanding the processes involved in the evolution of the present array of floral morphologies [1]. Golenberg and Freeman [18] argued that floral organ identity genes, B and C class genes, will not be instrumental in the sex-determination regulatory process in those species that achieve unisexuality by organ abortion. In those species, altered temporal or spatial expression of these genes will likely be a secondary outcome of the degeneration of the organs. Studies in Thalictrum dioicum [25] and Spinacia oleracea [26,27] demonstrate that some of the B and C class paralogues are alternatively expressed in either male or female flowers
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