Golenberg, E.M.: Floral Developmental Regulation and Sexual Lability in Spinach

In dioecious, cultivated spinach, Spinacia oleracea, the presence or absence of reproductive organs is determined by organ primordia initiation rather than by degeneration of existing organ primordia as found in maize, sorrel, and campion. Male and female flowers differentiate morphologically early in floral morphogenesis (stage 2) when sepal primordia are formed. Hence, sexual determination is established before stamen or carpel primordia are formed and appears to follow canalized developmental pathways.

Two lines of evidence demonstrate that sexual dimorphism in spinach is determined by facultative regulation of sex specific genes. First, we have determined by in situ hybridization that spinach B class homologs, SpAP3 and SpPI, are expressed throughout the male floral meristem before stamen primordia are formed, then. subsequently, become restricted to stamen primordia. Expression of either gene is not detectable in any stage of female flower development. This implies that sex determination occurs at or upstream of B class gene expression, and that downstream targets of the B class genes will be differentially expressed in males and females. This also implies that these expression patterns may control the development of sexual dimorphism in spinach. Second, in two cultivars, 30-40% of potentially female plants develop as males when GA₃ is applied, indicating that sex determination can be regulated upstream of the organ identity genes. Furthermore, a third cultivar does not exhibit a male bias in response to GA₃, indicating that genes upstream of the normal organ identity genes determine sex and are polymorphic. These results suggest that sexual lability itself can potentially evolve in this species.