Abstract
Dioecious species evolved from species with monomorphic sex systems in order to achieve overall fitness gains by separating male and female functions. As reproductive organs, unisexual flowers have different reproductive roles and exhibit conspicuous sexual dimorphism. To date, little is known about the temporal variations in and molecular mechanisms underlying the morphology and reproductive costs of dioecious flowers. We investigated male and female flowers of Salix paraplesia in three flowering stages before pollination (the early, blooming and late stages) via transcriptional sequencing as well as metabolite content and phenotypic analysis. We found that a large number of sex-biased genes, rather than sex-limited genes, were responsible for sexual dimorphism in S. paraplesia flowers and that the variation in gene expression in male flowers intensified this situation throughout flower development. The temporal dynamics of sex-biased genes derived from changes in reproductive function during the different flowering stages. Sexually differentiated metabolites related to respiration and flavonoid biosynthesis exhibited the same bias directions as the sex-biased genes. These sex-biased genes were involved mainly in signal transduction, photosynthesis, respiration, cell proliferation, phytochrome biosynthesis, and phenol metabolism; therefore, they resulted in more biomass accumulation and higher energy consumption in male catkins. Our results indicated that sex-biased gene expression in S. paraplesia flowers is associated with different reproductive investments in unisexual flowers; male flowers require a greater reproductive investment to meet their higher biomass accumulation and energy consumption needs.
Highlights
Sexual dimorphism in dioecious plants refers to sexual differences in primary and secondary sex characteristics[1,2]
At the early and blooming stages, the C:P ratios were higher in female flowers, while the C:N ratios were higher in male flowers
Dimorphism in gene expression patterns in flowers On average, 7.78 G transcriptome data were obtained from the samples of S. paraplesia from the three flowering stages
Summary
Sexual dimorphism in dioecious plants refers to sexual differences in primary and secondary sex characteristics[1,2]. Such divergences between males and females are the results of sexual selection for traits that influence individual fitness during the evolution process[3]. Cai et al Horticulture Research (2021)8:125 because (1) in some wind-pollinated species, males shed their flowers throughout the mating season to increase their pollination success rate[8] and (2) the numbers of male flowers and pollinated female flowers are random, resulting in varying reproductive allocation patterns between males and females at the whole-plant level[9,10]. Female-biased sex ratios are rarer than male-biased sex ratios in dioecious species, and the dominant status of the male-biased sex ratio is the consequence of reproductive sexual dimorphism[12]
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