Dioecious Plant Mercurialis Annua Research Articles

Epigenetic aspects of floral homeotic genes in relation to sexual dimorphism in the dioecious plant Mercurialis annua.

In plants, dioecy characterizes species that carry male and female flowers on separate plants and it occurs in about 6% of angiosperms; however, the molecular mechanisms that underlie dioecy are essentially unknown. The ability for sex-reversal by hormone application raises the hypothesis that the genes required for the expression of both sexes are potentially functional but are regulated by epigenetic means. In this study, proteomic analysis of nuclear proteins isolated from flower buds of females, males, and feminized males of the dioecious plant Mercurialis annua revealed differential expression of nuclear proteins that are implicated in chromatin structure and function, including floral homeotic proteins. Focusing on floral genes, we found that class B genes were mainly expressed in male flowers, while class D genes, as well as SUPERMAN-like genes, were mainly expressed in female flowers. Cytokinin-induced feminization of male plants was associated with down-regulation of male-specific genes concomitantly with up-regulation of female-specific genes. No correlation was found between the expression of class B and D genes and the changes in DNA methylation or chromatin conformation of these genes. Thus, we could not confirm DNA methylation or chromatin conformation of floral genes to be the major determinant regulating sexual dimorphisms. Instead, determination of sex in M. annua might be controlled upstream of floral genes by one or more sex-specific factors that affect hormonal homeostasis. A comprehensive model is proposed for sex-determination in M. annua.

YY males of the dioecious plant Mercurialis annua are fully viable but produce largely infertile pollen.

Summary The suppression of recombination during sex‐chromosome evolution is thought to be favoured by linkage between the sex‐determining locus and sexually antagonistic loci, and leads to the degeneration of the chromosome restricted to the heterogametic sex. Despite substantial evidence for genetic degeneration at the sequence level, the phenotypic effects of the earliest stages of sex‐chromosome evolution are poorly known.Here, we compare the morphology, viability and fertility between XY and YY individuals produced by crossing seed‐producing males in the dioecious plant Mercurialis annua, which has young sex chromosomes with limited X−Y sequence divergence.We found no significant difference in viability or vegetative morphology between XY and YY males. However, electron microscopy revealed clear differences in pollen anatomy, and YY males were significantly poorer sires in competition with their XY counterparts. Our study suggests either that the X chromosome is required for full male fertility in M. annua, or that male fertility is sensitive to the dosage of relevant Y‐linked genes.We discuss the possibility that the maintenance of male‐fertility genes on the X chromosome might have been favoured in recent population expansions that selected for the ability of females to produce pollen in the absence of males.

Do metrics of sexual selection conform to Bateman's principles in a wind-pollinated plant?

Bateman's principles posit that male fitness varies more, and relies more on mate acquisition, than female fitness. While Bateman's principles should apply to any organism producing gametes of variable sizes, their application to plants is potentially complicated by the high levels of polyandry suspected for plants, and by variation in the spatial distribution of prospective mates. Here we quantify the intensity of sexual selection by classical Bateman metrics using two common gardens of the wind-pollinated dioecious plant Mercurialis annua. Consistent with Bateman's principles, males displayed significantly positive Bateman gradients (a regression of fitness on mate number), whereas the reproductive success of females was independent of their ability to access mates. A large part of male fitness was explained by their mate number, which in turn was associated with males' abilities to disperse pollen. Our results suggest that sexual selection can act in plant species in much the same way as in many animals, increasing the number of mates through traits that promote pollen dispersal.

Early Sex-Chromosome Evolution in the Diploid Dioecious Plant Mercurialis annua.

Suppressed recombination allows divergence between homologous sex chromosomes and the functionality of their genes. Here, we reveal patterns of the earliest stages of sex-chromosome evolution in the diploid dioecious herb Mercurialis annua on the basis of cytological analysis, de novo genome assembly and annotation, genetic mapping, exome resequencing of natural populations, and transcriptome analysis. The genome assembly contained 34,105 expressed genes, of which 10,076 were assigned to linkage groups. Genetic mapping and exome resequencing of individuals across the species range both identified the largest linkage group, LG1, as the sex chromosome. Although the sex chromosomes of M. annua are karyotypically homomorphic, we estimate that about one-third of the Y chromosome, containing 568 transcripts and spanning 22.3 cM in the corresponding female map, has ceased recombining. Nevertheless, we found limited evidence for Y-chromosome degeneration in terms of gene loss and pseudogenization, and most X- and Y-linked genes appear to have diverged in the period subsequent to speciation between M. annua and its sister species M. huetii, which shares the same sex-determining region. Taken together, our results suggest that the M. annua Y chromosome has at least two evolutionary strata: a small old stratum shared with M. huetii, and a more recent larger stratum that is probably unique to M. annua and that stopped recombining ∼1 MYA. Patterns of gene expression within the nonrecombining region are consistent with the idea that sexually antagonistic selection may have played a role in favoring suppressed recombination.

Effects of pollination intensity on offspring number and quality in a wind‐pollinated herb

Summary Low pollination intensity may cause low seed set in plant populations and is thought to be responsible for evolutionary transitions from outcrossing to selfing, or from animal to wind pollination. Variation in pollination intensity may also affect seed quality both through its influence on the degree of pollen competition (with lower quality offspring produced under low pollen intensities) and through seed size–number trade‐offs (with plants under low pollination intensity producing fewer but larger seeds). Here, we use a field experiment to examine the effects of pollination intensity on both quantity and quality of progeny. We manipulated pollen receipt to stigmas of the wind‐pollinated dioecious plant Mercurialis annua by varying the distance of females from males. We then compared seed size and number, seedling growth and allocation to male and female function, for the progeny produced by females subjected to different pollination intensities. Our experiment revealed a reduction in pollen load with increasing distance to males, translating into large reductions in the number of seeds produced but only small effects on the performance of offspring. The main effect on offspring quality was through a seed size–number trade‐off, with pollen‐limited females producing fewer but larger seeds, which subsequently performed better. Sons and daughters were affected differently by this trade‐off, pointing to gender‐dependent effects of pollination intensity on progeny performance. Synthesis. Our results highlight the importance of pollination intensity on both the quantity and quality of progeny. Nevertheless, fitness calculations suggest that the enhanced quality of seed produced by pollen‐limited mothers was not sufficient to offset their losses in terms of quantity.

A test of the size-constraint hypothesis for a limit to sexual dimorphism in plants.

In flowering plants, many dioecious species display a certain degree of sexual dimorphism in non-reproductive traits, but this dimorphism tends to be much less striking than that found in animals. Sexual size dimorphism in plants may be limited because competition for light in crowded environments so strongly penalises small plants. The idea that competition for light constrains the evolution of strong sexual size dimorphism in plants (the size-constraint hypothesis) implies a strong dependency of the expression of sexual size dimorphism on the neighbouring density as a result of the capacity of plants to adjust their reproductive effort and investment in growth in response to their local environment. Here, we tested this hypothesis by experimentally altering the context of competition for light among male-female pairs of the light-demanding dioecious annual plant Mercurialis annua. We found that males were smaller than females across all treatments, but sexual size dimorphism was diminished for pairs grown at higher densities. This result is consistent with the size-constraint hypothesis. We discuss our results in terms of the tension between selection on size acting in opposite directions on males and females, which have different optima under sexual selection, and stabilizing selection for similar sizes in males and females, which have similar optima under viability selection for plasticity in size expression under different density conditions.

Phytohormone control of translatable RNA populations in sexual organogenesis of the dioecious plant Mercurialis annua L. (2n = 16)

A cell-free translation system was programmed with total, poly(A), non poly(A) and polysomal RNAs from male and female flowers of this plant with separated sexes. The peptide patterns obtained reflected differences in corresponding translatable RNAs. In total RNA products, three peptides were specific for males, two for females. One of the two male-specific polypeptides of high molecular weight was obtained from poly(A) RNAs and a female-specific one from non poly(A) RNAs. Differences between peptides common to both sexes reflected different concentrations of corresponding messengers. Similar results were obtained with polysomal RNAs. The male-specific RNAs were depending on high endogenous auxin concentrations while the female on active cytokinins. Cytokinin feminization of males induced the female-specific RNAs showing cytokinin action at pretranslational stages. Phytohormone roles are discussed.

Sexual differentiation in Mercurialis annua L. (2n=16): Evidence for a control system of nucleo-cytoplasmic relationships by the study of tyrosyl-tRNAs from ‘developmental mutants’ of male sterility

In ‘developmental mutants’ of staminogenesis carrying sensitive cytoplasm (male sterile strains of the dioecious plant Mercurialis annua), the increasing levels of tRNAs tyr aminoacylation as a function of the degree of sterility confirm the classification of strains previously obtained by means of genetics and physiology: the series begins with fertile males with normal cytoplasm (0.61), restored fertiles (0.74), semisteriles (0.74) and total steriles (0.84), (all with S cytoplasm) and ends with normal females (1.06). The fractionation of tyrosyl-tRNAs shows that two chloroplast tyrosine tRNAs isoacceptors absent in normal fertile staminogenesis and present in female organogenesis are always expressed in strains with S cytoplasm, in the presence or absence of fertility nuclear restorer genes. In S cytoplasm the chloroplast escapes from its normal regulation during sexual differentiation. In restored fertile males (S cytoplasm), the nuclear restorer genes do not suppress but only compensate the chloroplast disfunction that remains of female type.

Differential cytokinin binding to dioecious plant ribosomes

It has been reported that the administration of auxins, gibberellins and cytokinins to plants affects nuclear transcription [l-7]. Other observations sug gest that cytokinins also have an effect at the translational level [8]. Cytokinin binding to the ribosomes of Chinese cabbage [9], wheat germ and tobacco callus [lo] has been described and isolation of the specific receptors has been attempted. Based on our knowledge of sexual differentiation of the dioecious plant Mercurialis annua [ 111, we believe that genetically male meristem is a specific target organ of cytokinins. Exogenous cytokinins provoked obvious changes of these male meristems, including the appearance of certain tRNA species and enzymes specific to female flowers [ 121. The purpose of the present study was to detect differential cytokinin binding to ribosomes in male and female target cells. The binding of BAP, FAP, 2IP and Z, as well as their less active ribosides, were compared in order to detect any relation between physiological action and binding efficiency.

Isolation and localization by histoimmunology of isoperoxidases specific for male flowers of the dioecious species Mercurialis annua L.

Specific anodic isoperoxidases of male flowers of the dioecious plant Mercurialis annua were extracted, partially purified, and injected into rabbits. The serum against these specific peroxidases was used after absorption to localize these male isoenzymes during flower development by means of histoimmunology. (Indirect immunoperoxidase method). The peroxidases were characteristic of microsporogenesis and tapetum differentiation. Their synthesis started at an early stage of male organogenesis. They were not observed in other sites of peroxidase activity of male flowers (tunica, endothecium, filament, vascular anatomy) or in female flowers (embryo sac or nucellus). I propose that these isoenzymes constitute an early and specific marker of male organogenesis in higher plants.

Comparison between four isoaccepting transfer ribonucleic acids and corresponding synthetases in male and female flowers of the dioecious species Mercurialis annua L

Isoaccepting species of leucyl-, valyl-, seryl- and tyrosyl-tRNAs from male and female organs of the dioecious plant Mercurialis annua were compared by co-chromatography on RPC5 columns. Each organ possessed a specific pattern of tRNAs and aminoacyl-tRNA synthetases. Female tRNAs were quantitatively and qualitatively more numerous and the corresponding synthetases were more active. Implications of these data on developmental control are discussed.