Parthenogenesis maintains male sterility in a gynodioecious orchid

Pollination Syndrome, Florivory, and Breeding System of Satyrium nepalense var. ciliatum (Orchidaceae) in Central Yunnan, China

Research on Satyrium nepalense var. ciliatum (Lindl.) Hook. f. has primarily focused on populations in Northwestern Yunnan, with limited studies on pollination syndromes and insect behavior. In addition, it is geographically limited in its breeding system studies. Here, pollination syndromes, florivory, and breeding systems of S. nepalense var. ciliatum from Liangwang Mountain (Central Yunnan, China) were investigated through field work, microscope, scanning electron microscope (SEM), and parafin section. It was revealed that the pollination syndrome was possessing out-crossing, such as bright color, a developed rostellum, nectar glands in the spur, and food hairs at the lip base. The color and nectar attracted flower visitors, and florivory was observed. Some flower visitors pollinated their companion species. Ants were identified as floral visitors for the first time in Satyrium, although substantial pollination was not observed. Ants might be potential pollinators. S. nepalense var. ciliatum possessed a mixed breeding system, including selfing, out-crossing, and apomixis, with apomixis being predominant in nature. It is suggested that the pollination syndrome, florivory, and pollination competition would contribute to its mixed breeding systems, particularly leading to the occurrence of apomixis.

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.

Exploiting sterility and fertility variation in cytoplasmic male sterile vegetable crops

Cytoplasmic male sterility (CMS) has long been used to economically produce hybrids that harness growth vigor through heterosis. Yet, how CMS systems operate within commercially viable seed production strategies in various economically important vegetable crops, and their underlying molecular mechanisms, are often overlooked details that could expand the utility of CMS as a cost-effective and stable system. We provide here an update on the nature of cytoplasmic-nuclear interplay for pollen sterility and fertility transitions in vegetable crops, based on the discovery of components of nuclear fertility restoration and reversion determinants. Within plant CMS systems, pollen fertility can be rescued by the introduction of nuclear fertility restorer genes (Rfs), which operate by varied mechanisms to countermand the sterility phenotype. By understanding these systems, it is now becoming feasible to achieve fertility restoration with Rfs designed for programmable CMS-associated open reading frames (ORFs). Likewise, new opportunities exist for targeted disruption of CMS-associated ORFs by mito-TALENs in crops where natural Rfs have not been readily identified, providing an alternative approach to recovering fertility of cytoplasmic male sterile lines in crops. Recent findings show that facultative gynodioecy, as a reproductive strategy, can coordinate the sterility and fertility transition in response to environmental cues and/or metabolic signals that reflect ecological conditions of reproductive isolation. This information is important to devising future systems that are more inherently stable.

Changes in polyamine pattern mediates sex differentiation and unisexual flower development in monoecious cucumber (Cucumis sativus L.)

Changes in the levels of polyamines are associated with fundamental physiological processes such as embryogenesis, induction of flowering, fruit development and ripening, senescence, and responses to environmental stresses, but the role of polyamines in sex differentiation and unisexual flower development has not been deeply studied. To extend the knowledge on the regulatory mechanisms of flowering in monoecious plant (producing unisexual flowers), we investigated the morphogenesis and free polyamine levels in Cucumis sativus during sex differentiation and unisexual flower development in vitro using histocytological and biochemical methods. As shown in our study, floral development in vitro was undisturbed and flowers of both sexes were produced. Sex differentiation relied on preventing the development of generative organs of the opposite sex, as we observed carpel repression in male flowers and stamen repression in female flowers. Pollen viability was negatively correlated with female flower development on the same node. Biochemical analysis revealed increased accumulation of aliphatic amines (tri, tetra-amines) in generative (flower buds and flowers) compare to vegetative (axillary buds and leaves) organs. Undifferentiated floral buds contained elevated levels of agmatine, cadaverine, spermidine and spermine. Sex differentiation was associated with significantly decreased levels of agmatine and cadaverine. Our results showed that female flowers contained higher levels of total polyamine than male flowers. The increased level of cadaverine was associated with macrogametogenesis and female flower maturation. Putrescine was important for male flower development. Such results support the hypothesis that aliphatic amines are involved in unisexual flower development.

Gynomonoecy in a mycoheterotrophic orchid Eulophia zollingeri with autonomous selfing hermaphroditic flowers and putatively outcrossing female flowers

Most orchid species exhibit an extreme case of hermaphroditism, owing to the fusion of male and female organs into a gynostemium. Exceptions to this rule have only been reported from the subtribes Catasetinae and Satyriinae. Here, I report an additional orchidaceous example whose flowers are not always hermaphroditic. In several Japanese populations of Eulophia zollingeri (Rchb.f.) J.J.Sm, a widespread Asian and Oceanian orchid, some flowers possess both the anther (i.e., anther cap and pollinaria) and stigma, whereas others possess only the stigma. Therefore, pollination experiments, an investigation of floral morphology and observations of floral visitors were conducted to understand the reproductive biology of E. zollingeri in Miyazaki Prefecture, Japan. It was confirmed that E. zollingeri studied here possesses a gynomonoecious reproductive system, a sexual system in which a single plant has both female flowers and hermaphroditic flowers. In addition, hermaphroditic flowers often possess an effective self-pollination system while female flowers could avoid autogamy but suffered from severe pollinator limitation, due to a lack of agamospermy and low insect-mediated pollination. The present study represents the first documented example of gynomonoecy within Orchidaceae. Gynomonoecy in E. zollingeri may be maintained by the tradeoff in reproductive traits between female flowers (with low fruit set but potential outcrossing benefits) and hermaphroditic flowers (with high fruit set but inbreeding depression in selfed offspring). This mixed mating is probably important in mycoheterotrophic E. zollingeri because it occurs in shaded forest understorey with a paucity of pollinators.

The complete chloroplast genome of a gynodioecious deciduous orchid Satyrium ciliatum (Orchidaceae) female

The chloroplast (cp) genome sequence of a gynodioecious Satyrium ciliatum female type has been characterized using Illumina pair-end sequencing. The complete cp genome was 154,418 bp in length, containing a large single copy region (LSC) of 83,475 bp and a small single copy region (SSC) of 17,513 bp, which were separated by a pair of 26,715 bp inverted repeat regions (IRs). The genome contained 132 genes, with 113 unique genes, including 79 protein-coding genes, 30 tRNA genes, and 4 rRNA genes. The overall GC content is 37.18% with the values of the LSC, SSC, and IR regions are 34.90%, 30.15%, and 43.06%, respectively. Phylogenetic analysis suggested that the S. ciliatum is close to Platanthera japonica (MG925368) in subfamily Orchidoideae.

Geographic consistency and variation in conflicting selection generated by pollinators and seed predators

BACKGROUNDS AND AIMS

Floral traits that attract pollinators may also attract seed predators, which, in turn, may generate conflicting natural selection on such traits. Although such selection trade-offs are expected to vary geographically, few studies have investigated selection mediated by pollinators and seed predators across a geographic mosaic of environments and floral variation.

METHODS

Floral traits were investigated in 14 populations of the bumble-bee-pollinated herb, Pedicularis rex, in which tubular flowers are subtended by cupular bracts holding rain water. To study potentially conflicting selection on floral traits generated by pollinators and florivores, stigmatic pollen loads, initial seed set, pre-dispersal seed predation and final viable seed production were measured in 12-14 populations in the field.

KEY RESULTS

Generalized Linear Model (GLM) analyses indicated that the pollen load on stigmas was positively related to the exsertion of the corolla beyond the cupular bracts and size of the lower corolla lip, but so too was the rate of seed predation, creating conflicting selection on both floral traits. A geographic mosaic of selection mediated by seed predators, but not pollinators, was indicated by significant variation in levels of seed predation and the inclusion of two-, three- and four-way interaction terms between population and seed predation in the best model [lowest corrected Akaike Information Criterion (AICc)] explaining final seed production.

CONCLUSIONS

These results indicate opposing selection in operation: pollinators generated selection for greater floral exsertion beyond the bracts, but seed predators generated selection for reduced exsertion above the protective pools of water, although the strength of the latter varied across populations.

Male interference with pollination efficiency in a hermaphroditic orchid

Hermaphroditism can lead to both intra- and intersexual conflict between male and female gender functions. However, the effect that such gender conflicts have on pollination efficiency has seldom been investigated. By artificially reducing the number of available male gametes on an individual, we quantified whether male interference with pollination efficiency occurs in the self-compatible, moth-pollinated orchid Satyrium longicauda. We partially emasculated S. longicauda inflorescences and compared pollination success and fecundity in these plants to intact controls. Pollen in both groups of plants was colour-labelled so that its dispersal by pollinators could be tracked directly in the field. Intact flowers on partially emasculated inflorescences exported more pollen and received more cross-pollen and less self-pollen than those on intact inflorescences. Proportion of fruit set per plant was similar between the two treatments; however, fruits on partially emasculated plants had proportionally more viable seeds than those on intact controls. These results provide empirical evidence that male interference with pollination efficiency can occur in a hermaphroditic plant and that such interference can compromise fecundity. The most likely mechanism for such male interference is competition for placement on the proboscis of hawkmoth pollinators. Consequently, male competition for siring success may influence the evolution of sexual systems in hermaphroditic pollinator-dependent plants.

О РАСПРОСТРАНЕНИИ ГИНОДИЭЦИИ У ЦВЕТКОВЫХ РАСТЕНИЙ, "БОТАНИЧЕСКИЙ ЖУРНАЛ"

На основании литературных данных и собственных наблюдений составлен новый список гинодиэцичных растений, включающий в себя 1126 видов из 89 семейств покрытосеменных мировой флоры. Анализируются особенности распространения гинодиэции (женской двудомности) среди цветковых растений. В настоящее время у двудольных растений гинодиэция обнаружена у представителей 77 семейств, 278 родов и 1044 видов, а у однодольных — у 12 семейств, 25 родов и 82 видов. Гинодиэция ассоциируется главным образом с многолетними травами, умеренным климатом, энтомофильным опылением.

How much better are females? The occurrence of female advantage, its proximal causes and its variation within and among gynodioecious species

BACKGROUND

Gynodioecy is a reproductive system of interest for evolutionary biologists, as it poses the question of how females can be maintained while competing with hermaphrodites that possess both male and female functions. One necessary condition for the maintenance of this polymorphism is the occurrence of a female advantage, i.e. a better seed production or quality by females compared with hermaphrodites. Theoretically, its magnitude can be low when sterility mutations are cytoplasmic, while a 2-fold advantage is needed in the case of nuclear sterility. Such a difference is often thought to be due to reduced inbreeding depression in obligatory outcrossed females. Finally, variation in sex ratio and female advantage occur among populations of some gynodioecious species, though the prevalence of such variation is unknown.

SCOPE

By reviewing and analysing the data published on 48 gynodioecious species, we examined three important issues about female advantage. (1) Are reduced selfing and inbreeding depression likely to be the major cause of female advantage? (2) What is the magnitude of female advantage and does it fit theoretical predictions? (3) Does the occurrence or the magnitude of female advantage vary among populations within species and why?

CONCLUSIONS

It was found that a female advantage occurred in 40 species, with a magnitude comprised between 1 and 2 in the majority of cases. In many species, reduced selfing may not be a necessary cause of this advantage. Finally, female advantage varied among populations in some species, but both positive and negative correlations were found with female frequency. The role of reduced selfing in females for the evolution of gynodioecy, as well as the various processes that affect sex ratios and female advantage in populations are discussed.

Reduced recombination in gynodioecious populations of a facultative apomictic orchid

Many plants combine sexual reproduction with some form of asexual reproduction to different degrees, and lower genetic diversity is expected with asexuality. Moreover, the ratios of sexual morphs in species with gender dimorphism are expected to vary in proportion to the reproductive success of the sexual process. Hence, sex ratios can directly influence the genetic structure and diversity of a population. We investigated genotypic diversity in 23 populations of a facultative, apomictic gynodioecious orchid, Satyrium ciliatum, to examine the effect on genotypic diversity of variation in the frequency of females and in the amount of sexual reproduction. The study involved one pure female, seven gynodioecious (both females and hermaphrodites present) and 15 hermaphroditic populations. Pollinia receipt was higher in hermaphroditic than in gynodioecious populations. Analyses of variation in ISSRs demonstrated that genotypic diversity was high in all populations and was not significantly different between hermaphroditic and gynodioecious populations. We used character compatibility analysis to determine the extent to which recombination by sexual reproduction contributed to genotypic diversity. The results indicate that the contribution of recombination to genotypic diversity is higher in hermaphroditic than in gynodioecious populations, consistent with the finding that hermaphroditic populations received higher amounts of pollinia. Our finding of reduced recombination in gynodioecious populations suggests that maintenance of sex in hermaphrodites plays an important role in generating genotypic diversity in this apomictic orchid.