GENETICS OF GRACILARIA TIKVAHIAE (RHODOPHYCEAE). X. STUDIES ON A BISEXUAL CLONE12

A male done of the red alga Gracilaria tikvahiae McLachlan spontaneously produced a bisexual frond which remained bisexual in subsequent subcultures. Both male and female components of bisexual fronds were functional; however, some unusual results were obtained in crosses. When bisexual fronds were crossed with a normal haploid male, the resulting carpospores all developed into diploid male gametophytes. When bisexual plants were self fertilized, all the carpospores yielded diploid bisexual gametophytes. Only when bisexual plants were crossed to normal haploid females, did carpospores develop into diploid tetrasporophytes as they normally do. The F₁ gametophyte generation obtained from these tetrasporophytes, however, included not only females and males but also bisexual plants, in a 2:1:1 ratio. These results are consistent with the interpretation that bisexual plants have a recessive mutation of a gene other than the primary sex determining locus, and that this mutation is expressed only in male plants. It is suggested that the altered gene may ordinarily have a regulatory function in the maintenance of the dioecious condition.     

Reproductive ecology of the freshwater red alga Batrachospermum delicatulum (Batrachospermales, Rhodophyta) in three tropical streams

Batrachospermum delicatulum specimens from three stream segments were analyzed from a tropical region in south‐ eastern Brazil (20°18′– 20°49′S, 49°13′– 49°46′W). Physical and chemical parameters and the spatial placement of thalli were investigated along with the reproductive characteristics of the gametophytic phase. Sequence data of the cox 2‐ 3 spacer region was also utilized to evaluate genetic variation in individuals within and among stream segments. Gametophyte occurred under relatively diverse environmental conditions, whereas thalli abundance was weakly or not correlated to environmental variables within the stream segments. All specimens examined were dioecious. The ratio of male/female plants was relatively low (0.5 to 1.3) and male plants tended to occur as clumps (two or three plants together). High reproductive success was observed, as indicated by the occurrence of 100% fertilized (carposporophytic) female plants. This is similar to previous reports for this and other dioecious species, which is remarkable considering the relatively low proportion of male/female plants. Results support the two hypotheses to explain the high reproductive success in dioecious species. The occurrence of male plants in clumps was evidence for a strict spatial relationship (i.e. male plants located in upstream position of female plants in order to release spermatia, which would be carried by eddies through female plants). In contrast, the occurrence of male and female plants adjacent to each other allowed outcrossing among neighboring plants with intermingled male and female branches, which seemed more applicable to some situations (low turbulence habitats). The cox 2‐ 3 spacer region from the 18 individuals sequenced was 376 bp and the DNA sequence was identical with no base pair substitutions. Likewise, a previous study of another Batrachospermum species showed that the same haplotypes were present in all stream segments from the same drainage basin, even though the stream segments were a considerable distance apart. Short distance dispersal either by small birds or waterway connectivity might explain these findings.     

Sex ratio and gender stability in the dioecious plants of Israel

The hypothesis that in dioecious plants genders are usually stable and that the sex ratio is 1:l was tested in the dioecious species of Israel. The flora of Israel comprises some 2500 wild species, among which 43 are dioecious, one gynodioecious and one androdioecious. Forty-one out of these 45 species were examined during 1995-97. These species were examined in 120 populations comprising 7019 individuals, at 81 different localities. The majority of the species (33/41) manifested the expected 1:1 sex ratio. Of the eight species deviating from this ratio, a significant bias was repeatedly scored in five of them. Three other species of the willow family manifested sometimes strongly biased sex ratios or even monomorphic (unisexual) stands, apparently as a result of extensive vegetative cloning. The stability of the male and female morphs was also examined in the 41 species. Only eight individuals (from seven species) out of the 7019 individuals surveyed (0.1%) were found to be bisexual. In addition, in 27 populations belonging to 17 species, 909 males were tagged and revisited in fruiting time, and only three of them (0.3%) were found to set fruits. In addition not a single case of gender reversal between seasons was found. The field survey suggests that in dioecious plants sex determination and expression are usually effectively maintained. The very slight deviations from the 1:1 sex ratio and from the distinct male and female morphs suggest that sexual dimorphism in dioecious plants is usually stabile.     

When looks can kill: the evolution of sexually dimorphic floral display and the extinction of dioecious plants

Dioecious plants (with separate male and female individuals) more often have drab, inconspicuous flowers than related bisexual plants. Models indicate, however, that similar conditions favour the evolution of showy floral displays in dioecious and bisexual plants. One difference, however, is that dioecious plants may evolve floral displays that are sexually dimorphic. We show that males are more likely to evolve showy flowers than females in animal-pollinated plants, especially when pollinators are abundant. We demonstrate that this dimorphism places showy dioecious plants at a much higher risk of extinction during years of low pollinator abundance because pollinators may fail to visit female flowers. The higher extinction risk of showy dioecious plants provides an explanation for the fact that dioecious plants that do persist tend to have inconspicuous flowers and are more often wind pollinated. It may also help explain why dioecious plants are less species-rich than related bisexual plants.     

白背叶(大戟科)性系统和传粉生物学研究

为探讨野桐属(Mallotus)雌雄异株的进化和传粉机制,对白背叶(Mallotus apelta)野生居群的性系统和传粉生物学进行了研究。结果表明,所调查的白背叶居群均由雌性(F)、雄性(M)和少数两性(B)个体组成,平均性比为1：0.66：0.18(F：M：B);3种性别植株的基径大小差异不显著;雌株与两性植株的单花胚珠数、单果结籽数和自然结实率无显著差异;雄株与两性植株的花粉萌发率、花粉组织化学和花粉微形态特征也无显著差异,但雄株的单花花粉量是两性植株的1.26~1.63倍,且差异显著;雌株的异交结实率为96.67%,两性植株的异交结实率为76.00%,两者差异显著,说明居群内雌株的潜在种子生产力明显高于两性植株;野外观察到雄株和两性植株上的雄花具有访花者而雌株没有;雌株经套网处理后结实率超过30%而套袋处理不结实。这些表明白背叶具典型的亚雌雄异株性系统,雌株和雄株的适合度均高于两性植株;雌株以风媒传粉结实,两性植株可能兼有风媒和虫媒传粉特征。     

Complex sex determination in the stinging nettle Urtica dioica

The dioecious species Urtica dioica harbours wide variation in sex ratio of seeds. We conducted a series of crosses to analyse the genetic basis of sex determination in this species. Dutch populations of U. dioica contain low proportions of monoecious individuals beside male and female plants. Self-pollination of monoecious plants always yielded female, male and monoecious plants, generally in a ratio of one female to three male/monoecious individuals. This motivated us to write down a simple model in which gender is determined by one major sex-determination locus with four alleles. In the model males and monoecious plants have distinct genotypes but are both heterozygous at the sex-determination locus. We first made crosses among progeny obtained after self-pollination of monoecious plants. These crosses showed that the monoecious trait generally showed Mendelian inheritance and was passed on to the next generation via both pollen and seeds. Further crosses between monoecious plants and plants from dioecious system indicated that alleles from the dioecious system are often dominant. However, many exceptions to our genetic model are observed which suggest that dominance is incomplete and/or that more genes are involved in sex determination. We discuss to what extent sex determination genes explain the strongly biased seed sex ratios and argue that additional genes, for instance genes for female choice, must also be involved.     

Epigenetic control of sexual phenotype in a dioecious plant,Melandrium album

Melandrium album (syn.Silene latifolia) is a model dioecious species in which theY chromosome, present only in heterogametic males, plays both a male-determining and a strict female-suppressing role. We showed that treatment with 5-azacytidine (5-azaC) induces a sex change to androhermaphroditism (andromonoecy) in about 21% of male plants, while no apparent phenotypic effect was observed in females. All of these bisexual androhermaphrodites (with the standard male 24,AA +XY karyotype) were mosaics possessing both male and hermaphrodite flowers and, moreover, the hermaphrodite flowers displayed various degrees of gynoecium development and seed setting. Southern hybridization analysis with a repetitive DNA probe showed that the 5-azacytidine-treated plants were significantly hypomethylated in CG doublets, but only to a minor degree in CNG triplets. The bisexual trait was transmitted to two successive generations, but only when androhermaphrodite plants were used as pollen donors. The sex reversal was inherited with incomplete penetrance and varying expressivity. Based on the uniparental inheritance pattern of androhermaphroditism we conclude that it originated either by 5-azaC induced inhibition ofY-linked female-suppressing genes or by a heritable activation of autosomal female-determining/promoting genes which can be reversed, on passage through female meiosis, by a genomic imprinting mechanism. The data presented indicate that female sex suppression inM. album XY males is dependent on methylation of specific DNA sequences and can be heritably modified by hypomethylating drugs.     

Flower structure and sex expression of leaky dioecy in Eurya obtusifolia

The genus Eurya is described as strictly dioecious, but rare leaky plants have been found in some species, causing much confusion about the sex expression of the genus. Through field investigations from 2009 to 2011, morphologically hermaphroditic flowers in Eurya obtusifolia were discovered. In order to understand the sex expression of these plants, their stamens, pistils and ovules were carefully observed. Staining methods were used to assess the functional gender. The flowers of the leaky plants can be classified into six types: pistillate, staminate, cryptic pistillate, cryptic staminate, hermaphrodite and infertile. Leaky plants usually exhibit a combination of different kinds of flowers, making them either gynoecious, androecious, gynomonoecious or monoecious. Some individuals have infertile flowers. Bagging experiments verified the bisexual function of E. obtusifolia plants, some of which possess the ability to self‐pollinate. The flower morphology of leaky plants varied more than that of dioecious ones, and the number of ovules were significantly negatively correlated with the number of stamens. These plants show lower fitness than normal dioecious plants. This may owe to allocation tradeoffs or sexual genome conflicts. As leaky dioecy is rarely reported in this genus, E. obtusifolia is an important species to study in order to better understand the ecological adaptations and evolutionary pathways that lead to dioecy in Eurya. Our findings provide some evidence that dioecy of Eurya evolved from hermaphroditism, but further studies are still needed.     

Sex-specific physiological and growth responses to elevated atmospheric CO2 in Silene latifolia Poiret

Dioecy is found in nearly half of the angiosperm families, but little is known about how rising atmospheric CO₂ concentration will affect male and female individuals of dioecious species. We examined gender‐specific physiological and growth responses of Silene latifolia Poiret, a widespread dioecious species, to a doubled atmospheric CO₂ concentration in environmentally controlled growth chambers. Elevated CO₂ significantly increased photosynthesis in both male and female plants and by a similar magnitude. Males and females did not differ in net photosynthetic rate, but females had significantly greater biomass production than males, regardless of CO₂ concentrations. Vegetative mass increased by 39% in males and in females, whereas reproductive mass increased by 82% in males and 97% in females at elevated CO₂. As a result, proportionately more carbon was allocated to reproduction in male and female plants at elevated CO₂. Higher CO₂ increased individual seed mass significantly, but had no effect on the number or mass of seeds per female plant. Our results demonstrated that rising atmospheric CO₂ will alter the allocation patterns in both male and female S. latifolia Poiret plants by shifting proportionally more photosynthate to reproduction.     

Inter-sexual competition in a dioecious grass

Spatial segregation of the sexes (SSS) occurs in many dioecious angiosperms, but little data are available on the fitness advantages, if any, for males and females. We examined whether reciprocally transplanted male and female seedlings of Distichlis spicata, a dioecious grass species that exhibits extreme SSS, differed in their responses to microhabitats and competition treatments. Plants grown without conspecific competitors grew equally well in both male- or female-majority habitats, suggesting that male and female plants do not have differential resource needs at the juvenile life-history stage. However, plants subject to intra-sexual competition were significantly larger than plants subject to inter-sexual competition, suggesting that niche partitioning may occur in D. spicata.     

Pollination ecology and reproductive success in Jack-in-the-pulpit (Arisaema triphyllum) in Québec (Canada)

Pollination ecology and reproductive success of Jack-in-the-pulpit ( Arisaema triphyllum ) were studied in two natural populations in Québec, Canada. Individual A. triphyllum plants can be of three types: male, female or bisexual. In both populations studied, the presence of bisexual inflorescences was not negligible (13%), where 'female' and 'male' bisexual plants were categorised according to the relative number of stamens and ovaries. 'Male bisexual' plants produce only pollen and 'female bisexual' plants produce only fruit. Hence, A. triphyllum is a true dioecious plant, as each plant only reproduces through either the male or the female function. 'Female bisexual' plants were equivalent to female plants in terms of visitation rate by insects, fructification rate and production of berries and seeds. Neither agamospermy in female plants nor self-pollination in 'female bisexual' plants was found, thus A. triphyllum relies on insects for cross-pollination. Despite the long flowering cycle, a low visitation rate was documented: only 20–40% of inflorescences were visited, according to gender, by a mean of 1.5 insects. In this study, Mycetophilidae represented the most generically diversified and abundant family, as well as the most efficient insect pollinator, especially the genera Docosia and Mycetophila .     

性别和发育阶段对绒毛白蜡光合特征及叶功能性状的影响

雌雄异株植物资源分配模式上往往表现出显著的性别二态性,但在叶片光合及功能性状上是否有差异目前仍未有定论,且与发育阶段的关系尚不明确。阐明上述问题,能够进一步了解雌雄异株植物的生理生态特征,并为理解性别对性二态植物生长发育的影响机制提供理论依据。以雌雄异株绒毛白蜡(Fraxinus velutina Torr.)为研究对象,针对不同发育阶段不同性别植株进行光合特征及叶功能性状测定,采用双因素方差分析了不同发育阶段下雌雄植株光合能力及叶功能性状的性别间差异,采用Pearson检验了雌雄植株各叶功能性状之间的相关性,并采用标准化主轴分析(Standardized major axis regression, SMA)分析不同性别植株净光合速率与叶功能性状的相关性。结果表明性别和发育阶段显著影响植物个体的光合能力和叶功能性状。总体而言,雄树在坐果期和果实成熟期均表现出更强的净光合速率(Pn)、更高的比叶面积(SLA)、叶绿素含量(Chl)和叶氮含量(LNC);而雌树在果实膨大期表现出更强Pn、SLA和Chl。雌雄性别内Pn与SLA、Chl和LNC间均呈显著正相关(P<0.05),雄树的S...     

ISSR marker-assisted selection of male and female plants in a promising dioecious crop: jojoba (Simmondsia chinensis)

Simmondsia chinensis (Link) Schneider, a multipurpose and monogeneric dioecious shrub from arid zones, has emerged as a cash crop all over the globe. Its seed propagation poses severe problems due to its male-biased population: the male:female ratio is 5:1. Investigations have been carried out to generate a sex-specific Inter-simple sequence repeat (ISSR) marker for the early detection of male and female plants. Of the 42 primers analysed with a bulk sample of pooled male DNA and a bulk sample of pooled female DNA, only one primer, UBC-807, produced a unique ~1,200 base-pair fragment in the male DNA. To validate this observation, this primer was re-tested with individual male and female samples from eight cultivars. A similar unique ~1,200 bp fragment was present in the male individuals of all eight cultivars and completely absent in the female individuals tested. This is the first report of the use of ISSR markers to ascertain sex in physiologically mature S. chinensis plants.     

Dioecious Silene at the X-road: the reasons Y

Among the variety of breeding systems developed by flowering plants, those based on heteromorphic sex chromosomes are the most intellectually challenging in evolutionary terms. This is because, among other things, they enable us to compare sex determination processes between plants and animals. White campion (Silene latifolia, also named Lychnis or Melandrium) is dioecious and, much like us, females are homogametic (XX) and males are heterogametic (XY). Sexual dimorphism in white campion is controlled by two independent developmental pathways operating from the Y chromosome at very early developmental stages and within distinct regions of the flower. In addition, all basic steps in the evolution from the bisexual to the dioecious condition with heteromorphic sex chromosomes are known and available to experimentation in the genus Silene. This group of species has been under scrutiny for more than a century. Such an ideal experimental system enables us to tackle, with novel methodological tools, several classical questions in biology. These include the question of how sexual dimorphism evolved and how dimorphic development is controlled, as well as questions of how sex chromosomes evolve in the absence of meiotic recombination or how male-female genetic conflicts are generated. At the turn of the century, the time is now ripe to have a closer look. Received: 21 September 1999 / Accepted: 11 October 1999     

Evolution of male sterility mechanisms in gynodioecious and dioecious species ofCirsium (Cynareae,Compositae)

The genusCirsium comprises both gynodioecious and dioecious species. The observation of microsporogenesis in female plants ofC. montanum, C. oleraceum, C. palustre andC. spinosissimum shows that the male sterility is due to a degeneration of the tapetum. This degeneration occurs more or less early according to the species and, in the light of these results, a scheme of evolution in the male sterility mechanism is proposed. Furthermore, the male sterility mechanism inC. montanum is very similar to that previously found in female plants of the dioecious speciesC. arvense. This fact enhances the possibility of evolution of the dioecy ofC. arvense from the gynodioecy found in other species. According to these results, a general scheme of evolution of sexes in the genusCirsium is proposed.     

The true sex ratio in European Pseudocalliergon trifarium (Bryophyta: Amblystegiaceae) revealed by a novel molecular approach

Dioecious plants, including many bryophytes, rarely exhibit discernible sexual dimorphism before sexual maturity. Because many species and populations of dioecious bryophytes do not express their sex, it remains mostly unresolved whether expressing individuals reflect the ratios of genetically male and female plants. The present study assesses the population sex ratio of the wetland moss Pseudocalliergon trifarium in central and northern Europe. For the first time in a bryophyte, we estimate the sex ratio in a population by assessing directly both expressing and non‐expressing plants. Expressed gender ratio was assessed from herbarium specimens. Single shoots from non‐expressing specimens were sexed using a recently developed molecular sex marker. On the basis of the female and male frequencies in these two data sets and the overall proportion of expressing specimens, we estimate the European population sex ratio to be 1.93 : 1 (female/male). Expressed, non‐expressed, and population sex ratios are not significantly different from each other, suggesting that gender differences in rates of sex expression cannot account for the female bias. Earlier studies of P. trifarium failed to reveal gender‐specific growth rates or pre‐zygotic reproductive costs. Gender differences at the spore to protonemal stage, in mortality, or niche preferences could potentially explain the uneven sex ratio. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 132–140.     

Programmed cell death promotes male sterility in the functional dioecious Opuntia stenopetala (Cactaceae)

Background and Aims

The sexual separation in dioecious species has interested biologists for decades; however, the cellular mechanism leading to unisexuality has been poorly understood. In this study, the cellular changes that lead to male sterility in the functionally dioecious cactus, Opuntia stenopetala, are described.

Methods

The spatial and temporal patterns of programmed cell death (PCD) were determined in the anthers of male and female flowers using scanning electron microscopy analysis and histological observations, focusing attention on the transition from bisexual to unisexual development. In addition, terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling assays were used as an indicator of DNA fragmentation to corroborate PCD.

Key results

PCD was detected in anthers of both female and male flowers, but their patterns differed in time and space. Functionally male individuals developed viable pollen, and normal development involved PCD on each layer of the anther wall, which occurred progressively from the inner (tapetum) to the outer layer (epidermis). Conversely, functional female individuals aborted anthers by premature and displaced PCD. In anthers of female flowers, the first signs of PCD, such as a nucleus with irregular shape, fragmented and condensed chromatin, high vacuolization and condensed cytoplasm, occurred at the microspore mother cell stage. Later these features were observed simultaneously in all anther wall layers, connective tissue and filament. Neither pollen formation nor anther dehiscence was detected in female flowers of O. stenopetala due to total anther disruption.

Conclusions

Temporal and spatial changes in the patterns of PCD are responsible for male sterility of female flowers in O. stenopetala. Male fertility requires the co-ordination of different events, which, when altered, can lead to male sterility and to functionally unisexual individuals. PCD could be a widespread mechanism in the determination of functionally dioecious species.     

Variation in inflorescence size in a dioecious fig tree and its consequences for the plant and its pollinator fig wasp

The host-specific relationship between fig trees (Ficus) and their pollinator wasps (Agaonidae) is a classic case of obligate mutualism. Pollinators reproduce within highly specialised inflorescences (figs) of fig trees that depend on the pollinator offspring for the dispersal of their pollen. About half of all fig trees are functionally dioecious, with separate male and female plants responsible for separate sexual functions. Pollen and the fig wasps that disperse it are produced within male figs, whereas female figs produce only seeds. Figs vary greatly in size between different species, with female flower numbers varying from tens to many thousands. Within species, the number of female flowers present in each fig is potentially a major determinant of the numbers of pollinator offspring and seeds produced. We recorded variation in female flower numbers within male and female figs of the dioecious Ficus montana growing under controlled conditions, and assessed the sources and consequences of inflorescence size variation for the reproductive success of the plants and their pollinator (Kradibia tentacularis). Female flower numbers varied greatly within and between plants, as did the reproductive success of the plants, and their pollinators. The numbers of pollinator offspring in male figs and seeds in female figs were positively correlated with female flower numbers, but the numbers of male flowers and a parasitoid of the pollinator were not. The significant variation in flower number among figs produced by different individuals growing under uniform conditions indicates that there is a genetic influence on inflorescence size and that this character may be subject to selection.     

Gender in plants: sex chromosomes are emerging from the fog

Although most plants have flowers with both male and female sex organs, there are several thousands of plant species where male or female flowers form on different individuals. Surprisingly, the presence of well-established sex chromosomes in these dioecious plants is rare. The best-described example is white campion, for which large sex chromosomes have been identified and mapped partially. A recent study presented a comprehensive genetic and physical mapping of the genome of dioecious papaya. It revealed a short male specific region on the Y chromosome (MSY) that does not recombine with the X chromosome, providing strong evidence that the sex chromosomes originated from a regular pair of autosomes. The primitive papaya Y chromosome thus represents an early event in sex chromosome evolution. In this article, we review the current status of plant sex-chromosome research and discuss the advantages of different dioecious models.     

细枝柃性别表达特性及资源分配

已有的资料将柃木属(Eurya)描述为严格的雌雄异株植物, 性别变异现象极为少见。目前仅在柃木(E. japonica)和钝叶柃(E. obtusifolia)等少数种类中报道过两性花的存在。近几年笔者发现细枝柃(E. loquaiana)存在性别变异现象, 性别变异株上具有不同性别类型的花。该文从单花和植株水平分析了细枝柃的性别表达特性, 并对不同类型花的花部构件生物量分配进行比较分析。结果表明, 细枝柃具有6种类型的花, 从单花水平上看, 细枝柃性别有雌性、雄性及两性3种类型; 细枝柃性别在植株水平上体现较为复杂, 有雌株, 雄株, 雌花和两性花同株, 雄花和两性花同株, 雌雄异花同株及雌花、雄花、两性花同株6种类型; 在细枝柃花部构件生物量分配中, 雄花(包括雄株花和变异株雄花)花部构件生物量分配中雄蕊生物量的分配低于雌花(包括雌株花和变异株雌花)中雌蕊生物量的分配; 两性花中, 雄蕊生物量分配低于雌蕊, 这是其优化资源分配的手段, 进而获取最大适合度收益。