Morphological variation in the flowers of Jacaratia mexicana A. DC. (Caricaceae), a subdioecious tree

The Caricaceae is a small family of tropical trees and herbs in which most species are dioecious. In the present study, we extend our previous work on dioecy in the Caricaceae, characterising the morphological variation in sexual expression in flowers of the dioecious tree Jacaratia mexicana . We found that, in J. mexicana , female plants produce only pistillate flowers, while male plants are sexually variable and can bear three different types of flowers: staminate, pistillate and perfect. To characterise the distinct types of flowers, we measured 26 morphological variables. Our results indicate that: (i) pistillate flowers from male trees carry healthy-looking ovules and are morphologically similar, although smaller than, pistillate flowers on female plants; (ii) staminate flowers have a rudimentary, non-functional pistil and are the only flowers capable of producing nectar; and (iii) perfect flowers produce healthy-looking ovules and pollen, but have smaller ovaries than pistillate flowers and fewer anthers than staminate flowers, and do not produce nectar. The restriction of sexual variation to male trees is consistent with the evolutionary path of dioecy from hermaphrodite ancestors through the initial invasion of male-sterile plants and a subsequent gradual reduction in female fertility in cosexual individuals (gynodioecy pathway), but further work is needed to confirm this hypothesis.     

Flower structure and potential bisexuality in Freycinetia reineckei (Pandanaceae), a species of the Samoa Islands

In Freycinetia reineckei the staminate flower (on the staminate spikes) comprises 3 or 4 (sometimes 2) stamens and a pistillode with 2 (sometimes 4) carpellodes, and the pistillate flower (on the pistillate spikes) is formed of a pistil with 2 (sometimes 4) carpels and of 3 or 4 (sometimes 2) staminodes. This perfect floral homology, also observed in all the other species that were studied with both pistillate and staminate material, strongly suggests that the flower of Freycinetia is basically and potentially bisexual, and may explain the occasional sexual lability and bisexuality of that flower (occurrence of both pistillate and staminate inflorescences, and/or of bisexual inflorescences with bisexual flowers and/or unisexual flowers, on the same individuals) in some species, and also the frequent occurrence of bisexual spikes in this species. These may be partitioned into pistillate, staminate, mixed and sterile zones. In the pistillate zones the flowers have the same aspect and structure as the pistillate flowers. In the staminate zones the flowers generally comprise 3 or 4 (sometimes 2) stamens and a ‘semi-pistil’ some have both stamens and staminodes. The semi-pistils are intermediate between pistils and pistillodes in length, aspect and structure, but always have placentas and ovules. In the mixed zones the flowers are generally formed of a pistil and 3 or 4 (sometimes 2) stamens, and are therefore true hermaphrodite flowers; some have both stamens and staminodes. In the sterile zones the flowers comprise a semi-pistil and 3 or 4 (sometimes 2) staminodes. The staminodes are anatomically very similar to the stamens, especially in the staminate, mixed, and sterile zones, in which they exhibit a wide range of variation in length, aspect and structure. The perfect floral homology as generic character on one hand, and the occasional bisexuality both with and without bisexual flowers and other aspects of sex expression (e.g. occurrence of both pistillate and staminate shoots on the same individuals) in some species on the other hand, seem to indicate that Freycinetia is a basically monoecious, sex changing genus.     

FUNCTIONAL DIOECY AND ANDROMONOECY IN SOLANUM

Field and laboratory studies of 19 diclinous species endemic to Australia help to clarify the nature and evolution of andromonoecy, androdioecy, and dioecy in the genus Solanum. Ten species are andromonoecious; typically these species bear inflorescences with a single, large basal hermaphroditic flower and 12–60 distal, smaller staminate flowers. We suggest that the andromonoecious condition was derived from hermaphroditic-flowered ancestors in part by hemisterilization of flowers but largely by addition of staminate flowers. The resultant larger inflorescences are hypothesized to serve both to attract and to entrain pollinators, yielding more or higher-quality seed set in hermaphroditic flowers and/or greater dispersion of pollen from staminate flowers. We suggest that andromonoecy may also serve to reduce selling. Nine other species are morphologically androdioecious but functionally dioecious. In these species, staminate flowers, like those of the andromonoecious species, bear anthers with copious tricolporate pollen and a highly reduced gynoecium. The morphologically hermaphroditic flowers are functionally pistillate and borne singly in inflorescences, and they bear anthers with inaperturate pollen. The inaperturate pollen, although viable, never germinates and is hypothesized to be retained in pistillate flowers as a reward to pollinators in the nectarless Solanum flowers. All other species of Solanum studied with pollen dimorphism in which one pollen morph is inaperturate are also best treated as functionally dioecious. We conclude that there is no evidence for androdioecy in Solanum. A review of other families suggests that there is little support for this unusual breeding system in any other angiosperm group either. Preliminary analyses suggest that andromonoecy and dioecy are polyphyletic in Solanum. Furthermore, dioecy is as likely to have arisen from hermaphroditic as from andromonoecious ancestors.     

MORPHOLOGY,VASCULAR ANATOMY AND EMBRYOLOGY OF PISTILLATE AND STAMINATE FLOWERS OF ASPARAGUS OFFICINALIS

Flowers of three pistillate (female), two heterogametic staminate (male) and two homogametic male genotypes of Asparagus officinalis L. were compared for morphology and vascular anatomy of the flower and for embryological development to the stage of mature ovules and pollen. Flowers are liliaceous, the staminate with rudimentary pistils and the pistillate with collapsed anthers. The uncomplicated vascular pattern differs between staminate and pistillate flowers only in the size and degree of maturation of bundles to stamens and carpels. Longer styles appear to be correlated with a greater extent of ovule development in ovaries of staminate flowers. Microsporogenesis in males is normal with wall development corresponding to the Monocotyledonous type. The tapetum is glandular and binucleate, cytokinesis successive, the tetrads isobilateral or occasionally decussate, and the mature pollen grain two-celled. A pair of heteromorphic, possibly sex, chromosomes was observed in heterogametic male plants. Anther development is initially the same in pistillate flowers, but the tapetum degenerates precociously followed by collapse of microspore mother cells. In pistillate flowers the ovules are hemitropous, bitegmic, and slightly crassinucellate. Megasporogenesis-megagametogenesis conforms to the Polygonum type. In staminate flowers ovule development is like that in pistillate flowers until degeneration starts in nucellar and integumentary cells at the chalazal end. Ovules in both homogametic male genotypes rarely complete meiosis, while in the heterogametic males it is normally completed with about one ovule in 20 flowers forming a mature megagametophyte. Since manipulation of sex expression in Asparagus could be important in developing inbred male and female lines for breeding purposes, those aspects of the morphological and embryological observations presented which might be useful in planning experiments to induce sex changes are discussed briefly.     

Pistillate and staminate flower development in dioecious Pistacia vera (Anacardiaceae)

The development of staminate and pistillate flowers in the dioecious tree species Pistacia vera L. (Anacardiaceae) was studied by scanning electron microscopy with the objective of determining organogenetic patterns and phenology of floral differentiation. Flower primordia are initiated similarly in trees of both sexes. Stamen and carpel primordia are initiated in both male and female flowers, and the phenology of organ initiation is essentially identical for flowers of both sexes. Vestigial stamen primordia arise at the flanks of pistillate flower apices at the same time functional stamens are initiated in the staminate flowers. Similarly, a vestigial carpel is initiated in staminate flowers at the same time the primary, functional carpel is initiated in pistillate flower primordia. Differences between the two sexes become apparent early in development as, in both cases, development of organs of the opposite sex becomes arrested at the primordial stage. Male flowers produce between four and six mature functional stamens and female flowers produce a gynoecium with one functional and two sterile carpels.     

Flower morphology and biology of Myrsine laetevirens, structural and evolutionary implications of anemophily in Myrsinaceae

This study deals with the phenology, pollination biology and floral morphology of Myrsine laetevirens , a neotropical dioecious tree. In Punta Lara (Argentina), its flowering period occurs during January-February. Both pistillate and staminate flowers are small, with a yellowish-green perianth and produce neither nectar nor odour. Staminate flowers have five stamens and a vestigial gynoecium while pistillate flowers, have non-functional anthers and a conspicuous stigma. The floral characteristics of staminate and pistillate plants are related to the syndrome of anemophily. Fruit set in inflorescences covered with mesh bags and observations prove that animals are not involved in the pollination process. A provisional cladistic analysis of Myrsinaceae shows that dioecy evolved as one of the most recent apomorphies of Myrsine and is part of the anemophilous syndrome.     

Comparative floral development in male and female plants of Palmer amaranth (Amaranthus palmeri)

Premise

Characterizing the developmental processes in the transition from hermaphroditism to unisexuality is crucial for understanding floral evolution. Amaranthus palmeri, one of the most devastating weeds in the United States, is an emerging model system for studying a dioecious breeding system and understanding the biological traits of this invasive weed. The objectives of this study were to characterize phases of flower development in A. palmeri and compare organogenesis of flower development in female and male plants.

Methods

Flower buds from male and female plants were dissected for light microscopy. Segments of male and female inflorescences at different stages of development were cut longitudinally and visualized using scanning electron microscopy.

Results

Pistillate flowers have two to three styles, one ovary with one ovule, and five obtuse tepals. Staminate flowers have five stamens with five acute tepals. Floral development was classified into 10 stages. The distinction between the two flower types became apparent at stage four by the formation of stamen primordia in staminate flowers, which developed female and male reproductive organs initially, as contrasted to pistillate flowers, which produced carpel primordia only. In staminate flowers, the putative carpel primordia changed little in size and remained undeveloped.

Conclusions

Timing of inappropriate organ termination varies across the two sexes in A. palmeri. Our study suggests that the evolution of A. palmeri from a cosexual ancestral state to complete dioecy is still in progress since males exhibited transient hermaphroditism and females produced strictly pistillate flowers.     

Cryptic dioecy and leaky dioecy in endemic species of Dombeya (Sterculiaceae) on La Re´union

The high frequency of dioecy on oceanic islands such as Hawaii and New Zealand has attracted a great deal of attention from plant evolutionary biologists. One reason suggested for the high prevalence of dioecy on oceanic islands is that taxa considered truly dioecious may have occasional hermaphrodite flowers, i.e., show leaky dioecy. In this study, we quantified the presence and distribution of leaky dioecy in a group of congeneric endemic species of the genus Dombeya (Sterculiaceae) on La Re´union island (Indian Ocean). All eight species show cryptic dioecy. Five species show strict dioecy and three species show leaky dioecy due to the presence of male trees that set fruit. Species with strict dioecy and large populations tend to occur in mid- to high-altitude moist tropical cloud forest, whereas species in smaller populations at lower altitude and in semidry tropical forest tend to show leaky dioecy. Two reasons for this differential distribution of strict dioecy and leaky dioecy are discussed. First, environmental variation along the altitudinal gradient, biotic and/or abiotic, may influence the breeding system. Second, leaky dioecy may be favored in lowland populations due to the small size and disturbed nature of such populations.     

Heterochrony and its role in sex determination of cryptically dioecious Consolea (Cactaceae) staminate flowers

Ovule development, megasporogenesis, and megagametogenesis were studied in six cryptically dioecious species of Consolea. All species showed uniform development typical for the Opuntioideae. Ovule development proceeds acropetally, but shows developmental asynchrony across floral morphs. At anthesis, female morph ovules are functional and available for fertilization, whereas staminate flower ovules are senescing and incapable of being fertilized. In occasional plants of some species, staminate flowers may reach anthesis with a few functional apical ovules capable of seed formation. Such plants are described as inconstant/leaky males. Ovule fertility differences across morphs are interpreted as resulting from heterochronic ovule development and senescence, although variation in embryo sac longevity cannot be ruled out. Significantly, ovule abortion follows a common pattern and timing in staminate flowers of both male morphs in all species. Thus, on the basis of this uniformity, a common origin for the cryptically dioecious breeding system in Consolea is hypothesized. Furthermore, staminate expression in Consolea appears to be controlled by a common, genetically determined heterochronic ovule developmental programme affecting the relative timing of ovule receptivity and flower opening. This is the first report of heterochrony as a mechanism of male sex determination.  © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society , 2008, 156 , 305–326.     

Gender variation in Actinidia deliciosa,the kiwifruit

Summary Flower and fruit characters were measured in ten female, five male and five fruiting male selections of A. deliciosa var deliciosa (A. Chev) Liang and Ferguson. Flowers from female vines had functional pistils, which contained many ovules. Stamens appeared to be fully developed but produced only empty pollen grains. Flowers from male vines had functional stamens that produced high percentages of pollen grains with stainable cytoplasmic contents. Pistils did not contain ovules and were generally small with vestigial styles. Fruiting male vines had both staminate and bisexual flowers. Staminate flowers were similar to those found on strictly male vines. Bisexual flowers produced ovules and stainable pollen. Pistils were smaller than in pistillate flowers. Although the three flower sexes differed in style length, ovary dimensions and ovules per carpel, staminate and bisexual flowers were similar in number of flowers per inflorescence, stamen filament length, pollen stainability, inflorescence rachis length and carpel number, and differed from pistillate flowers in these characters. The three flower sexes had similar sepal and petal numbers. The fruit of fruiting males were considerably smaller than those of females. Low ovule number appears to be the major factor limiting fruit size in the fruiting males studied. Prospects for developing hermaphroditic kiwifruit cultivars through breeding are discussed.     

Leaky dioecy in <Emphasis Type="Italic">Diospyros</Emphasis> (Ebenaceae) endemic to the Island of Mauritius

Dioecy, a rather rare phenomenon in the plant kingdom seems to be more prevalent on oceanic islands. The high incidence of dioecy on these islands could result from dioecious colonists among which a small percentage show leaky dioecy, which is an ability to self-fertilise. In this study, we report the occurrence of leaky dioecy in one of the 11 extant Diospyros species endemic to Mauritius. Female flowers on the leaky dioecious plants were artificially pollinated and bagged. Populations of D. egrettarum, D. leucomelas, D. melanida, D. revaughanii, D. tessellaria were all male-biased with a ratio of at least 2:1. Leaky dioecy occurred only in one Diospyros species, D. egrettarum where hermaphrodite plants represented 2% of the populations studied. Seeds collected from them had the same germination rate (approximately 40%) as the ones obtained from strictly unisexual female plants of D. egrettarum. The fact that leaky dioecy led to the production of fertile seeds opens the possibility that a single pioneer Diospyros plant could have played a major role in the establishment of reproductively viable populations in Mauritius.     

Subdioecy in Consolea spinosissima (Cactaceae): breeding system and embryological studies

The breeding system and the embryology of Consolea spinosissima, a tree-like opuntioid endemic to Jamaica, were investigated. Morphological and embryological studies revealed that the species is subdioecious, with three sexual morphs present in the 150 × 120 m plot studied at Hellshire Hills, Jamaica. The female morph has pistillate flowers with open stigma lobes, no pollen grains, and sets fruit. The male morph has cryptic staminate flowers with closed stigma lobes, viable pollen grains, and a nonfunctional gynoecium that does not set seed. The weak hermaphrodite morph has low fruit set and "perfect" flowers that superficially resemble the functionally staminate flowers of the male morph. These perfect flowers reach anthesis with viable pollen grains, with no or only a few functional ovules, and with the style supporting pollen tube growth. Embryological studies showed that the critical stage for sex determination occurs earlier in pistillate than in staminate and perfect flowers. Anthers of pistillate flowers abort prior to microspore tetrad formation, whereas ovules of the staminate and perfect flowers degenerate after the complete maturation of the embryo sac. Based on flower structure and embryological data, we hypothesize that the ancestor of C. spinosissima is/was hermaphroditic.     

Garcinia brasiliensis: insights into reproductive phenology and sexual system in a Neotropical environment

Sexual systems play a key role in the ecology and evolution of plant populations. Research on Garcinia L. has shown the occurrence of variations in the sexual systems of this genus, with some authors considering it as dioecious, and for some species as gynodioecious or androdioecious. The population of G. brasiliensis in the restinga of Maricá is dioecious with two individuals characterized as monoecious. Anatomical analysis showed that one has pistillate and sterile staminate flowers, being an unusual female and the other has staminate and perfect flowers (functional pollen grains and ovules), being andromonoecious. In addition, field observations, controlled hand-pollinations, reproductive phenology and anatomical analysis showed no evidence of apomixis in this Neotropical species, different from that provided in the literature for paleotropical species. However, asexual reproduction occurs through ramets suggesting that in this environment of restinga the seedlings are subjected to considerable limitations, and vegetative propagation becomes an important alternative for the occupation of the environment. Flowering episodes occurred throughout the study period, featuring a subannual and long-lasting flowering pattern mainly due to multiple blooms of male plants. The floral characters observed in G. brasiliensis are often associated with a generalist pollination system. Apis mellifera and Trigona spinipes are the effective pollinators of G. brasiliensis flowers. Our findings provide new insights into the reproductive biology of Garcinia.     

ARE TRIOECY AND SEXUAL LABILITY IN ATRIPLEX CANESCENS GENETICALLY BASED?: EVIDENCE FROM CLONAL STUDIES

Prior studies have alternatively considered floral phenotypes in Atriplex canescens as trioecious (having three sexual genders) and/or dioecious and having a “leaky genetical switch.” Clones transplanted from three populations and grown in common gardens reveal the existence of two distinctly different genetic controls regulating gender expression. In some clones gender is fixed as male (staminate) or female (pistillate), while in other clones gender varies, ranging from a mixture of male and female ramets to simultaneous hermaphrodites with various proportions of male and female flowers. For clones which vary their sex expression, variation occurs within irrigation treatments, between treatments and over time, as a consequence of the combined effects of genotype plus environment. The magnitude of sex change is also a product of the interaction of genetics and environment. Some clones have been repeatedly examined for 20 years.     

Anther structure and pollen development in Melicoccus lepidopetalus (Sapindaceae): An evolutionary approach to dioecy in the family

Anther and pollen development in staminate and pistillate flowers of dioecious Melicoccus lepidopetalus (Sapindaceae) were examined by light and electron microscopy. Young anthers are similar in both types of flowers; they consist of epidermis, endothecium, two to four middle layers and a secretory tapetum. The microspore tetrads are tetrahedral. The mature anther in staminate flowers presents compressed epidermal cells and endothecium cells with fibrillar thickenings. A single locule is formed in the theca by dissolution of the septum and pollen grains are shed at two-celled stage. The mature anthers of pistillate flowers differ anatomically from those of staminate flowers. The epidermis is not compressed, the endothecium does not develop fibrillar thickenings, middle layers and tapetum are generally persisting, and the stomium is nonfunctional. Microspore degeneration begins after meiosis of microspore mother cells. At anthesis, uninucleate microspores and pollen grains with vegetative and generative nuclei with no cytokinesis are observed. Some pollen walls display an abnormal exine deposition, whereas others show a well formed exine, although both are devoid of intine. These results suggest that in the evolution towards unisexuality, the developmental differences of anther wall tissues and pollen grains between pistillate and staminate flowers might become more pronounced in a derived condition, such as dioecy.     

Reproductive biology of the dioecious Canary Islands endemic Withania aristata (Solanaceae)

We confirmed functional dioecy of Withania aristata via field and greenhouse studies. Male flowers are significantly larger. Female flowers bear stamens with no pollen; males bear 220?000 grains. Stigmata of male flowers senesce in buds. Anatomical observations confirm more ovules in females and an ovarian nectary in both sexes. We detected nectar in female flowers in the greenhouse but found no nectar in males. Thus, males offer pollen and females nectar. Females bear large numbers of fruits and, infrequently, male plants bear few significantly smaller fruits with few seeds. Outcrosses of females (self crosses impossible without pollen) yielded fruits in young buds, older buds, and open flowers. Self crosses of male flowers succeeded only with very young buds. Although functionally dioecious, this species manifests self-compatibility; however, no fruits are produced autonomously. Bee species (Lassioglossum, Amegilla, Apis) visit flowers and mature buds. Bud visits in which bees force petal tips apart, coupled with self-compatibility, may explain infrequent fruit on males. Thus, dioecy in W. aristata seems to have evolved from self-compatible ancestors, that leaky dioecy may have been favored during colonization, and, that despite autogamy and a low floral visition rate, this endemic enjoys a high rate of reproductive success.     

麻疯树雌雄花中MADS-BOX基因的表达分析

麻疯树(Jatropha curcas)种子含油率高,种子中的油类物质可作为生物柴油被开发和利用,是极具潜力的生物质能源树种之一。麻疯树雌雄异花,在自然条件下雄花数量通常远远大于雌花,这大大限制了种子和油的产量,因此开展麻疯树性别分化与花发育分子机理的研究具有重要意义。该研究选取10个麻疯树的MADS-BOX基因(JcAGL1,JcAGL6,JcAGL9,JcAGL11,JcAGL15,JcAGL61-3,JcAGL62-1,JcAGL62-6,JcAGL62-7,JcAGL80-2),提取麻疯树早期发育各个阶段的雌雄花总RNA,并反转录成cDNA,采用实时荧光定量方法,探索早期发育不同阶段的麻疯树雌雄花目的基因的表达情况。结果表明:目的基因在发育起始的雌雄花中的表达具有差异,比如JcAGL6和JcAGL15在雄花中表达量要高于雌花,而JcAGL1,JcAGL9和JcAGL11在雌花中的表达量要高于雄花,这说明花原基中目的基因表达会直接或间接决定性别分化的方向;在之后的发育过程中,目的基因的表达情况在雌雄花中有所不同:随着花的发育,目的基因在雌雄花中的表达量变化存在差别,这反应出麻疯树雌雄花发育中目的基因表达模式上的差异;另外,也能看出在此过程中各个目的基因又发挥着不同的功能。该研究结果为进一步探究麻疯树雌雄花发育相关基因的表达提供了理论依据,为了解麻疯树性别分化和花发育的分子机理奠定了基础。