Variation in the breeding system of two sympatricDombeya species on La Réunion island

The high frequency of dioecy on oceanic islands has attracted a great deal of attention in plant evolution. In this study we quantify variation in the breeding system of two sympatricDombeya species on La Réunion island. BothDombeya species are cryptically dioecious, each sex retaining non-functional morphological structures of the other sex. One species is strictly dioecious, whilst the other shows leaky dioecy, with 8/13 males producing fruit during two years of study. Males that set fruit did so following outcrossing but not after self-pollination despite the growth of self-pollen tubes in the style. This suggests the occurrence of a late-acting self-incompatibility system. Several morphologically intermediate individuals are recognized and suggested to be a result of interspecific hybridization. These purported hybrids are hermaphrodite and self-compatible, consistent with a complementary breakdown in dioecy and self-compatibility.     

Synchronized dichogamy and dioecy in neotropicalLauraceae

In hermaphrodite neotropicalLauraceae a highly evolved dichogamous system is present which represents a kind of temporal dioecy. This system involves the existence of two flower morphs which are characterized by reciprocal phases of receptivity of the stigmas and pollen release. In some genera (Persea, Cinnamomum), nectar is produced as a reward for the flower visitors, while in other genera (Aniba, Clinostemon, Licaria), nectar is absent and pollen seems to be the only reward. This implies that in this case the flowers in the female stage must be deceptive flowers. In dioecious species of the generaOcotea andNectandra, both the male and female flowers attract the visitors with nectar. The pollen-ovule ratio of theLauraceae is comparatively low. — The type of reproductive system that characterizes theLauraceae, comprising functional dioecy, small, inconspicuously coloured flowers, pollination by small bees, and large, one-seeded fruits dispersed by birds, is quite prominent among trees of various families in the tropical lowland forest. The relationship between the different modes of flowering within theLauraceae and the causes for the correlation of their reproductive traits are discussed.     

Agouticarpa, a new neotropical genus of Tribe Gardenieae (Rubiaceae)

Agouticarpa, a new genus of the informalAlibertia group (Rubiaceae), is described and illustrated. It comprises six species, and occurs from Costa Rica to Bolivia. Three species are here described as new:Agouticarpa grandistipula, A. hirsuta, andA. velutina. Additionally, three new combinations are made:A. curvifolia andA. williamsii (previously inGenipa) andA. isernii (previously inAlibertia). Agouticarpa is characterized by being dioecious, having elliptic to abovate, membranaceous stipules, male flowers in a branched dichasial or thyrse-like inflorescence, a poorly developed cup-shaped calyx, pollen grains with 3–7 apertures, and large globose fruits.     

Pollen morphology and functional dioecy inSolanum (Solanaceae)

Dioecy has evolved independently several times in the large, mostly tropical genusSolanum. In all cases of dioecy inSolanum functionally male flowers have normal anthers, normal pollen and reduced stigmas while functionally female flowers have stigmas and anthers that appear normal but contain non-functional, usually inaperturate pollen. The inaperturate pollen has living cytoplasm, but apparently never germinates and it has been hypothesised that the pollen in these functionally female flowers is retained as a pollinator reward. Pollen morphology is compared in twelve of the thirteen known dioecious species ofSolanum, and some stages in the the development of inaperturate pollen in the anthers of functionally female flowers ofSolanum confertiseriatum of Western Ecuador are examined. Observations on the development and morphology of inaperturate pollen in functionally female flowers ofSolanum are related to hypotheses about the evolution of dioecy in the genus.     

A molecular and morphological investigation of Batrachospermum arcuatum (Batrachospermales, Rhodophyta) in China

Batrachospermum arcuatum specimens were analysed from seven stream segments in North China. Morphological characteristics were observed and cluster analysis was used to evaluate the divergence among thalli from. Sequence data of the rbcL gene (chloroplast gene) and cox2-3 spacer region (mitochondrial gene) were also utilized to evaluate genetic variation in specimens among stream segments. The specimens from four of the streams were monoecious, while the individuals at the other three locations were dioecious. Cluster analysis showed that the monoecious specimens were not separated from the dioecious specimens, based on morphology, but rather the specimens were grouped by geographical closeness and habitat similarity. Likewise, the combined analyses of rbcL and the cox2-3 spacer data from provided more evidence that breeding system (monoecy vs. dioecy) is not a good morphological character to distinguish species.     

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.     

Mating System in a Low-density Natural Population of the Dioecious Wind-pollinated Patagonian Cypress

The mating system of a species is an essential factor that determines the genotypic frequencies of its populations in each generation. Thus, knowledge in this regard is important in the application of practical measures in conservation and forest tree breeding. Austrocedrus chilensis (D.DON) PIC. SER. ET BIZZARRI is a member of the Cupressaceae native to the Andean–Patagonian Forest of Argentina and Chile with regional economic importance. It is a dioecious tree with pollen and seeds distributed by wind. Isozyme markers were utilized to study the mating system of this species through the survey of individual effective pollen clouds in a natural low-density population. If random mating occurs, effective pollen clouds that pollinate single seed trees are expected to be genetically homogeneous. The classical HardyWeinberg test of panmixia was also performed. Although it failed to reject random mating, the analysis of the pollen clouds showed that this mating pattern does not occur. Evidence of locally distributed heterogeneous pollen clouds is presented. The two methods are critically compared.     

Mating System and Clonal Architecture: A Comparative Study in Two Marine Angiosperms

In the present study, we compared the clonal architecture between two seagrass species, the dioecious Cymodocea nodosa and the hermaphroditic, self-compatible Zostera noltii, in order to verify the hypothesis that clonal growth strategies and resulting genet architecture are associated with mating system in clonal plants. It is expected that self-incompatible species should be associated to a guerrilla growth form, because of pollen limitation due to obligate outbreeding, while the ecologically advantageous phalanx strategy can be adopted in self-compatible species. Genotypic diversity and heterozygosity were also assessed in the two species. Sampling has been conducted in mixed stands, collecting shoots of the two species at the same points of the sampling grid, in order to even out any effects of environmental heterogeneity. Species-specific microsatellite loci have been used as molecular markers to identify clones and assess their spatial distribution in both species. As expected, we found an intermingled configuration of genets in the dioecious C. nodosa while Z. noltii was characterized by a clumped, `phalanx-type' distribution of clones. C. nodosa was characterized by higher genotypic diversity with regard to Z. noltii, while heterozygosity levels were comparable in the two species. Coordinating Editor: Dr J. Tuomi     

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.     

Patterns of molecular evolution in dioecious and non‐dioecious Silene

Dioecy (i.e. having separate sexes) is a rather rare breeding system in flowering plants. Such rareness may result from a high probability of extinction in dioecious species because of less efficient dispersal and the costs of sexual selection, which are expected to harm dioecious species' survival on the long term. These handicaps should decrease the effective population size () of dioecious species, which in turn should reduce the efficacy of selection. Moreover, sexual selection in dioecious species is expected to specifically affect some genes, which will evolve under positive selection. The relative contribution of these effects is currently unknown and we tried to disentangle them by comparing sequence evolution between dioecious and non‐dioecious species in the Silene genus (Caryophyllaceae), where dioecy has evolved at least twice. For the dioecious species in the section Melandrium, where dioecy is the oldest, we found a global reduction of purifying selection, while on some, male‐biased genes, positive selection was found. For section Otites, where dioecy evolved more recently, we found no significant differences between dioecious and non‐dioecious species. Our results are consistent with the view that dioecy is an evolutionary dead end in flowering plants, although other scenarios for explaining reduced cannot be ruled out. Our results also show that contrasting forces act on the genomes of dioecious plants, and suggest that some time is required before the genome of such plants bears the footprints of dioecy.     

Viscum album in Japan: Chromosomal translocations,maintenance of heterozygosity and the evolution of dioecy

The genusViscum is very suitable for study of structural rearrangements in chromosomes, having very large chromosomes, low basic number and very little polyploidy. An extensive survey of the dioecious speciesV. album (n=10) in Japan has revealed the widespread occurrence of several different chromosomal translocation complexes. Male plants are always heterozygous for large sex-associated translocation complexes, having 6II ⊙8 (six bivalents and a ring-of-eight) or 5II ⊙10 or rarely 4II ⊙12 at meiosis. Female plants are homozygous for these complexes, usually having 10II. There is also a floating ⊙4 which occurs in both male and female plants. Female plants may be heterozygous for another ⊙4 or ⊙6, which do not occur in male plants. Models are presented to account for the relationship between all of the translocations involved. The high levels of translocation heterozygosity are probably important in maintaining heterozygosity in the species for large complexes of adaptive genes. However the sex-associated permanent translocation heterozygosity may have originally been established as a mechanism to stabilize dioecy based on non-allelic unlinked genes for maleness and femaleness.     

Critical studies on the sexuality of southern Argentinian Ericaceae

Summary The family Ericaceae is considered to be mostly hermaphroditic and only in a few cases unisexual or dioecious. Five Argentinian species (Gaultheria antarctica Hook, f., G. caespitosa P. et E., G. phillyreaefolia (Pers.) Sleumer, G. tenuifolia (Phil.) Sleumer and Pernettya insana (Molina) Gunckel), previously described as hermaphroditic, were found by us to have two types of flowers: female and male. Female flowers have both a rudimentary androecium with stamens, in which the anthers are small, collapsed and without pollen grains or are reduced to filaments only, and a perfect gynoecium with well-developed ovules filling the carpel locules. The stigma is expanded and of the wet, papillate type, with copious to slight secretion. The stigma protrudes above the anthers. Male flowers have a well-developed androecium included within the corolla and a reduced gynoecium. Anthers are as long as the filaments, except in G. caespitosa. Pollen is shed as tetrads which are 100% viable. Ovule development ranges from total absence, passing through aborted ovules of various sizes to apparently normal ones. Stigmata are neither expanded nor papillate, except in Pernettya insana. Only in two species do the style and stigma extend beyond the level of the anthers. Heterostyly does not occur in any of the five species studied. Functional dioecy thus characterizes the five species considered and is reported for the first time in the genus Gaultheria.     

Dioecy inHonkenya peploides var.major (Caryophyllaceae)

Sexual difference and pollination system inHonkenya peploides var.major were investigated. The present analyses indicate that sex expression of this plant is functionally dioecious, rather than androdioecious, as described previously. Its flowers are largely pollinated by small bees, hover-flies, flies, and ants. Female flowers were morphologically characterized by conspicuously longer styles and smaller petals, and had no pollen grains, while male flowers had shorter styles, larger petals, and longer filaments, and produced viable pollen grains. Although the male flowers possess many ovules in an ovary, they set fruits at a very low frequency in natural conditions and did not produce fruits by pollination experiments. We suggest that the dioecy of this species has been derived from hermaphrodite through gynodioecy, and not directly from hermaphrodite.     

Evolutionary transitions among dioecy,androdioecy and hermaphroditism in limnadiid clam shrimp (Branchiopoda: Spinicaudata)

Examinations of breeding system transitions have primarily concentrated on the transition from hermaphroditism to dioecy, likely because of the preponderance of this transition within flowering plants. Fewer studies have considered the reverse transition: dioecy to hermaphroditism. A fruitful approach to studying this latter transition can be sought by studying clades in which transitions between dioecy and hermaphroditism have occurred multiple times. Freshwater crustaceans in the family Limnadiidae comprise dioecious, hermaphroditic and androdioecious (males + hermaphrodites) species, and thus this family represents an excellent model system for the assessment of the evolutionary transitions between these related breeding systems. Herein we report a phylogenetic assessment of breeding system transitions within the family using a total evidence comparative approach. We find that dioecy is the ancestral breeding system for the Limnadiidae and that a minimum of two independent transitions from dioecy to hermaphroditism occurred within this family, leading to (1) a Holarctic, all‐hermaphrodite species, Limnadia lenticularis and (2) mixtures of hermaphrodites and males in the genus Eulimnadia. Both hermaphroditic derivatives are essentially females with only a small amount of energy allocated to male function. Within Eulimnadia, we find several all‐hermaphrodite populations/species that have been independently derived at least twice from androdioecious progenitors within this genus. We discuss two adaptive (based on the notion of ‘reproductive assurance’) and one nonadaptive explanations for the derivation of all‐hermaphroditism from androdioecy. We propose that L. lenticularis likely represents an all‐hermaphrodite species that was derived from an androdioecious ancestor, much like the all‐hermaphrodite populations derived from androdioecy currently observed within the Eulimnadia. Finally, we note that the proposed hypotheses for the dioecy to hermaphroditism transition are unable to explain the derivation of a fully functional, outcrossing hermaphroditic species from a dioecious progenitor.     

Genetic and physical maps around the sex-determining <Emphasis Type="Italic">M</Emphasis>-locus of the dioecious plant asparagus

Asparagus officinalis L. is a dioecious plant. A region called the M-locus located on a pair of homomorphic sex chromosomes controls the sexual dimorphism in asparagus. The aim of this work was to clone the region determining sex in asparagus from its position in the genome. The structure of the region encompassing M should be investigated and compared to the sex-determining regions in other dioecious model species. To establish an improved basis for physical mapping, a high-resolution genetic map was enriched with AFLP markers closely linked to the target locus by carrying out a bulked segregant analysis. By screening a BAC library with AFLP- and STS-markers followed by chromosome walking, a physical map with eight contigs could be established. However, the gaps between the contigs could not be closed due to a plethora of repetitive elements. Surprisingly, two of the contigs on one side of the M-locus did not overlap although they have been established with two markers, which mapped in a distance as low as 0.25 cM flanking the sex locus. Thus, the clustering of the markers indicates a reduced recombination frequency within the M-region. On the opposite side of the M-locus, a contig was mapped in a distance of 0.38 cM. Four closely linked BAC clones were partially sequenced and 64 putative ORFs were identified. Interestingly, only 25% of the ORFs showed sequence similarity to known proteins and ESTs. In addition, an accumulation of repetitive sequences and a low gene density was revealed in the sex-determining region of asparagus. Molecular cytogenetic and sequence analysis of BACs flanking the M-locus indicate that the BACs contain highly repetitive sequences that localize to centromeric and pericentromeric locations on all asparagus chromosomes, which hindered the localization of the M-locus to the single pair of sex chromosomes. We speculate that dioecious Silene, papaya and Asparagus species may represent three stages in the evolution of XX, XY sex determination systems. Given that asparagus still rarely produces hermaphroditic flowers and has homomorphic sex chromosomes, this species may be an ideal system to further investigates early sex chromosome evolution and the origins of dioecy.     

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.     

The morphology of stipules and inflorescences in <Emphasis Type="Italic">Geissois sensu stricto</Emphasis> (<Emphasis Type="Italic">Cunoniaceae</Emphasis>)

Summary   Geissois sensu stricto (i.e. excluding the Australian species of this genus) has 19 species restricted to islands in the south-west Pacific and is the sole group in Cunoniaceae in which the stipules are intrapetiolar (axillary) in the adult foliage. In apical buds, paired opposite stipules are coherent round their margins and are either clearly intrapetiolar in relation to the most distal pair of developed leaves (Type I, most species), or they appear interpetiolar due to the abortion of the pair of leaves associated with them and the failure of the internode proximal to them to elongate (Type II, a few species). Stipules are often accrescent and the largest in the family occur in this group, reaching 10 cm long in some species. The ornithophilous racemose inflorescences of members of this group normally consist of either axillary triads (G. hirsuta only) or apparently simple, unbranched racemes which are ramiflorous or occasionally axillary. However, bract scars on the axes proximal to the flowers show that both types of inflorescence module usually consist of two metamers. The structure of the inflorescences and stipules in Geissois sensu stricto supports the distinction between this group and the Australian species traditionally included in Geissois sensu lato.     

Dioecy Is Associated with High Genetic Diversity and Adaptation Rates in the Plant Genus Silene

About 15,000 angiosperm species (∼6%) have separate sexes, a phenomenon known as dioecy. Why dioecious taxa are so rare is still an open question. Early work reported lower species richness in dioecious compared with nondioecious sister clades, raising the hypothesis that dioecy may be an evolutionary dead-end. This hypothesis has been recently challenged by macroevolutionary analyses that detected no or even positive effect of dioecy on diversification. However, the possible genetic consequences of dioecy at the population level, which could drive the long-term fate of dioecious lineages, have not been tested so far. Here, we used a population genomics approach in the Silene genus to look for possible effects of dioecy, especially for potential evidence of evolutionary handicaps of dioecy underlying the dead-end hypothesis. We collected individual-based RNA-seq data from several populations in 13 closely related species with different sexual systems: seven dioecious, three hermaphroditic, and three gynodioecious species. We show that dioecy is associated with increased genetic diversity, as well as higher selection efficacy both against deleterious mutations and for beneficial mutations. The results hold after controlling for phylogenetic inertia, differences in species census population sizes and geographic ranges. We conclude that dioecious Silene species neither show signs of increased mutational load nor genetic evidence for extinction risk. We discuss these observations in the light of the possible demographic differences between dioecious and self-compatible hermaphroditic species and how this could be related to alternatives to the dead-end hypothesis to explain the rarity of dioecy.     

Die Stipeln derIrvingioideae undRecchioideae und ihre systematische Wertung nebst Bemerkungen zur Holzanatomie und Palynologie

TheSimaroubaceae generally have no true stipules. The stipule-like appendages of some genera proved to be pseudo- or metastipules (Weberling & Leenhouts 1965). There seem to be some exceptions, however: the generaCadellia (incl.Guilfoylia) andRecchia on the one hand, and theIrvingioideae on the other. As these taxa, with exception ofRecchia, have simple leaves, there are no indications that their stipule-like appendages might be pseudo- or metastipules. In regard to their position and ontogeny these appendages behave completely like true stipules. Assuming the view ofForman, one could conceive a morphological line from the long, broadly inserted axillary stipules of mostIrvingioideae to the small scaly triangular stipules ofIxonanthoideae. The similarities between the stipules ofIrvingioideae andErythroxylaceae (already emphasized byHallier and others), become even more evident when their ontogeny is investigated. TheIrvingioideae, therefore, might be regarded as a separate family (perhaps with some relation to theErythroxylaceae,Hallier) or as a subfamily ofIxonanthaceae (Forman).—In addition to data on stipules some results on the palynology and shoot anatomy of the generaCadellia (incl.Guilfoylia) andRecchia are reported. Their relationship with theSimaroubaceae also appears doubtful. If they are to be included, they represent a somewhat isolated group near the base of the family which otherwise has lost its stipules.

Herrn Univ.-Prof. Dr.Walter Leinfellner zum 70. Geburtstag gewidmet.     

Dioecy in the endemic genusDendrocacalia (Compositae) on the Bonin (Ogasawara) Islands

Dendrocacalia crepidifolia Nakai (Compositae, Senecioneae), the only species of this arboreal genus endemic to Haha Island in the Bonin Islands, was found to be dioecious. Male flowers differ from female ones in having a stunted style (style in female exserted from corolla and deeply bifurcating) and anthers filled with fertile pollen (anthers in the female lacking pollen). The size of the corolla and number of florets per head were similar between male and female flowers. The crown area of this arboreal species was also similar in male and female plants. The sex ratio was 0.55 male, not significantly different from 0.5. Both sexes produced nectar of similar sugar concentration (ca. 50%). The flowers are pollinated by feral honeybees (Apis mellifera), but they are thought to have been pollinated by small, lesshairy, endemic solitary bees before honeybees were introduced and subsequently became the dominant bee species on the island. The evolution of dioecy ofDendrocacalia on the island is thought to stem from the deleterious effects of inbreeding that are inherent in plants with geitonogamy. The increased geitonogamy on the island has resulted from increased woodiness (i.e., increased number of flowers per plant) and the original dependence on endemic bee pollinators, which are now endangered.