Widespread functional android in Mercurialis annua L. (Euphorbiaceae)

The widespread coexistence of male and monoecious (cosexual) plants in Spanish, Portuguese and Moroccan populations of Mercurialis annua , an annual wind-pollinated ruderal, represents an important case of functional androdioecy, a rare breeding system in plants and animals. In M. annua , both males and cosexes disperse fully competent pollen. Quantitative gender varies discontinuously between males and cosexes, with males producing a mean of 6.09 times as much pollen as cosexes. It appears that gender is determined by a simple developmental switch, with male and cosexual inflorescences differing markedly in morphology: staminate flowers are borne on erect peduncles in males and in tight spiral clusters around a subsessile pistillate flower in cosexes. Males do not differ from cosexes in their biomass, but they are significantly taller, principally as a result of their greater internode lengths. The cosexual inflorescence is strongly protogynous so that outcrossing is favoured in dense stands, but seed-set is assured in cosexes isolated from prospective mates because of their ability to self-fertilize. Males typically occur at frequencies of less than about 30% in androdioecious populations, in accordance with theoretical predictions for functional androdioecy. In the genus Mercurialis , dioecy is the ancestral condition and monoecy and androdioecy, which occur in polyploid populations of M. annua , are derived. I argue here that androdioecy is most likely to evolve in plants (1) from dioecy, (2) in wind-pollinated species, and (3) in species with a colonizing habit. These predictions are also consistent with the limited published data available for other species.     

Mouse autosomal trisomy: two's company, three's a crowd.

Autosomal trisomy causes a large proportion of all human pregnancy loss and so is a significant source of lethality in the human population. The autosomal trisomy syndromes each have a different phenotype and are probably caused by the effects of specific genes that are present in three copies, rather than the normal two. Identifying these genes will require the application of classical genetic and new genome-manipulation approaches. Recent advances in chromosome engineering are now allowing us to create precisely defined autosomal trisomies in the mouse, and so provide new routes to identifying the critical, dosage-sensitive genes that are responsible for these highly deleterious, yet very common, syndromes.     

Two's company,three's a crowd: new insights on spruce budworm species boundaries using genotyping‐by‐sequencing in an integrative species assessment (Lepidoptera: Tortricidae)

Species delimitation requires an assessment of varied traits that can contribute to reproductive isolation, as well as of the permanence of evolutionary differentiation among closely related lineages. Integrative taxonomy, including the combination of genome‐wide molecular data with ecological data, offers an effective approach to this issue. We use genotyping‐by‐sequencing together with a review of ecological divergence to assess the traditionally recognized species status of three closely related members of the spruce budworm species complex, Choristoneura fumiferana (Clemens), C. occidentalis Freeman (=C. freemani Razowski) and C. biennis Freeman, each of which is a major defoliator of conifer forests. We sampled a broad region of overlap between these three taxa in Alberta and British Columbia (Canada) where potential for gene flow provides a strong test of the durability of divergence among lineages. A total of 2218 single nucleotide polymorphisms (SNPs) were assayed, and patterns of differentiation were evaluated under the biological, ecological, genotypic cluster and phylogenetic species concepts. Choristoneura fumiferana was genetically distinct with substantial barriers to genetic exchange with C. occidentalis and C. biennis. Conversely, divergence between C. occidentalis and C. biennis was limited to a small subset of outlier loci and was within the range observed within any one of the taxa. Considering both population genetic and ecological patterns of divergence, C. fumiferana should continue to be recognized as a distinct species, and C. biennis ( syn.n. ) should be treated as a subspecies (C. occidentalis biennis Freeman, 1967) of C. occidentalis, thereby automatically establishing the nominate name C. occidentalis occidentalis Freeman, 1967 for univoltine populations of this species.     

Hybridization, polyploidy, and the evolution of sexual systems in Mercurialis (Euphorbiaceae)

Hybridization and polyploidy are widely believed to be important sources of evolutionary novelty in plant evolution. Both can lead to novel gene combinations and/or novel patterns of gene expression, which in turn provide the variation on which natural selection can act. Here, we use nuclear and plastid gene trees, in conjunction with morphological data and genome size measurements, to show that both processes have been important in shaping the evolution of the angiosperm genus Mercurialis, particularly a clade of annual lineages that shows exceptional variation in the sexual system. Our results indicate that hexaploid populations of M. annua, in which the rare sexual system androdioecy is common (the occurrence of males and hermaphrodites) is of allopolyploid origin involving hybridization between an autotetraploid lineage of M. annua and the related diploid species M. huetii. We discuss the possibility that androdioecy may have evolved as a result of hybridization between dioecious M. huetii and monoecious tetraploid M. annua, an event that brought together the genes for specialist males with those for hermaphrodites.     

Responses of carbon acquisition traits to irradiance and light quality in Mercurialis annua (Euphorbiaceae): evidence for weak integration of plastic responses

It is often suggested that traits will be integrated, either because of pleiotropy or because natural selection may favor suites of integrated traits. Plant responses to different environments can provide evidence of such integration. We grew Mercurialis annua plants in high-density stands in high irradiance, in neutral shade, and in high red to far-red (R:FR) shade, resulting in environments of high irradiance, low R:FR; low irradiance, low R:FR; and low irradiance, high R:FR. We measured gas exchange, leaf morphology, stem elongation, and biomass traits and tested the prediction that traits within each functional group would show higher trait integration, as evidenced by high correlations among traits within environments, higher correlations of trait plasticity, and lower plasticity of trait correlations. Overall, we found evidence of only moderate integration for some groups of traits. Functionally related groups of traits, or pairs of traits, could be strongly integrated by one criterion but weakly integrated by another of the criteria. Stem elongation traits, though often observed to be strongly integrated in other taxa, showed little evidence of integration. Internode traits exhibited a novel pattern of responses to low R:FR, with increased elongation of the hypocotyl, decreased elongation of the first internode, and no change in the second internode. We propose that these responses to light are more likely to be the result of natural selection than the consequence of constraints imposed by pleiotropy.     

MAPKAP kinase-2: three's company at the G(2) checkpoint

Elegant studies in fission yeast by and in mammalian cells by offer new insights into the mechanism through which stress-induced p38 activation inhibits mitotic entry in eukaryotic cells.     

Characterization of microsatellite loci and reliable genotyping in a polyploid plant, Mercurialis perennis (Euphorbiaceae)

For many applications in population genetics, codominant simple sequence repeats (SSRs) may have substantial advantages over dominant anonymous markers such as amplified fragment length polymorphisms (AFLPs). In high polyploids, however, allele dosage of SSRs cannot easily be determined and alleles are not easily attributable to potentially diploidized loci. Here, we argue that SSRs may nonetheless be better than AFLPs for polyploid taxa if they are analyzed as effectively dominant markers because they are more reliable and more precise. We describe the transfer of SSRs developed for diploid Mercurialis huetii to the clonal dioecious M. perennis. Primers were tested on a set of 54 male and female plants from natural decaploid populations. Eight of 65 tested loci produced polymorphic fragments. Binary profiles from 4 different scoring routines were used to define multilocus lineages (MLLs). Allowing for fragment differences within 1 MLL, all analyses revealed the same 14 MLLs without conflicting with merigenet, sex, or plot assignment. For semiautomatic scoring, a combination of as few as 2 of the 4 most polymorphic loci resulted in unambiguous discrimination of clones. Our study demonstrates that microsatellite fingerprinting of polyploid plants is a cost efficient and reliable alternative to AFLPs, not least because fewer loci are required than for diploids.     

Resource defence in female pollinating fig wasps: two's a contest, three's a crowd

Female pollinating fig wasp (foundress) reproduction is often reduced when multiple foundresses oviposit in figs, owing to oviposition site limitation or the extra time spent searching for unused sites. In species where foundresses cannot re-emerge from figs (meaning that all reproduction takes place in a single fig), they might therefore be expected to defend sites from competitors. For the first time, we explicitly documented defence by foundresses. Larger Platyscapa awekei foundresses (pollinators of Ficus salicifolia) in two-foundress figs prevented smaller foundresses from ovipositing by holding them in their mandibles while ovipositing themselves, and deposited the same number of eggs as when ovipositing in isolation. Smaller foundresses deposited around 30% fewer eggs. The onset of defence in two-foundress figs depended on larger foundress body size, implying that they made decisions based on their own size relative to the population mean (i.e. the average probability of being successful). Defence was less common in three- than in two-foundress figs, probably because the presence of a third competitor reduced the benefits accrued. We discuss why foundresses use such tactics, and identify other species in which defence may occur. Qualitative behavioural observations and comparative morphology suggest that defence is common, occurring in four pollinator genera. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.      

Chromosome and breeding system evolution of the genus Mercurialis (Euphorbiaceae): implications of ITS molecular phylogeny

 The internal transcribed spacers (ITS1 and ITS2) of nuclear ribosomal DNA were amplified and sequenced from 19 samples representing all species of the genus Mercurialis and two outgroup species, Ricinus communis and Acalypha hispida. The length of ITS1 in the ingroups ranged from 223 to 246 bp and ITS2 from 210 to 218 bp. Sequence divergence between pairs of species ranged from 1.15% to 25.88% among the ingroup species in the combined data of ITS1 and ITS2. Heuristic phylogenetic analyses using Fitch parsimony on the combined data of ITS1 and ITS2 with gaps treated as missing generated 45 equally parsimonious trees. The strict consensus tree was principally concordant with morphological classification. Within the genus, the ITS sequences recognised two main infrageneric clades: the M. perennis complex including three Eurasian stoloniferous species (M.␣leiocarpa, M. ovata and M. perennis) and the western Mediterranean group including eight both annual and perennial species. Of the western Mediterranean clade, the annual and perennial species grouped respectively into two different groups, and the annual life form is revealed as a synapomorphic character derived from perennial, whereas in the Eurasian clade ITS phylogeny suggested M. leiocarpa as basal clade sister to M.␣perennis and M. ovata. ITS phylogeny failed to resolve the relationships among the different cytotypes of M. ovata and M. perennis. ITS phylogeny also suggested rapid karyotypic evolution for the genus. The karyotypic divergence among the perennial species of western Mediterranean region did not corroborate the nucleotide sequence divergence among the species. Optimisation of chromosome numbers onto the ITS phylogeny suggested x=8 to be the ancestral basic chromosome number of the genus. ITS phylogeny confirmed that the androdioecy of M. ambigua is derived from dioecy. The nucleotide heterozygosity and additivity in ITS sequences clearly confirm the interspecific hybridisation in the genus Mercurialis. Received December 22, 2001; accepted May 21, 2002?Published online: November 14, 2002 Address of the authors: Martin Kr?henbühl, Yong-Ming Yuan (correspondence) and Philippe Küpfer, Institut de Botanique, Laboratoire de botanique évolutive, Université de Neuchatel, Emile-Argand 11, CH-2007 Neuchatel, Suisse. (e-mail: yong-ming.yuan@unine.ch)     

Predominant expression of a peroxidase gene in staminate flowers of Mercurialis annua

A direct correlation between the feminizing process of male plants of Mercurialis annua by benzylaminopurine and the disappearance of male-specific peroxidases was established. In order to understand the process of hormonal regulation of the sexual differentiation and peroxidase activity, two peroxidase cDNA clones were selected from a male flower cDNA library with degenerated oligonucleotide probes encoding a peroxidase-specific site. These clones contained three peroxidase-specific sequences and an Eco RI restriction site. The two Eco RI fragments of one cDNA clone were subcloned and used as probes to analyse the expression of corresponding gene(s). The northern blot analysis showed hybridization on a single mRNA band with a predominant expression in male flowers. The feminizing treatment with a cytokinin induced the progressive disappearance of the hybridization signal in correlation with the disappearance of male specific isoperoxidases.     

Geographic variation in the breeding system and the evolutionary stability of trioecy in Pachycereus pringlei (Cactaceae)

The Sonoran Desert columnar cactus Pachycereus pringlei has a geographically variable, non-hermaphroditic breeding system. It is trioecious (separate males, females and hermaphrodites) in the northern two-thirds of its range in Sonora, Mexico, and in the southern three-quarters of its range in Baja California, Mexico, and is gynodioecious (separate females and hermaphrodites) elsewhere. Trioecy occurs near known maternity roosts of its major pollinator, the nectar-feeding bat Leptonycteris curasoae; gynodioecy occurs>50km from known bat roosts. The observed geographic patterns cannot be explained by limited gene flow or by the geographic distributions of diurnal avian pollinators. Our field observations plus a theoretical analysis suggest that the abundance of chiropteran pollinators plays an important role in the maintenance of trioecy in this plant. Under pollinator limitation, trioecy can be a stable breeding system in this species. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sexual dimorphism in intra- and interspecific competitive ability of the dioecious herb Mercurialis annua

Males and females of dioecious plant species often show different responses to competition with individuals of the same or opposite gender, but almost no data are available on the outcome of competition with members of other species. Here, we show that male and female individuals of the wind-pollinated herb Mercurialis annua are sexually dimorphic in both their intraspecific and interspecific competitive abilities. In a controlled experiment, we found that both sexes of M. annua were negatively affected by interspecific competition, but the sensitivity of males and females depended on the identity of their competitor species, with females tending to suppress the aboveground growth of competitor species more than males. Further, we found that intrasexual and intersexual competition affected the aboveground growth of males but not that of females: only males showed a significant reduction in growth when growing with conspecific competitors (male or female). We discuss our results with reference to related studies that suggest that males and females of M. annua have different resource requirements for reproduction, which in turn affect their competitive abilities.     

Sexual dimorphism in a dioecious population of the wind-pollinated herb Mercurialis annua: the interactive effects of resource availability and competition

Background and Aims

Male-biased sex allocation commonly occurs in wind-pollinated hermaphroditic plants, and is often positively associated with size, notably in terms of height. Currently, it is not well established whether a corresponding pattern holds for dioecious plants: do males of wind-pollinated species exhibit greater reproductive allocation than females? Here, sexual dimorphism is investigated in terms of life history trade-offs in a dioecious population of the wind-pollinated ruderal herb Mercurialis annua.

Methods

The allocation strategies of males and females grown under different soil nutrient availability and competitive (i.e. no, male or female competitor) regimes were compared.

Key Results

Male reproductive allocation increased disproportionately with biomass, and was greater than that of females when grown in rich soils. Sexual morphs differentially adjusted their reproductive allocation in response to local environmental conditions. In particular, males reduced their reproductive allocation in poor soils, whereas females increased theirs, especially when competing with another female rather than growing alone. Finally, males displayed smaller above-ground vegetative sizes than females, but neither nutrient availability nor competition had a strong independent effect on relative size disparities between the sexes.

Conclusions

Selection appears to favour plasticity in reproductive allocation in dioecious M. annua, thereby maintaining a relatively constant size hierarchy between sexual morphs. In common with other dioecious species, there seems to be little divergence in the niches occupied by males and females of M. annua.     

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

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

Genetic diversity and structure of a Mediterranean endemic plant in Corsica (<Emphasis Type="Italic">Mercurialis corsica</Emphasis>, Euphorbiaceae)

The island of Corsica is a Mediterranean hotspot of plant biodiversity characterized by a high rate of plant endemism, but also by a lack of studies combining genetic diversity and conservation. In Corsica, the dioecious and Corso-Sardinian endemic Mercurialis corsica Cosson (Euphorbiaceae) occurs across a wide ecological gradient, but the number of populations have decreased considerably over the last century. The main aim of this study was to examine the patterns of genetic diversity occurring in the Corsican populations of M. corsica, depending on their location and demographic structure. The rDNA sequences did not show the existence of any polymorphism, whereas the cpDNA sequences revealed the divergence of the western Corsican populations. By contrast, when the AFLP markers were examined, although significant levels of differentiation were detected between populations, no distinct geographical patterns were observed except for the pronounced isolation of the Cap Corse genotypes. No significant correlations were found to exist between population size and the genetic diversity indexes used. The results of this study suggest that M. corsica has undergone a complex gene flow history involving past population admixtures, followed by fragmentation processes resulting in population differentiation but no geographical patterns of isolation. These results support the existence of three evolutive conservation units which have to be monitored in priority to determine whether the current pattern of demographic structure is still declining or has stabilized.     

Pollen morphology of the Flueggeinae (Euphorbiaceae,Phyllanthoideae)

A total of 129 species from the subtribe Flueggeinae of the tribe Phyllantheae (Euphorbiaceae, Phyllanthoideae) were investigated using light and scanning electron microscopy, and 10 species using transmission electron microscopy, in order to evaluate the relationships between the eight constituent genera: Breynia, Flueggea, Glochidion, Margaritaria, Phyllanthus, Reverchonia, Richeriella, and Sauropus. Of these genera, Flueggea, Margaritaria and Richeriella share pollen with a prolate spheroidal meridional outline and a 3-colporate aperture system. Pollen of Reverchonia is also 3-colporate, but differs from that of the Flueggea alliance by its clearly prolate shape, tilioid ornamentation and absence of costae endopori. Breynia and Sauropus have 4–12 and 3–16-colporate pollen, respectively, with diploporate colpi. Two pollen types are recognised in Breynia, and four in Sauropus, one of which supports the recognition of Sect. Hemisauropus. Glochidion pollen is 3–6-colporate, and similar to that of Breynia in having reticulate sculpture with Y-shaped sexine structures, but it has monoporate colpi. Of the genus Phyllanthus, only species with pollen with diploporate colpi have been studied. Seven types are described. Diploporate Phyllanthus pollen can be distinguished from that of Breynia and Sauropus by its distinct colpus margins consisting of parallel muri. Colpal irregularities and endoaperture configurations in the subtribe are discussed, and pollen morphological trends are hypothesised. Placed in the successiform aperture series, the Flueggea alliance and Reverchonia form a basal group. Glochidion is considered intermediate, giving rise to the Breynia-Sauropus group. The relationship with Phyllanthus remains unclear.     

Synopsis of the tribe Hureae (Euphorbioideae, Euphorbiaceae)

A synopsis of the tribe Hureae is presented with nomenclatural updates, a discussion of diagnostic features, and summaries of geographical distributions. This study is based on the analysis of approximately 300 voucher specimens, including collections and photographs of types, in addition to bibliographic documentation. Seventeen species distributed in three genera were recognized: Algernonia (11 species), Hura (2), and Ophthalmoblapton (4). All species are American with the majority distributed within the Atlantic Forest, particularly in southeastern Brazil. A key for the identification of genera and species is provided along with illustrations, information on the geographic distributions and conservation status. Lectotypification for Algernonia leandrii is proposed.