Showing papers in "Plant Species Biology in 2000"

Taxonomy, anatomy and evolution of physical dormancy in seeds

Abstract: Physical dormancy (PY) is caused by a water-impermeable seed or fruit coat. It is known, or highly suspected, to occur in nine orders and 15 families of angiosperms (sensuAngiosperm Phylogeny Group 1998), 13 of which are core eudicots. The Zingiberales is the only monocot order, and Cannaceae (Canna) the only monocot family, in which PY is known to occur. Six of the nine orders, and 12 of the 15 families, in which PY occurs are rosids. Furthermore, six of the families belong to the Malvales. The water-impermeable palisade layer(s) of cells are located in the seed coats of 13 of the families, and in the fruit coats of Anacardiaceae and Nelumbonaceae. In all 15 families, a specialized structure is associated with the water-impermeable layer(s). The breaking of PY involves disruption or dislodgment of these structures, which act as environmental ‘signal detectors’ for germination. Representatives of the nine angiosperm orders in which PY occurs had evolved by the late Cretaceous or early Tertiary (Paleogene). Anatomical evidence for PY in fruits of the extinct species Rhus rooseae (Anacardiaceae, middle Eocene) suggests that PY had evolved by 43Ma, and probably much earlier. We have constructed a conceptual model for the evolution of PY, and of PY+ physiological dormancy (PD), within Anacardiaceae. The model begins in pre-Eocene times with an ancestral species that has large, pachychalazal, non-dormant (ND), recalcitrant seeds. By the middle Eocene, a derived species with relatively small, partial pachychalazal, orthodox seeds and a water-impermeable endocarp (thus PY) had evolved, and by the Oligocene, PD had been added to the seed (true seed + endocarp) dormancy mechanism. It is suggested that climatic drying (Eocene), followed by climatic cooling (Eocene–Oligocene transition), were the primary selective agents in the development of PY. An evolutionary connection between PY and recalcitrance is suggested by the relatively high concentration of these two character states in the rosids. Phylogenetic data and fossil evidence seem to support the PY→(PY+PD) evolutionary sequence in Anacardiaceae, which also may have occured in Leguminosae.

Smoke as a germination cue

Abstract: Fire is a major environmental selective force that influences plant communities in many parts of the world. Reproductive strategies have evolved as adaptations to the various factors generated by, and/or associated with, fire. This is particularly true for seeds, in which strategies have evolved that respond to both the physical and chemical germination cues that may be associated with fires. Some of these cues are clearly of a primary nature, while others may play a secondary and/or highly specific, or even permissive, role in the germination process. Clearly, there must be considerable interaction between the physical and chemical cues generated by wild fires. Smoke is the most striking chemical factor. Smoke released from burning vegetation contains a chemical signal that triggers germination of both fire climax and non-fire climax species from different parts of the world. It is used in horticulture to stimulate seed germination of wildflower species and can break dormancy and improve germination of vegetable crops, such as lettuce and celery. Smoke can be applied to seeds immediately before sowing, or the seeds may be pretreated and stored until conditions are appropriate for sowing. Both smoke and aqueous smoke-water are active in this respect. The active constituent(s) is volatile, thermostable, water soluble and long-lasting in aqueous solution and in the soil. Attempts to identify the active compound(s) and to determine the mechanism(s) of action have been unsuccessful. Smoke treatment is effective on many seeds that have a light requirement for germination. A very clear concentration effect, resembling that of hormonal responses has been established with aqueous smoke solutions. Smoke extracts interact with gibberellins, cytokinins, abscisic acid and ethylene in photoblastic and in thermodormant seeds. However, despite these interactions it remains questionable whether smoke acts via hormones in stimulating seed germination. There is a clear connection between smoke and life history traits. Smoke may well be the overriding trigger for germination in relation to specific growth habits, regeneration strategies, seed storage, seed sizes, dispersal modes and structures for a large number of species growing in fire-prone habitats. It is becoming increasingly clear that smoke as a germination (or growth regulating) cue must have evolved as part, or as a consequence of fire, as an evolutionary factor. As such, it is probably a very old development and serves as an additional protection mechanism to ensure germination at optimal times for seedling survival.

Pollen presentation and pollination syndromes, with special reference to Penstemon.

Abstract: Pollen presentation theory (PPT) allows for a re-examination of some classic themes in pollination biology. Here, we outline its implications in the context of bee- and birdadapted species of Penstemon and Keckiella (Scrophulariaceae). PPT models the optimal schedule of pollen presentation, based on the frequency of visits by pollinators, and the capacities of those pollinators to remove and deposit pollen. High visitation rates, high removal and low deposition all favor plants that present pollen in many small doses. Dosing is achieved through gradual opening of anthers and through anthers opening only narrowly. We hypothesize that bees have higher rates of removal and lower rates of deposition than birds; therefore, bee-pollinated species should have anthers that open more gradually and less completely than bird-pollinated species. Before presenting preliminary results that affirm this prediction, we critically discuss the characterization of species by pollination syndrome. PPT sheds new light on why plants may specialize on particular pollinators. Stebbins’ most effective pollinator can be recast as the pollinator that deposits more of the pollen that it removes, thereby making other visitors into conditional parasites. Pollinator shifts might occur when a pollinator with low removal and high deposition becomes abundant; the plants would then be selected to discourage their previous pollinators who are now parasites. Bird-pollination may favor anthers that open quickly and widely, thereby making bees wasteful parasites. Bee-pollination may favor anthers that open slowly and narrowly, thereby making birds ineffective pollinators. In paired comparisons of closely related species, the hummingbird-visited species were redder, had narrower or longer floral tubes, more exserted anthers and stigmas, less pronounced landing platforms, more inclined orientation, produced more nectar of a lower concentration, and had anthers that dehisce faster and more extensively.

Ecological and evolutionary trends in wetlands: Evidence from seeds and seed banks in New South Wales, Australia and New Jersey, USA

Abstract: Aquatic plants include a variety of life forms and functional groups that are adapted to diverse wetland habitats. Both similarities and differences in seed and seed-bank characteristics were discovered in comparisons of Australian (New South Wales) temporary upland wetlands with a North American (New Jersey) tidal freshwater marsh having both natural and constructed wetlands. In the former, flooding and drying are unpredictable and in the latter water levels vary diurnally and substrate is constantly moist. The hydrologic regimen provides the overriding selective force, with climate an important second factor. Other factors related to water level, such as oxygen availability, temperature and light, vary spatially and temporally, influencing germination processes, germination rates and seedling establishment. Seed and seed-bank characteristics (size, desiccation and inundation tolerance, germination cues and seed-bank longevity and depletion) differ, with the Australian temporary wetland being more similar to the small-seeded persistent seed bank of the constructed wetland site than to the natural tidal freshwater site with its larger seeds, transient seed bank and seasonal spring germination. Some non-spring germination can occur in the tidal constructed wetland if the soil is disturbed. In contrast, seeds in the temporary Australian wetlands germinated in response to wet/dry cycles rather than to season. Functional groups (e.g. submerged, amphibious) are more diverse in the Australian temporary wetlands, where all species tolerate drying. We suggest that the amphibious zone, with its hydrologic gradient, is the site of selection pressure determining establishment of wetland plants from seed. In this zone, multiple selective factors vary spatially and temporally.

Environmental factors and survival strategies of annual plant species in the Negev desert, Israel.

Abstract: Several environmental factors have pronounced influences on the complementary sets of survival strategies, at different stages of the plant life cycle, of some of the most common annual plants in the Negev Desert, Israel. Plasticity of seed germination may spread germination over time and thus reduce the risk to species survival. This plasticity may be affected by day length during seed development and maturation and by position of seeds on mother plants. Post-seed maturation dry storage temperatures may affect the duration of after-ripening. The tiny, dust-like seeds produced by some plants that are dispersed by wind after maturation may escape massive seed predation and remain in large seed banks in the soil. Seeds of other species are protected in lignified structures in aerial seed banks on dead, dry, lignified mother plants. Seed dispersal strategies from these aerial seed banks by rain may also prevent massive seed collection by animals. The ‘cautious’ or ‘opportunistic’ germination strategy and seedling drought tolerance may increase seedling survival in many of the plant species under the unpredictable amounts and distribution of rain in these desert areas. Factors such as amount of rain, temperature, relative humidity, time of the beginning of soil wetting (day or night), seed size and history and the depth of seeds in the soil may affect germination percentages. Day length and soil water content during plant development may have a pronounced influence on plant age at first flowering, seed yield and plant life span.

Adaptive advantages of aerial seed banks

Abstract: Prolonged storage of seeds on the plant is called serotiny. It is well represented in fireprone, nutrient-poor and seasonally dry, woody vegetation in Australia, South Africa and North America. Serotiny maximizes the number of seeds available for the next generation by storing successive seed crops and protecting them from granivores, agents of decay and fire heat. It is an advantage when annual seed production is insufficient for self-replacement in any single year and greatly increases the range of fire frequencies tolerated. While serotiny is a form of environmentally enforced dormancy, it has little in common with soil storage of seeds in terms of its ecology and trade-offs. Our models have shown that optimal levels of serotiny depend on fire frequency in relation to lifespan of the species, probability of interfire recruitment and the extent of fluctuations in fire intervals. Serotiny is adaptive when the cue for seed release, usually fire, also creates superior conditions for seedling establishment. For serotiny to be an advantage over soil storage, it is essential that any fire is followed by reliable recruitment conditions. Intermediate levels of serotiny are favored as fire intervals approach the lifespan of the species, or if there is a substantial resource cost at short intervals. As the probability of interfire recruitment increases, maximum population growth shifts from strong serotiny under short fire intervals to no serotiny at long fire intervals. As variation about the mean fire interval increases, the optimal level of serotiny declines slightly.

Molecular systematics of Lilium and allied genera (Liliaceae): phylogenetic relationships among Lilium and related genera based on the rbcL and matK gene sequence data

Abstract: Coding regions of the rbcL and matK genes of cpDNA were sequenced to analyze phylogenetic relationships of the family Liliaceae sensu stricto, including the major 16 genera of Medeoloideae and Lilioideae of the Liliaceae, in reference to several genera such as Scoliopus, Uvularia, Disporum, and Trillium used as outgoups. The results were congruent with the taxonomic concept of Liliaceae sensu stricto recently proposed by Tamura (1998). The inter- and infrageneric relationships in the genus Lilium and allied taxa were then analyzed based upon the rbcL and matK gene sequencing data, using Medeola and Erythronium as outgroups. The rbcL gene has evolved more slowly than matK and its phylogenetic resolution has been poor as a result of the low base substitution rates; whereas the matK gene has shown a much higher base substitution: 104 variable sites (including 80 informative sites) out of 1641 base pairs were detected. In addition, a remarkably high number of indels, i.e. 19 insertion/deletion events, were detected in the matK gene, which provided us with new evidence for structural changes of this gene within the genus Lilium and allied taxa. Phylogenetic analyses based on the majority rule of the sequence data of matK gene revealed that the genus Lilium consists of three different major clades, including taxa that were placed into different sections by earlier taxonomic treatments, and thus the results of molecular systematic analysis was not congruent with sectional delimitations of the genus Lilium based on the morphological characters. Nomocharis pardanthina and Nomocharis saluenensis were ingroup taxa of Lilium. Notholirion, Cardiocrinum, and Fritillaria turned out to be sister groups to Lilium. An evaluation of the morphological and life-history characteristics was also attempted in light of the molecular phylogeny.

Some current approaches to the evolution of plant–herbivore interactions

Abstract: ‘Coevolution’ between plants and herbivorous arthropods has several meanings: cospeciation, reciprocal adaptation, and a history of ‘escape-and-radiation’ Few well documented examples of each are known Most evolutionary research on insect-plant interactions concerns the adaptations of insects to plants or of plants to insects, but little of it expressly addresses reciprocal adaptation Modern phylogenetic research confirms that host associations in many clades of specialized insects are evolutionarily conservative An example from leaf beetles (Ophraella) is presented, in which the historical conservatism of host shifts is mirrored by patterns of paucity of genetic variation in the ability to use novel hosts In several species of Ophraella, genetic variation was more often discerned in responses to plants closely related to the insect’s normal host plant than to more distantly related plants Thus availability of genetic variation might bias the evolution of host shifts, and account for the phylogenetic pattern The difficult problem of showing that chemical and other resistance features of plants evolved for their defensive function is slowly yielding to investigation One difficulty is that most insect herbivores are thought to be usually too rare to impose appreciable selection Insects collectively could exert quite strong selection on resistance characters if these characters have genetically correlated effects across many species of herbivores (ie, if selection were ‘diffuse’) Surprisingly, the little available evidence does not suggest that pervasive genetic correlations are common I conclude that more, and more multidisciplinary, research is needed to understand the evolution of plant resistance and defense

Spatial genetic structure and non-equilibrium demographics within plant populations.

Abstract: Spatial genetic structure within plant populations is primarily determined by the effects of limited seed and pollen dispersal. In the absence of selfing, spatial genetic structure and limited pollen flow are generally considered to be the primary controlling factors of the dynamics of biparental inbreeding and inbreeding depression. However, in some cases spatial structure may depend on specific demographic processes. Most theoretical studies have focused on the long-term or equilibrium structure of genetic variation, which allows, if demographic considerations are simple enough, a system to be collapsed into a single standardized measure (such as Wright's neighborhood size) of total pollen and seed dispersal distances. Specific demographic considerations may take many forms and strengths. In this paper we examine some particular cases that illustrate a wide range of effects on spatial genetic structure, and identify some of the most critical parameters. For example, Cecropia obtusifolia is a canopy-gap-filling, dioecious species. Perennially, the highly limited availability of habitat forces the removal of seedlings and the associated strong but ephemeral genetic structure, leaving one or a few trees to fill in each canopy gap. The structure among reproductive adults is minor and in line with standard amounts and forms expected for a wind-pollinated species. In contrast, Silene dioica is a dioecious, bee-pollinated, gravity-seed-dispersed perennial, and a colonizer or early successional species. In certain regions of Sweden, S. dioica grows primarily on uplifting islands that undergo rapid succession, and S. dioica exhibits exhibits extreme levels of spatial autocorrelation, caused by highly limited seed and pollen flow at a specific spatial scale that corresponds to demographic patches. Moreover, the types of genotypic concentrations in space vary, being dominated by matrilineal relationships. The availability of habitat rapidly increases, then rapidly decreases (with analogous changes in population growth rates), and, combined with a long-life, causes the genetic structure to exist on such a time scale that it appears to be the main form of within-population genetic structure for the species in this region. Clonal reproduction may have marked effects on the spatial genetic structure of plant populations. For monoecious, self-compatible species, clonal groups may increase effective selfing, whereas in dioecious plants any effects on the development of IBD may be more subtle. We examine a number of dioecious plants with a variety of spatial distributions of clones and populations, existing for various time scales. The demographics of clonal structure may interact with spatial-temporal demographics and structure of the genetic variation.

Modes and mechanisms of speciation in pteridophytes : Implications of contrasting patterns in ferns representing temperate and tropical habitats

Abstract: Discovering how biological diversification results in species is one of the primary challenges facing evolutionary biologists. In the ferns, evidence indicates that dissimilar speciation modes and mechanisms may differentiate some temperate and tropical groups. The Polypodium sibiricum group contains three related diploid species that all inhabit rock outcrops in temperate forests. Although differing lettle in gross leaf morphology and joined by the distinctive morphological synapomorphy of sporangiasters, these three species have an average interspecific genetic identity developed from isozymic com-parisons of only 0.460. A likely mode of speciation is that periodic glaciation pushed Po. sibiricum populations south and, with the retreat of the glaciers, southern populations persisted, evolved diagnostic traits, and ultimately erected postzygotic barriers to interbreeding. This hypothesis follows a classic allopatric speciation model and interspecific distinctions may have been reinforced through contact mediated by subsequent ice ages. In contrast, a monophyletic group of four diploid, epiphytic Pleopeltis species centered in Mexico has an isozymically-determined average interspecific genetic identity value of 0.849. In spite of this high value, these species show greater morphological discrimination than do the Polypodium species. Although the species ranges overlap, they appear to occupy ecologically discrete habitats. These Pleopeltis species may have originated through adaptation to different ecological zones and developed individual morphologies in the process. The high interspecific genetic identity values among the Pleopeltis species suggest a relatively recent and/or rapid process. These hypotheses should be tested by further biosystematic investigations and the discovery of additional monophyletic assemblages with similar patterns of speciation.

Adaptive traits regulating dormancy and germination of invasive species

Abstract: Weed invasion in cropland can be estimated by measuring specific constancy (Number of fields of occurence/Number of fields surveyed). Since the beginning of this century, many floristic surveys have been carried out in fields with similar soil types in the Pampas of Argentina. Thus, weed species with high constancy in maize or wheat crops over more than 60 years could be identified. Convergence among species of any trait having a selective advantage in the cropping environment would be expected. However, no evidence of such convergence was found in seed bank persistence or in dormancy and germination characteristics of summer or winter annuals selected as model species: Amaranthus retroflexus, Datura ferox, Echinochloa crus-galli, Tagetes minuta, Avena fatua, Galinsoga parviflora. On the other hand, changes in dormancy and germination traits were recorded in seeds of E. crus-galli and A. fatua in different agricultural systems, suggesting an adaptive value for invasion of human disturbed habitats. We used a simulation approach to investigate the importance of ecophysiologic seed phenotypes for the invasion of plants into specific human disturbed habitats. Results of the simulations showed that rainfall pattern and soil type interactions changed relative fitness of the tested phenotypes.

Pollinator limitation and resource limitation of seed production in the Scotch broom, Cytisus scoparius (Leguminosae)

Abstract: Pollinator limitation and resource limitation of seed production were examined in flowers of a natural population of Scotch broom, Cytisus scoparius, by taking advantage of the floral characteristics that flowers tripped open by an effective pollinator were easily distinguishable from unvisited flowers. In total, 40.26% of flowers were visited by effective pollinators, 28.91% were pollinated and 13.75% matured fruits. Therefore, 71.80% of visited flowers were pollinated and 47.58% of pollinated flowers matured fruits. Thus, the most limiting factor for fruit production was pollinator visitation rate (pollinator limitation), and secondarily the process from pollination to fruiting further constrained fruit production (resource limitation). The pollinator visitation rate was significantly higher for plants growing in a sunny habitat than in a shady one, and pollinator limitation was more severe in the shady habitat than the sunny one. The proportion of fruit produced to pollinated flowers was not significantly different between the sunny and shady habitats, but was lower on larger plants, indicating that resource limitation was more severe on larger plants producing many more flowers.

Some thoughts on the evolution and ecology of recalcitrant seeds.

Abstract: There are substantial scientific reports on the basic physiology and desiccation sensitivity of recalcitrant seeds, but ecological and evolutionary aspects of their biology have received scant attention Recalcitrant seeds are shed hydrated, are desiccation sensitive and have a short lifespan In vegetative tissue, desiccation sensitivity is probably the ancestral state, but tolerance is thought to have evolved early and a number of times independently It is difficult to see evolutionary relationships among species producing recalcitrant seeds However, it is suggested that early evolved seeds were desiccation sensitive and that desiccation tolerance is a derived characteristic Desiccation sensitivity and short lifespan of recalcitrant seeds places constraints on the range of environmental conditions in which reproductive success can occur Species producing recalcitrant seeds are common in humid tropical forests, where the seeds of climax species germinate and form a seedling bank, rather than contributing to the soil seed bank However, there is a wide range in post-shedding physiology among recalcitrant seed species, and recalcitrant seeds do occur in habitats with seasonal climates Here, regeneration strategies may be more specialized

Gene trees : A powerful tool for exploring the evolutionary biology of species and speciation

Abstract: Evolutionary trees can be constructed from the haplotypes observed with molecular surveys of sequence or restriction site variation. Such gene trees can be constructed regardless of whether or not all of the individual specimens came from one or many species. Hence, these gene trees can straddle the species/population interface, thereby providing a powerful tool for studying the meaning of species and the process of speciation. We illustrate how historical approaches using gene trees can be used to separate the effects of population structure from population history, in order to rigorously test the species status of a group, and to test hypotheses about the process of speciation. A worked example of species status in the Piriqueta caroliniana complex is presented. Species status is evaluated under the cohesion species concept that defines a species as an evolutionary lineage with boundaries arising from the forces that create reproductive communities. Such forces are collectively called cohesion mechanisms and consist of two main subtypes: (i) genetic exchangeability, and (ii) ecological interchangeability. To make this definition operational, populations that behave as separate evolutionary lineages are first identified. A method is reviewed for inferring lineages using explicit statistical criteria from geographic overlays upon gene trees. Once lineages have been identified, the next step is to use the cohesion mechanisms to identify candidate traits that should contribute to genetic exchangeability and/or ecological interchangeability. The cohesion species are then identified by performing overlays upon gene trees in order to identify significant transitions in the candidate traits. Cohesion species are recognized only when statistically significant reproductive/ecological transitions occur that are concordant with the lineages defined earlier. This data-rich method of recognizing species automatically generates much information about the biogeography, population structure, historical events, and ecology and/or reproductive biology of the group under study. In turn, this information provides much insight into the process of speciation. It also makes the criteria, data, methods of analysis and degree of support for the species inference completely explicit, thereby avoiding confusion, inconsistency and artificial controversies that plague much of the literature on species concepts.

Genetics of hybrid inviability and sterility in Drosophila: The Drosophila melanogaster–Drosophila simulans case

Abstract: How reproductive isolation is established between populations is a fundamental question in the study of speciation. To answer this question, mechanisms of prezygotic and postzygotic isolation have been analyzed in many species pairs from diverse taxa. Because of its genetic manipulability, the genus Drosophila is suitable for such analyses. Since hybrid inviability and sterility was noticed in crosses between Drosophila melanogaster and Drosophila simulans 90 years ago, this species pair has contributed much to our knowledge of genetic mechanisms of reproductive isolation. The history of genetic analyses of postzygotic isolation in this species pair is reviewed here.

Reproductive biology of a North American subalpine plant: Corydalis caseana A. Gray ssp. brandegei (S. Watson) G. B. Ownbey

Abstract: Corydalis caseana ssp. brandegei (Fumariaceae) is a perennial plant that grows in moist, subalpine regions of south central Colorado, USA. Prior to this study, nothing was known of its reproductive biology. The most numerous visitors (59%), and the only known pollinators, were long-tongued bumblebees (Bombus appositus). Twenty-nine percent of visits were from short-tongued nectar-robbing bumblebees (Bombus occidentalis). Hummingbirds also visited the flowers but they did not pollinate them. Corydalis caseana flowers remained open and in good condition for approximately 4 days. During that time, in the absence of visitors, nectar containing 35% sugar accumulated at a rate of approximately 1 mL per day. Corydalis caseana has a mixed-mating system. It is self-fertile, but the self-fertilized flowers produce fewer seeds per fruit than the outcrossed flowers (a mean of 2.9 compared with a mean of 4.7). Results suggest a possibility of inbreeding depression.

From pollen dynamics to adaptive dynamics

Abstract: On plants with many flowers, bumblebees tend to visit more flowers in sequence. This induces pollen transfer between flowers on the same plant (geitonogamy). Consequently, in self-compatible plants the selfing rate increases with the number of simultaneously open flowers on the plant, and pollen export to other plants in the population can be a decelerating function of the number of flowers. I argue that geitonogamy is important in relation to two phenomena. First, it may explain the low number of seeds per flower and the size-dependent sex allocation observed in some species. Applying sex allocation theory to the boraginaceous Cynoglossum officinale and Echium vulgare shows that hermaphroditism is stable in both species. The predicted evolutionarily stable values for seeds per flower are close to observed values in the field. The model generally predicts that seeds per flower increase with plant size. This prediction is fine for C. officinale but E. vulgare defies the theory. Second, geitonogamy is important in the evolution of dioecy, as already suggested by Darwin. With abiotic pollination the export of pollen to other plants is likely to be proportional to the number of flowers produced, while it decelerates in animal-pollinated plants. Dioecy can then evolve gradually in a species with abiotic pollination, provided that inbreeding depression exceeds 50%, geitonogamous selfing increases with the number of flowers and that genetic variation exists. With complete pollen discounting as in the models of geitonogamous pollination by animals, hermaphroditism is always stable. The model explains Darwin’s observation that dioecy is more common in wind-pollinated species and in species in which individuals are large, reproducing with many flowers.

Size–number trade-off and allocation to flower production in animal-pollinated flowers

Abstract: The evolutionarily stable size of attractive structures of a flower was analyzed theoretically to examine the factors that select for the differences in the size of attractive structures of a flower depending on the sexuality and self-fertilization rate. In the first analysis, it was assumed that the size–number trade-off for flowers is non-linear (the size of individual flowers increases less rapidly than in a linear trade-off with a decrease in the number of flowers, as would occur during a linear trade-off) and the frequency of pollinator visits per flower depends on the size of attractive structures of individual flowers, but is independent of the number of flowers on the plant. In this case the size of attractive structures is larger in male flowers than in female flowers in dioecious plants, and this size decreases with an increase in the self-fertilization rate for hermaphroditic plants (where both are consistent with the general trends observed). In the second analysis, it was assumed that the size–number trade-off for flowers is linear and the frequency of pollinator visits per flower depends not only on the size of attractive structures of individual flowers, but also on the number of flowers on the plant. However, the results were inconsistent with the general trends in this case; namely, that the size of the attractive structures is smaller in male flowers than in female flowers in dioecious plants, and this size increases with an increase in self-fertilization rate for hermaphroditic plants. I therefore conclude that the non-linearity in size–number trade-offs for flowers is a more useful factor to use to explain the general trends in the size of attractive structures of a flower. This also suggests that the attractive function hypothesis, that is, the differences in fitness gains through allocation to attractive structures between males and females and among plants with different self-fertilization rates, are important factors, but alone cannot explain the general trends.

Phylogenetic placement of the enigmatic Western Australian genus Emblingia based on rbcL sequences

Abstract: Assignment of the enigmatic Australian genus Emblingia to a particular family or order has been difficult. Informative morphological characters have not as yet been found to place Emblingia conclusively into a family, though it does share a number of attributes with the Capparaceae and Resedaceae. As a result, in the past it has been put in various families (Capparaceae, Sapindaceae, Goodeniaceae and Polygalaceae), representing a number of orders, as well as in its own family, the Emblingiaceae. The current molecular study, using rbcL, shows strong support for the placement of Emblingia within the Brassicales, and possibly sister to the Resedaceae. Further morphological and molecular studies within the Brassicales are needed before finalizing the familial placement of this genus. At this time, we consider treatment of Emblingia as a monotypic family, Emblingiaceae, within the order Brassicales the most satisfactory solution.

Evolution of interactions between plants and their pollinators

Abstract: Our theoretical understanding of plant reproductive character evolution often only implicitly includes pollinator behavior and the community ecological context of pollination. Reciprocal reservations apply to studies of pollinator evolution. I recast selection models to incorporate interaction between plants of one species with the pollinators of another, emphasizing primarily the plant perspective. I then include pollination and community processes, such as optimal foraging and interspecific competition for pollinators. This highlights how many models implicitly assume that a plant species exerts a monopoly on its pollinators. Such an assumption is at odds with the observation of widespread generalist interactions. Reconsideration of previous literature surveys and studies of plant–pollinator interaction illustrate this perspective.

Molecules, morphology and maps: New directions in evolutionary genetics

Abstract: With the characterization of a complete nucleotide sequence of a plant genome within sight, we are getting access to the ultimate source of data for evolutionary genetics. This achievement could hardly have been imagined twenty years ago. The rapid progress in methods is generating novel results that require a reassessment of our ideas about the relationship between the evolution of genomes and organisms, and of the relationship between genotype and phenotype. The widespread incongruence between chloroplast and nuclear (organismal) phylogenies was not previously suspected. It shows that introgression based on ‘wide crosses’ is a regular process in natural populations, and geneflow among species and genera must influence the distribution of genes and alleles as much as linear, phyletic inheritance. Speciation from hybrids at the diploid level seems to be stabilized by the creation of new epistatic interactions. Speciation via allopolyploidy also involves more than the additive action of the combined genomes; rearrangements and homogenization of non-coding repetitive sequences seems to occur quickly, and repetitive rDNA sequences of the combined genomes are homogenized to one of the parental sequences or to a recombinant sequence. Diploidization of allopolyploid genomes involves the evolution of redundant genes. This has profound effects on morphological evolution. Extrapolation from the developmental genetics of model systems (e.g. Arabidopsis) has shown that single mutations in regulatory genes can explain major differences in diagnostic characters between taxa. This seems to be in conflict with the general assumption that morphological evolution proceeds gradually by the selection of allelic polymorphisms in populations. Where plants with major morphological character differences can be crossed to produce fertile offspring, genetic analysis aided by molecular marker maps confirms that major regulatory genes are involved. At the same time, such experiments suggest a hypothesis integrating saltatory evolution and population genetics. The key factor is genetic redundancy through duplicate or modifier genes that reduce the phenotypic penetrance of the major gene. A low incidence of the new, at first unadapted phenotype, possibly mainly under environmental stress conditions, facilitates selection for the proper genetic background and for a new ‘integrated genotype’. Such episodes of saltatory evolution are likely to be linked to reticulate evolution; wide hybrids are likely to break up the protective modifier networks and expose major gene effects, while polyploidy immediately generates genetic redundancy

Differentiation of the timing of flower abortion in Tilia japonica

Abstract: The sex expression of Tilia japonica is functional andromonoecy (an andromonoecious system with abortion of perfect flowers). Some of protandrous flowers are aborted after shedding pollen grains and thus are functionally male flowers (FMF). When both male and female organs are retained in a flower, the plants can more flexibly regulate sex allocation, or resource allocation to male and female functions than true andromonoecious plants, which is considered as an adaptive significance of functional andromonoecy. However, the plants have to pay some costs of female organ in FMF. For functional andromonoecious plants, there is a conflict between flexibility and cost in determining the timing of flower abortion, or arrest investing in female functions in a flower. We clarified that (i) differentiation of the timing of flower abortion caused two types of FMF: short-style (SS) and normal-style (NS) flowers; (ii) SS can be produced at a lower cost than NS, and (iii) plants having NS can flexibly regulate sex allocation following a sudden change in conditions. Hence, we concluded that differentiation of the timing of flower abortion makes a compromise of the conflict. We also found that NS is more frequent than SS, and we discuss the adaptive significance of keeping potential fertility.

Response of Trifolium repens to a mosaic of bare and vegetated patches

Abstract: In herbaceous vegetation, plants are subjected to a spatial heterogeneous distribution of light. Trifolium repens was tested on its ability to avoid vegetated patches by change of stolon growth direction, reduced branching or reduced growth. Treatments consisted of a no grass control, grass on the west side and grass on the north side. When Trifolium was growing beside a grass patch on the west side, plants reduced branching but did not change direction nor was there reduced growth in stolons that were growing towards/in the vegetated patch compared to the control plants. When the grass patch was situated to the north of the plant, plants did not show reduced branching except of the primary stolon, nor was there change of growth direction or reduced growth. The biomass and number of ramets were reduced in the vegetated patch of the west treatment and the patch adjacent to the grass patch. In the north treatment biomass and number of ramets were only reduced in the vegetated patch. Compared to the control, total plant biomass was reduced in the west-grass-patch treatment but not in the north-grass-patch treatment. Reduced branching near grass patches could be an added mechanism by which to avoid vegetated patches; it is not very effective, however, because biomass production did not increase in the open patches but was only reduced in the vegetated patches.