Early Flowering Plants Research Articles

An early infructescenceHyrcantha decussata(comb. nov.) from the Yixian Formation in northeastern China

The continuing study of early angiosperms from the Yixian Formation (approximately 125 Ma) of northeastern China has yielded a second early angiosperm genus. This report is a detailed account of this early flowering plant and recognizes earlier reports of similar fossils from Russia and China. Entire plants, including roots, stems, and branches terminating in fruits are presented and reconstructed. Evidence for a possible aquatic nature of this plant is presented. The relationship of Hyrcantha ("Sinocarpus") to the eudicots is discussed. The presence of this second early angiosperm genus, now known as a whole plant, is important in the discussion of its systematics and the ecology of the earliest angiosperms.

Mapping and characterization of FLC homologs and QTL analysis of flowering time in Brassica oleracea

The FLC gene product is an inhibitor of flowering in Arabidopsis. FLC homologs in Brassica species are thought to control vernalization. We cloned four FLC homologs (BoFLCs) from Brassica oleracea. Three of these, BoFLC1, BoFLC3 and BoFLC5, have been previously characterized. The fourth novel sequence displayed 98% sequence homology to the previously identified gene BoFLC4, but also showed 91% homology to BrFLC2 from Brassica rapa. Phylogenetic analysis showed that this clone belongs to the FLC2 clade. Therefore, we designated this gene BoFLC2. Based on the segregation of RFLP, SRAP, CAPS, SSR and AFLP loci, a detailed linkage map of B. oleracea was constructed in the F(2) progeny obtained from a cross of B. oleracea cv. Green Comet (broccoli; non-vernalization type) and B. oleracea cv. Reiho (cabbage; vernalization type), which covered 540 cM, 9 major linkage groups. Six quantitative trait loci (QTL) controlling flowering time were detected. BoFLC1, BoFLC3 and BoFLC5 were not linked to the QTLs controlling flowering time. However, the largest QTL effect was located in the region where BoFLC2 was mapped. Genotyping of F(2 )plants at the BoFLC2 locus showed that most of the early flowering plants were homozygotes of BoFLC-GC, whereas most of the late- and non-flowering plants were homozygotes of BoFLC-Rei. The BoFLC2 homologs present in plants of the non-vernalization type were non-functional, due to a frameshift in exon 4. Moreover, duplications and deletions of BoFLC2 were detected in broccoli and a rapid cycling line, respectively. These results suggest that BoFLC2 contributes to the control of flowering time in B. oleracea.

Hybrids and fruit set in a mixed flowering-time population of Gymnadenia conopsea (Orchidaceae)

We have recently found that the morphologically determined subspecies Gymnadenia conopsea ssp conopsea in Sweden includes early and late flowering individuals. We were interested in the interactions between the flowering time groups; if there were gene flow between them and if so this was detrimental or advantageous. A spatially mixed population of early and late flowering individuals was studied using three microsatellite loci. We measured patterns in genetic differentiation and inferred occurrence of hybridisation and introgression. Variation in flowering time, fertility and relative and absolute fruit set was measured. The pattern of introgression between flowering-time groups differed between loci. In two of the three investigated loci, allele separation was distinct between early and late flowering plants and one genetically obvious hybrid was infertile. In the third locus, several alleles were shared between the two flowering time variants. The degree of introgression was associated to fruit set failure, which was higher in the late flowering plants and lower in early flowering plants. A small group of early flowering individuals with somewhat delayed flowering compared to the main group was genetically distinct and had lower relative and absolute fruit set. This group was not genetically intermediate, but rather constituting an independent group, with lower fruit set possibly caused by absence of pollinators. There seem to be a strong barrier against introgression into the late flowering group which is kept genetically distinct and less diverse. The early flowering group is diverse, includes two subgroups and seems to benefit from gene flow.

A New Test Detects Selection for Early Flowering in a Much-Studied Weed

Plants use a wide range of reproductive strategies to get around the fact that they can't pull up stakes when conditions deteriorate. The life cycle of flowering plants precisely tracks local daylight and temperature cycles to optimize flowering and boost reproductive success. The timing of flowering is crucial. As any temperate-gardener knows, a delicate plant that flowers too early in the season is doomed to perish with the next frost. Much has been learned about the genes plants use to synchronize flowering with favorable environmental conditions by studying Arabidopsis thaliana. This slight mustard weed grows throughout the Northern Hemisphere. Reflecting adaptations to local environments, plants from different locations flower at different times when grown under the same light and temperature conditions. In particular, many plants take a very long time to flower unless induced to do so by prolonged exposure to cold in a process known as vernalization (the same process that forces spring bulbs into early bloom). This requirement for vernalization has been linked to the FRIGIDA (FRI) gene, based on observations that plants with nonfunctional FRI variants, or alleles, flower early without forcing. Two of these loss-of-function alleles— friCol and friLer—are linked to many of the early-flowering plants found in Europe. There's a good chance that a gene associated with a trait directly related to reproductive success would show signs of selective pressure—and that's just what a new study shows. Christopher Toomajian, Magnus Nordborg, and their colleagues developed a novel genomics-based approach to detect selection and provide evidence that the two FRI alleles are under selection for rapid flowering in Arabidopsis. A standard approach for detecting selection on a particular genomic region relies on the theoretical predictions of the neutral theory of molecular evolution. The problem with this approach, Toomajian et al. argue, is that many other forces besides selection can cause a deviation in the data from what is expected under simple neutral models. And with the plethora of genomic polymorphism data, they explain, it's possible to forgo the models and compare genome-wide patterns of variation instead. If the pattern at a region of interest differs radically from the genomic pattern, the region may be under selection. A 2002 study of polymorphism around the FRI locus found that plants carrying one of the early flowering alleles also shared long blocks of identical chromosomal regions, or haplotypes. Building on those results, the researchers looked for patterns of haplotype sharing in genomic data from 96 Arabidopsis plants to see if the length of these haplotypes was typical, in which case the region probably wasn't under selection, or unusual, in which case it probably was. To compare haplotype sharing around the FRI alleles with sharing at thousands of other loci, the researchers developed a new test, called the pairwise haplotype sharing (PHS) score. This score includes a function that controls for population structure: since pairs of individuals from the same population are more closely related than those from different populations, they're more likely to share long haplotypes and could bias the results. PHS scores were calculated for all alleles found in the dataset, including the two FRI alleles, which had abnormally high haplotype sharing scores compared with the “vast majority” of alleles. Thirty-one other alleles also showed higher-than-average PHS scores, which the researchers plan to investigate as possible selection candidates in a future study. They also measured haplotype sharing using another test developed by Pardis Sabeti and colleagues, called extended haplotype homozygosity, which relies on the relationship between an allele's population frequency and its linkage to surrounding loci to determine its likelihood of being under selection given its age. This measure, they show, identifies multiple alleles from very similar sets of individuals that tend to come from the same population. This property can erroneously attribute haplotype sharing caused by population structure to selection. But this potential drawback shouldn't be a problem for organisms with a less complex population structure, like humans. The researchers acknowledge that scanning the genome for haplotype sharing can't pinpoint the target of selection, but these results strongly suggest an adaptive role for the FRI alleles. While the evidence for selection is stronger for friCol than for frILer, it may be that selection on frILer happened much earlier, since both alleles have similar effects on the plant. This possibility is supported by the estimated age of the alleles, which the researchers place at 800 generations ago for friCol and 3,200 generations ago for frILer. Thus, if one assumes at least one generation per year, the loss-of-function FRI alleles emerged on the heels of the last glacial retreat, some 13,000 years ago. The researchers speculate that the evolution of these early flowering alleles, which turn an annual weed into a rapid reproducer, reflects selection for “weediness” in response to agriculture—an intriguing possibility that future studies can explore.

Pollinator Importance and Temporal Variation in a Population of Phlox divaricata L. (Polemoniaceae)

Temporal variation in climatic conditions is a regular occurrence for most early spring flowering plants in temperate regions, often producing variation in available pollinators. Evaluating the effectiveness and importance of pollinators to a plant population provides an indication of the relative influence of each visitor on a population's reproductive success and selection on floral traits. This study examined pollinator activity and its effect on seed set in a population of Phlox divaricata L. (Polemoniaceae), investigating variation over the 24-h day and the flowering season for two consecutive years. Eleven species, representing three orders of insects, were documented visiting the study population, although lepidopterans (four species, particularly Hemaris diffinis and Megalographa biloba) accounted for 90% of all visitation. The highest visitation rates (and resulting seed set) occurred during the morning and afternoon diurnal time periods and the evening crepuscular time period. The relative frequencies of the different visitors varied over the 24-h day and between flowering seasons. Most visitors that could be evaluated were found to be effective pollinators. Thus, a pollinator's importance to reproductive success was generally dictated by its visitation frequency, H. diffinis being the most important pollinator. This study highlights the importance of considering temporal variation in effective pollinators in species experiencing variable environmental conditions.

Long-term response of spring flora to chronic herbivory and deer exclusion in Great Smoky Mountains National Park, USA

Chronic herbivory by white-tailed deer ( Odocoileus virginianus Zimmerman) can have profound impacts on the function and structure of forest ecosystems. We examined the combined influence of intense herbivory associated with a deer population eruption and chronic herbivory by the post eruption population on the spring flora of Cades Cove, Great Smoky Mountains National Park. During the 1970s the deer population reached a peak of 43 deer per km 2, from which it has slowly declined in recent decades. To examine the influence of intense herbivory, we compared the abundance and flowering rates of early flowering plants in Cades Cove to a nearby reference site with similar bedrock geology, vegetation, and disturbance history but contrasting history of deer abundance. Our results suggest that significant changes ( p ⩽ 0.05) in the diversity, evenness, and species richness of the spring flora occurred during the eruptive phase. Trillium spp. and other liliaceous species appeared to be disproportionately impacted. Comparisons between control and exclosure plots established after the deer population eruption indicate that recovery has been largely restricted to species that were able to persist under intense herbivory. These species have increased in number in exclosures, suggesting continued impacts by deer on the plant community outside the exclosures. Little to no recolonization by browse sensitive species was observed. Consequently, to restore the natural diversity of early flowering plants once present in Cades Cove, active restoration may be necessary in addition to maintaining deer densities below current levels.

Effects of ski piste preparation on alpine vegetation

Summary Ski resorts increasingly affect alpine ecosystems through enlargement of ski pistes, machine‐grading of ski piste areas and increasing use of artificial snow. In 12 Swiss alpine ski resorts, we investigated the effects of ski piste management on vegetation structure and composition using a pairwise design of 38 plots on ski pistes and 38 adjacent plots off‐piste. Plots on ski pistes had lower species richness and productivity, and lower abundance and cover of woody plants and early flowering species, than reference plots. Plots on machine‐graded pistes had higher indicator values for nutrients and light, and lower vegetation cover, productivity, species diversity and abundance of early flowering and woody plants. Time since machine‐grading did not mitigate the impacts of machine‐grading, even for those plots where revegetation had been attempted by sowing. The longer artificial snow had been used on ski pistes (2–15 years), the higher the moisture and nutrient indicator values. Longer use also affected species composition by increasing the abundance of woody plants, snowbed species and late‐flowering species, and decreasing wind‐edge species. Synthesis and applications. All types of ski piste management cause deviations from the natural structure and composition of alpine vegetation, and lead to lower plant species diversity. Machine‐grading causes particularly severe and lasting impacts on alpine vegetation, which are mitigated neither by time nor by revegetation measures. The impacts of artificial snow increase with the period of time since it was first applied to ski piste vegetation. Extensive machine‐grading and snow production should be avoided, especially in areas where nutrient and water input are a concern. Ski pistes should not be established in areas where the alpine vegetation has a high conservation value.

Form, function and environments of the early angiosperms: merging extant phylogeny and ecophysiology with fossils

The flowering plants--angiosperms--appeared during the Early Cretaceous period and within 10-30 Myr dominated the species composition of many floras worldwide. Emerging insights into the phylogenetics of development and discoveries of early angiosperm fossils are shedding increased light on the patterns and processes of early angiosperm evolution. However, we also need to integrate ecology, in particular how early angiosperms established a roothold in pre-existing Mesozoic plant communities. These events were critical in guiding subsequent waves of angiosperm diversification during the Aptian-Albian. Previous pictures of the early flowering plant ecology have been diverse, ranging from large tropical rainforest trees, weedy drought-adapted and colonizing shrubs, disturbance- and sun-loving rhizomatous herbs, and, more recently, aquatic herbs; however, none of these images were tethered to a robust hypothesis of angiosperm phylogeny. Here, we synthesize our current understanding of early angiosperm ecology, focusing on patterns of functional ecology, by merging recent molecular phylogenetic studies and functional studies on extant 'basal angiosperms' with the picture of early angiosperm evolution drawn by the fossil record.

Lotus burttii Takes a Position of the Third Corner in the Lotus Molecular Genetics Triangle

In order to consolidate molecular genetic system in Lotus japonicus and to further access the biological diversity in Lotea, we introduce here Lotus burttii B-303 derived from West Pakistan as the third crossing partner of the Gifu ecotype (B-129-S9) for a genetic analysis. L. burttii is a relatively small and early flowering plant with non-shattering behavior. The general chromosome morphology is very similar to Gifu, and fluorescence in situ hybridization (FISH) analysis revealed that the short arm of chromosome 1 in L. burttii is comparable to that of Gifu, indicating that the translocation event involving chromosomes 1 and 2, which was observed in L. japonicus Miyakojima MG-20, is not present in L. burttii. In addition L. burttii has a higher level of DNA polymorphism compared to Gifu and MG-20 enabling design of codominant markers such as SSR, CAPS and dCAPS. Using an F2 population from a cross between Gifu and L. burttii, codominant makers that co-segregated at the translocation site could be expanded. In order to normalize the genetic background, L. burttii was inbred for nine generations and the germplasm L. burttii B-303-S9 was established.

Scarlet gilia resistance to insect herbivory: the effects of early season browsing, plant apparency, and phytochemistry on patterns of seed fly attack

Pollen-limited plants are confronted with a difficult tradeoff because they must present showy floral displays to attract pollinators and yet must also minimize their apparency to herbivores. In these systems, traits that increase pollinator visitation may also increase herbivore oviposition and overall plant resistance may therefore be constrained to evolve largely as a correlated response to selection on plant apparency or vigor. We used a family-structured quantitative genetic experiment to evaluate the importance of ungulate browsing, flowering date and plant height (traits that are related to overall vigor), and variation in a putative phytochemical defense (cucurbitacin production) on patterns of seed fly attack in a scarlet gilia population. We found significant genetic variation in the amount of insect damage plants experience in the field, providing evidence that resistance may evolve. In addition, we found that browsing reduced seed fly attack and that oviposition is strongly related to plant size and flowering date; large, early flowering plants experience high attack. In addition, we found that high cucurbitacin production was correlated with low seed fly damage, although this effect was relatively weak.

Chrysanthemum stunt viroid disturbs the photoperiodic response for flowering of chrysanthemum plants

Chrysanthemum ( Dendranthema grandiflorum Kitam.) is one of the qualitative short-day flowering plants. Therefore, the flowering of chrysanthemum can usually be controlled by photoperiod. However, it was noted that 'Piato' plants infected by the chrysanthemum stunt viroid (CSVd) flowered autonomously even under long-day conditions. In this study, CSVd-free and CSVd-infected plants were prepared by culturing different-sized dissected shoot apical meristems (SAMs) of 'Piato'. Using these CSVd-free and CSVd-infected plants, we clarified the relationship between CSVd infection and the autonomous flowering of 'Piato'. Under natural short-day conditions, the flowering of plants regenerated from SAMs containing leaf primordia (LPs) was 1 month earlier than plants regenerated from LP-free SAMs. CSVd was detected from these early flowering plants by reverse transcription-polymerase chain reaction. On the other hand, CSVd was not detected in plants regenerated from LP-free SAMs. CSVd-infected and CSVd-free plants were grown under long-day conditions simulated by night-break lighting at 22:00 p.m. to 2:00 a.m. All CSVd-infected plants flowered autonomously even under long-day conditions; on the other hand, CSVd-free chrysanthemum plants maintained their vegetative growth. When the CSVd-free plants were inoculated with CSVd by grafting them to CSVd-infected rootstocks, they flowered autonomously even under night-break lighting. In this study, the results suggest that CSVd may control the qualitative development process, flowering, i.e. CSVd can induce the autonomous flowering of chrysanthemum.

EVOLUTIONARY GENETICS OF RESISTANCE AND TOLERANCE TO NATURAL HERBIVORY IN ARABIDOPSIS THALIANA

Resistance and tolerance are widely viewed as two alternative adaptive responses to herbivory. However, the traits underlying resistance and tolerance remain largely unknown, as does the genetic architecture of herbivory responses and the prevalence of genetic trade-offs. To address these issues, we measured resistance and tolerance to natural apical meristem damage (AMD) by rabbits in a large field experiment with recombinant inbred lines (RILs) of Arabidopsis thaliana (developed from a cross between the Columbia × Landsberg erecta ecotypes). We also measured phenological and morphological traits hypothesized to underlie resistance and tolerance to AMD. Recombinant inbred lines differed significantly in resistance (the proportion of replicates within an RIL that resisted herbivory), and early flowering plants with tall apical inflorescences were more likely to experience damage. Tolerance (the difference in fitness between the damaged and undamaged states), also differed significantly among RILs, with some lines overcompensating for damage and producing more fruit in the damaged than undamaged state. Plastic increases in basal branch number, basal branch height, and senescence date in response to damage were all associated with greater tolerance. There was no evidence for a genetic trade-off between resistance and tolerance, an observation consistent with the underlying differences in associated morphological and phenological characters. Selection gradient analysis detected no evidence for direct selection on either resistance or tolerance in this experiment. However, a statistical model indicates that the pattern of selection on resistance depends strongly on the mean level of tolerance, and selection on tolerance depends strongly on the mean level of resistance. These observations are consistent with the hypothesis that selection may act to maintain resistance and tolerance at intermediate levels in spatially or temporally varying environments or those with varying herbivore populations.

Effects of transgenic glufosinate-tolerant oilseed rape (Brassica napus) and the associated herbicide application on eubacterial and Pseudomonas communities in the rhizosphere

A containment experiment was carried out in order to evaluate possible shifts in eubacterial and Pseudomonas rhizosphere community structures due to the release of genetically modified Basta-tolerant oilseed rape and the associated herbicide application. Treatments included cultivation of the transgenic plant as well as of the wild-type cultivar in combination with mechanical removal of weeds and the application of the herbicides Basta (active ingredient: glufosinate) and Butisan S (active ingredient: metazachlor). Rhizosphere soil was sampled from early and late flowering plants as well as from senescent plants. A culture-independent approach was chosen to characterize microbial communities based on denaturing gradient gel electrophoresis of 16S rRNA gene fragments amplified from rhizosphere DNA using eubacterial and Pseudomonas-specific PCR primers. Dominant pseudomonads in the rhizosphere were analyzed by sequence analysis. Whole community and Pseudomonas electrophoresis fingerprints revealed slightly altered microbial communities in the rhizosphere of transgenic plants; however, effects were minor as compared to the plant developmental stage-dependent shifts. Both herbicides caused transient changes in the eubacterial and Pseudomonas population structure, whereas differences due to the genetic modification were still detected at the senescent growth stage. The observed differences between transgenic and wild-type lines were most likely due to unintentionally modified plant characteristics such as altered root exudation.

Effects of transgenic glufosinate-tolerant oilseed rape ( Brassica napus) and the associated herbicide application on eubacterial and Pseudomonas communities in the rhizosphere

A containment experiment was carried out in order to evaluate possible shifts in eubacterial and Pseudomonas rhizosphere community structures due to the release of genetically modified Basta-tolerant oilseed rape and the associated herbicide application. Treatments included cultivation of the transgenic plant as well as of the wild-type cultivar in combination with mechanical removal of weeds and the application of the herbicides Basta (active ingredient: glufosinate) and Butisan S (active ingredient: metazachlor). Rhizosphere soil was sampled from early and late flowering plants as well as from senescent plants. A culture-independent approach was chosen to characterize microbial communities based on denaturing gradient gel electrophoresis of 16S rRNA gene fragments amplified from rhizosphere DNA using eubacterial and Pseudomonas-specific PCR primers. Dominant pseudomonads in the rhizosphere were analyzed by sequence analysis. Whole community and Pseudomonas electrophoresis fingerprints revealed slightly altered microbial communities in the rhizosphere of transgenic plants; however, effects were minor as compared to the plant developmental stage-dependent shifts. Both herbicides caused transient changes in the eubacterial and Pseudomonas population structure, whereas differences due to the genetic modification were still detected at the senescent growth stage. The observed differences between transgenic and wild-type lines were most likely due to unintentionally modified plant characteristics such as altered root exudation.

The advantage of early flowering in the spring ephemeral annual plant Floerkea proserpinacoides.

• Internal conflicts in the allocation of resources lead to trade-offs or compromises that can influence fitness and the evolution of specific life-history traits. • Here, the reproductive schedule of Floerkea proserpinacoides plants, a spring ephemeral annual of the deciduous forests of eastern North America, was manipulated to determine the presence of potential conflicts between growth and reproduction, and the advantage of early reproduction. • Plants on which flowering was delayed had a greater relative growth rate (RGR) and vegetative biomass than control plants after 8wks (when control plants started to senesce). Delayed-flowering plants took 1week longer to senesce, which allowed them to mature almost as many seeds as did control plants in 8wks. • However, late flowering in F. proserpinacoides may be maladaptive. With reduced light availability through canopy closure, early flowering plants can rapidly divert most of the resources accumulated in the stem and leaves to seed maturation. Thus, the advantage of maintaining a higher RGR by delaying flowering is associated with the greater disadvantage of necessitating more time to mature seeds.

MODIFYING LISIANTHUS TRAITS BY GENETIC ENGINEERING

The control of flowering time is an essential issue for most ornamentals. The molecular basis of this trait has been extensively studied in model plants, yielding useful information on the role of various genes which could potentially be used in heterologous systems. For instance, overexpression of the meristem-identity gene LEAFY (LFY) from Arabidopsis has previously been shown to be sufficient to reduce flowering time in plants as diverse as Arabidopsis itself, tobacco and aspen. We tested the effect of LFY overexpression on flower development in lisianthus plants transformed with LFY cDNA under constitutive control of the 35S CaMV promoter (35S::LFY, provided by Dr. Weigel). F1 progenies from two different transformed plants flowered in average one to two weeks earlier than control plants at two consecutive years. The number of nodes to the first flower as well as the number of flowers per inflorescence were reduced in the early-flowering plants, suggesting a faster flower development. Normal fertile flowers were obtained from all plants. These results indicate that flowering time can me manipulated by transgenic means. Lisianthus transformations with genes acting upstream to LFY are currently being made. The possibility of introducing a fragrance to lisianthus flowers by genetic engineering is being investigated using genes from the scented plant Clarkia breweri: Acetyl CoA benzyl alcohol acetyl transferase (BEAT) and S-linalool synthase (LIS). These two genes code for key enzymes in the synthesis of the aroma compounds benzyl acetate and S-linalool respectively. Aroma modification of the fruit of transgenic tomato plants carrying a LIS construct has already been demonstrated. The effect of BEAT and LIS transgenes on tobacco and lisianthus fragrance is under investigation.

Flowering time in oil flax can be influenced by microgametophytic selection

The effect of selection for pollen competitive ability in F1hybrids on days to flowering and some other morphological traits was studied in oil flax. Various intensity of selection on the rate of pollen tube growth was ensured by the change of the time intervals from pollination to the deletion of a part of the style. In the experimental variants the upper half of the style of F1hybrids was cut off at 40, 60, and 120 minutes, in the control styles being native. It was shown that F2 population structure essentialy changed for some morphological traits independence of competition intensity between the microgametophytes. The percentage of early flowering genotypes increased with shortening the period from pollination till style excision. In the treatment where styles were cut off 40 min after pollination the frequency of early flowering plants was highest while the frequency of those plants in 120 min treatment approached the control. Moreover, some treatments considerably influenced the phenotypic ratio in the F2 population for such traits as petal colour and number of side shoots. It is concluded that a positive relationship exists between pollen competitive ability and early flowering in oil flax. The approach based on selection of fast growing pollen tubes that increases the frequency of early flowering plants could be used in oil flax breeding for early ripeness.

Variation of pollen limitation in the early flowering Mediterranean geophyte Narcissus assoanus (Amaryllidaceae).

A recent literature review indicates that pollen limitation of female fertility is a common feature of flowering plants. Despite the ecological and evolutionary significance of pollen limitation, most studies have only examined fertility in a single population at one time. Here we investigate pollen limitation of fruit and seed set in five populations of Narcissus assoanus, a self- sterile, insect-pollinated geophyte, over 2-3 years in southern France. In common with many early spring flowering plants, pollinator visitation to N. assoanus is often infrequent. Supplemental hand-pollination of flowers with outcross pollen significantly increased overall fruit and seed set by 11% and 19%, respectively. Four of the five populations experienced some pollen limitation during the study. For a given year, there was significant variation in pollen limitation among populations. Two of the populations were pollen limited in one year but not in other years in which they were studied. Seed:ovule ratios for open- and hand-pollinated flowers averaged 0.29 and 0.33, respectively. While hand pollination significantly increased the seed:ovule ratio, the low value obtained indicates that the majority of ovules in flowers do not mature seeds despite hand pollination. The role of genetic and environmental factors governing low seed:ovule ratios in N. assoanus is discussed.

Toward a new synthesis: major evolutionary trends in the angiosperm fossil record.

Angiosperm paleobotany has widened its horizons, incorporated new techniques, developed new databases, and accepted new questions that can now focus on the evolution of the group. The fossil record of early flowering plants is now playing an active role in addressing questions of angiosperm phylogeny, angiosperm origins, and angiosperm radiations. Three basic nodes of angiosperm radiations are identified: (i) the closed carpel and showy radially symmetrical flower, (ii) the bilateral flower, and (iii) fleshy fruits and nutritious nuts and seeds. These are all coevolutionary events and spread out through time during angiosperm evolution. The proposal is made that the genetics of the angiosperms pressured the evolution of the group toward reproductive systems that favored outcrossing. This resulted in the strongest selection in the angiosperms being directed toward the flower, fruits, and seeds. That is why these organs often provide the best systematic characters for the group.

Directional selection on initial flowering date in Phlox drummondii (Polemoniaceae)

The fitness effects due to initial flowering date in Phlox drummondii were determined for three populations in central Texas (USA) over 3 yr (1990-1992). Mean fitness (seed set) always decreased with the later initiation of flowering. The likelihood of a plant fruiting differed with flowering date in five of the six instances (population by year combinations). Though plants that initiated flowering later tended to have spent more time in the vegetative stage and tended to die later in the year than did earlier flowering plants, this was not sufficient to overcome the reproductive penalties of flowering late. Plants that initiated flowering later in the season spent less time in the adult phase and were smaller. The mean number of flowers, fruits, and seeds per flowering plant always decreased with later flowering. Fruit set was negatively correlated with flowering date in four of the six population by year combinations. Nonparametric fitness functions were used to summarize predicted fitness among different initial flowering dates for each population on a yearly basis. Predicted mean fitness always declined nonlinearly with later flowering; the earliest flowering plants always had the highest predicted fitness. These fitness functions describe directional selection for the early initiation of flowering.