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DNA amplification patterns in maize endosperm nuclei during kernel development.

Increased DNA levels in centrally located endosperm nuclei are shown to be related to endosperm development in Zea mays. Mitotic activity sharply decreases in endosperm cells 10-12 days after pollination. At this time nuclear size and DNA content per nucleus (where C = haploid content) sharply increase until peak levels are reached at about 14-18 days after pollination. Mean DNA content per endosperm nucleus in strain A188 was shown by Feulgen cytophotometry to increase to about 90C by this peak stage, with the pattern being remarkably consistent over four consecutive growing seasons. Some individual nuclei achieved levels of >200C. Most other strains compared during one growing season averaged even higher peak levels of DNA per nucleus than did A188. Individual nuclei in those strains reached levels as high as 690C. A decrease in DNA level was observed in older endosperms with most strains. Endosperm mutant strains did not show a significant reduction in DNA. Opaque-2 mutants in several backgrounds achieved higher levels of DNA per nucleus. DNA levels from F(1) endosperms did not indicate heterosis. Regardless of differences in DNA content, the pattern of DNA increasing as development proceeds followed by a DNA decrease was observed for most strains. Cytological studies reveal much variation in chromatin strandedness, a maximum of three nucleoli, a maximum of three nucleolar organizer regions, and approximately 30 diffuse chromatin masses in older endosperm tissue. A form of DNA amplification, perhaps polytenization, appears to be occurring during endosperm development.

Journal Article↗

Oldest fossil flowers of hamamelidaceous affinity, from the Late Cretaceous of New Jersey.

Exceptionally well-preserved staminate inflorescences, pistillate inflorescences, and detached stamens with important phylogenetic and paleoecological implications have been discovered from the Turonian (ca. 88.5-90.4 million years B.P.) Raritan Formation of New Jersey. The fossils have a combination of floral and pollen characters found in various genera of modern entomophilous and anemophilous Hamamelidaceae and anemophilous Platanus (Platanaceae). The floral characters of the fossils, including a sepal cup, staminal tube, and apparently nectariferous staminodes, indicate that this taxon was probably insect pollinated. The juxtaposition of character complexes in an extinct taxon from disparate modern taxa provides an interesting phylogenetic perspective on the origins of Hamamelidaceae and is a striking example of a fossil that is a mosaic of familial level characters relative to modern taxa. Of even broader interest, however, is the occurrence of staminodal nectaries that have structural characters intermediate between the fossil's functional stamens and modern hamamelidaceous petals. This transitional staminode morphology in the context of the other fossil characters suggests a staminodal origin of petals in the hamamelid-rosid lineage. This hypothesis is supported by the apparent staminode position within the fossil flowers where petals are found in modern genera. The character complex of morphologically transitional staminodes, a staminal tube, and sepal cup can be viewed as prehypanthial, lacking only fusion of the staminal tube to the sepal cup. The appearance of the character complex embodied in these flowers during the late mid-Cretaceous may signal the early stages of the relationship between specialized pollinators, such as bees, and the hamamelid-rosid-asterid lineage of angiosperms, arguably one of the most important events in angiosperm radiation.

Journal Article↗

Floral and ecological isolation between Aquilegia formosa and Aquilegia pubescens.

Habitat preference and pollination syndrome have been suggested as major factors in reproductive isolation among plant species. The columbine genus Aquilegia contains species that have been used as classic examples of reproductive isolation due to ecological and floral factors. In this analysis Aquilegia formosa, Aquilegia pubescens, and natural hybrid populations between these two species were assayed for genetic and morphological variation. Clinal variation was evident for three "random amplified polymorphic DNA" loci and five morphological characters along a transect extending from a lower altitude A. formosa population, through an intermediate hybrid population, to a higher altitude A. pubescens population. Similar clinal variation was also discovered for a transect that included A. formosa-like, hybrid, and A. pubescens-like populations at a single elevation. The change in the frequency of both sets of markers was closely associated with change in habitat. The molecular markers indicate the presence of bidirectional introgression between these two species. In contrast, there was apparently selection against introgression of four of the five alternate floral characters. Selection against the incorporation of floral characters from one species into the other species was suggested by the introgression of the DNA markers with little or no introgression of the four floral characters. These findings suggest the importance of adaptations associated with both pollination syndromes and habitat preference on species integrity.

Adaptation, Biological↗

Dynamics of maize endosperm development and DNA endoreduplication.

Endosperm development in Zea mays is characterized by a period of intense mitotic activity followed by a period in which mitosis is essentially eliminated and the cell cycle becomes one of alternating S and G phases, leading to endoreduplication of the nuclear DNA. The endosperm represents a significant contribution to the grain yield of maize; thus, methods that facilitate the study of cellular kinetics may be useful in discerning cellular and molecular components of grain yield. Two mathematical models have been developed to describe the kinetics of endosperm growth. The first describes the kinetics of mitosis during endosperm development; the second describes the kinetics of DNA endoreduplication during endosperm development. The mitotic model is a modification of standard growth curves. The endoreduplication model is composed of six differential equations that represent the progression of nuclei from one DNA content to another during the endoreduplication process. Total nuclei number per endosperm and the number of 3C, 6C, 12C, 24C, 48C, and 96C nuclei per endosperm (C is the haploid DNA content per nucleus) for inbred W64A from 8 to 18 days after pollination were determined by flow cytometry. The results indicate that the change in number of nuclei expressed as a function of the number of days after pollination is the same from one yearly crop to another. These data were used in the model to determine the endosperm growth rate, the maximum nuclei number per endosperm, and transition rates from one C value to the next higher C value. The kinetics of endosperm development are reasonably well represented by the models. Thus, the models provide a means to quantify the complex pattern of endosperm development.

Cell Nucleus↗

Fertilization-independent seed development in Arabidopsis thaliana.

We report mutants in Arabidopsis thaliana (fertilization-independent seed:fis) in which certain processes of seed development are uncoupled from the double fertilization event that occurs after pollination. These mutants were isolated as ethyl methanesulfonate-induced pseudo-revertants of the pistillata phenotype. Although the pistillata (pi) mutant has short siliques devoid of seed, the fis mutants in the pi background have long siliques containing developing seeds, even though the flowers remain free of pollen. The three fis mutations map to loci on three different chromosomes. In fis1 and fis2 seeds, the autonomous endosperm nuclei are diploid and the endosperm develops to the point of cellularization; the partially developed seeds then atrophy. In these two mutants, proembryos are formed in a low proportion of seeds and do not develop beyond the globular stage. When FIS/fis plants are pollinated by pollen from FIS/FIS plants, approximately 50% of the resulting seeds contain fully developed embryos; these seeds germinate and form viable seedlings (FIS/FIS). The other 50% of seeds shrivel and do not germinate; they contain embryos arrested at the torpedo stage (FIS/fis). In normal sexual reproduction, the products of the FIS genes are likely to play important regulatory roles in the development of seed after fertilization.

Arabidopsis↗

Flower color variation: a model for the experimental study of evolution.

We review the study of flower color polymorphisms in the morning glory as a model for the analysis of adaptation. The pathway involved in the determination of flower color phenotype is traced from the molecular and genetic levels to the phenotypic level. Many of the genes that determine the enzymatic components of flavonoid biosynthesis are redundant, but, despite this complexity, it is possible to associate discrete floral phenotypes with individual genes. An important finding is that almost all of the mutations that determine phenotypic differences are the result of transposon insertions. Thus, the flower color diversity seized on by early human domesticators of this plant is a consequence of the rich variety of mobile elements that reside in the morning glory genome. We then consider a long history of research aimed at uncovering the ecological fate of these various flower phenotypes in the southeastern U.S. A large body of work has shown that insect pollinators discriminate against white phenotypes when white flowers are rare in populations. Because the plant is self-compatible, pollinator bias causes an increase in self-fertilization in white maternal plants, which should lead to an increase in the frequency of white genes, according to modifier gene theory. Studies of geographical distributions indicate other, as yet undiscovered, disadvantages associated with the white phenotype. The ultimate goal of connecting ecology to molecular genetics through the medium of phenotype is yet to be attained, but this approach may represent a model for analyzing the translation between these two levels of biological organization.

Journal Article↗

Parthenocarpic apple fruit production conferred by transposon insertion mutations in a MADS-box transcription factor.

Fruit development in higher plants normally requires pollination and fertilization to stimulate cell division of specific floral tissues. In some cases, parthenocarpic fruit development proceeds without either pollination or fertilization. Parthenocarpic fruit without seed has higher commercial value than seeded fruit. Several apple (Malus domestica) mutants (Rae Ime, Spencer Seedless and Wellington Bloomless) are known to produce only apetalous flowers that readily go on to develop into parthenocarpic fruit. Through genetics, a single recessive gene has been identified to control this trait in apple. Flower phenotypes of these apple mutants are strikingly similar to those of the Arabidopsis mutant pistillata (pi), which produces flowers where petals are transformed to sepals and stamens to carpels. In this study, we have cloned the apple PI homolog (MdPI) that shows 64% amino acid sequence identity and closely conserved intron positions and mRNA expression patterns to the Arabidopsis PI. We have identified that in the apetalous mutants MdPI has been mutated by a retrotransposon insertion in intron 4 in the case of Rae Ime and in intron 6 in the case of Spencer Seedless and Wellington Bloomless. The insertion apparently abolishes the normal expression of the MdPI gene. We conclude that the loss of function mutation in the MdPI MADS-box transcription factor confers parthenocarpic fruit development in these apple varieties and demonstrates another function for the MADS- box gene family. The knowledge generated here could be used to produce parthenocarpic fruit cultivars through genetic engineering.

Amino Acid Sequence↗

Immunocytochemical localization of allergenic proteins from mature to activated Zygophyllum fabago L. (Zygophyllaceae) pollen grains.

Zygophyllum fabago L. (Zygophyllaceae) can be found in the Middle East, in North Africa and in the arid zones of the Mediterranean region. It easily establishes itself in new regions, and is considered an invasive plant. They undergo ambophilous pollination, as there is a relationship between this type of pollination and its allergenic incidence. A combination of transmission electron microscopy with immunocytochemical methods was used to localize allergenic proteins during hydration and activation processes. Germination was induced in vitro for 1,2,4,6, and 30 min. The activated proteins reacting with antibodies present in human sera from allergenic patients are found in the cytoplasm, intine, exine and exudates from the pollen grains. The activation time plays an important role on the labelling intensity. Labelling of allergenic proteins was abundant at 1 and 2 min of activation, and decreased at 4 and 6 min. The rapid activation and release of the allergenic proteins appears to be the main cause of allergenic activity of Z. fabago pollen grains.

Allergens↗

Starch branching enzymes in sorghum (Sorghum bicolor) and barley (Hordeum vulgare): comparative analyses of enzyme structure and gene expression.

A genomic clone for starch branching enzyme (SBE) IIb was isolated from a sorghum bacterial artificial chromosome (BAC) library. The promoter and 5' flanking sequence, the first four exons and introns as well as the last exon and the 3' untranslated region were sequenced. The tentative transcription start site of sorghum sbeIIb was mapped based on alignment with the maize sbeIIb gene. The exon-intron structure of the 5' portion of sorghum sbeIIb was similar to that of maize sbeIIb but differed from that of barley sbeIIb. Specifically, the intronic BbI element involved in the endosperm specific expression of barley sbeIIb was lacking in the sorghum gene. A cDNA clone for sorghum sbeIIb was reverse PCR amplified and found to encode an 803 amino acids long protein. The amino acid sequence of sorghum SBEIIb was compared to that of sorghum SBEI and corresponding enzymes in barley. The overall identity in amino acid sequence was 54% in the central portion of the enzymes. A major difference between the SBEII and SBEI isoforms was a 67 amino acids-long C-terminal extension in the SBEIs. The spatial and temporal expression patterns of sorghum sbeIIb was determined and compared with those of the sorghum gene for SBEI and the barley genes for SBEIIB and SBEI. All four genes exhibited a seed specific expression. However, while barley sbeIIb and sbeI transcripts were detected exclusively in the endosperm, the sorghum genes were expressed also in the embryo. The activity of sorghum sbeIIb and sbeI exhibited a late onset, with a peak of transcription at around 22 days after pollination. This is similar to the pattern of barley sbeI but different from that of barley sbeIIb, which showed a peak of transcription at 12 days after pollination.

1,4-alpha-Glucan Branching Enzyme↗

Biogeography and floral evolution of baobabs (Adansonia, Bombacaceae) as inferred from multiple data sets.

The phylogeny of baobab trees was analyzed using four data sets: chloroplast DNA restriction sites, sequences of the chloroplast rpl16 intron, sequences of the internal transcribed spacer (ITS) region of nuclear ribosomal DNA, and morphology. We sampled each of the eight species of Adansonia plus three outgroup taxa from tribe Adansonieae. These data were analyzed singly and in combination using parsimony. ITS and morphology provided the greatest resolution and were largely concordant. The two chloroplast data sets showed concordance with one another but showed significant conflict with ITS and morphology. A possible explanation for the conflict is genealogical discordance within the Malagasy Longitubae, perhaps due to introgression events. A maximum-likelihood analysis of branching times shows that the dispersal between Africa and Australia occurred well after the fragmentation of Gondwana and therefore involved overwater dispersal. The phylogeny does not permit unambiguous reconstruction of floral evolution but suggests the plausible hypothesis that hawkmoth pollination was ancestral in Adansonia and that there were two parallel switches to pollination by mammals in the genus.

Animals↗

Symmetry in Flowers: Diversity and Evolution.

This article traces research on floral symmetry back to its beginnings. It brings together recent advances from different fields that converge in floral symmetry and new unpublished material on diversity and development of floral symmetry. During floral development, symmetry may change: monosymmetric flowers may have a polysymmetric early phase; polysymmetric flowers may have a monosymmetric or even asymmetric early phase; more than one symmetry change is also possible. In Lamiales s.l. (comprising the model plant Antirrhinum, where the cycloidea gene produces monosymmetric flowers with the adaxial side of the androecium reduced), taxa also occur in which the androecium is reduced on both sides, adaxial and abaxial. As a trend in asymmetric flowers, enantiomorphy (with two mirror-image morphs) at the level of individuals seems to occur only in groups in which the flowers are predominantly of a relatively simple construction. In contrast, one morph is fixed at the level of species or higher taxa in groups with more complicated flowers. This is indicated by the apparent lack of enantiomorphy in corolla contortion in asterids but its predominance in rosids with contort flowers, or by the apparent lack of enantiomorphy in the pollination organs of asymmetric flowers in Faboideae but its presence in asymmetric flowers in Caesalpinioideae. To study the evolution of the diverse symmetry patterns, a concerted approach from different fields including molecular developmental genetics, pollination biology, and comparative diversity research is necessary.

Journal Article↗

A theoretical investigation of the evolution and maintenance of mirror-image flowers.

Enantiostyly is the deflection of the female sex organ either to the left or to the right side of the floral axis, resulting in mirror-image flowers. Two types of enantiostyly occur: in monomorphic enantiostyly, individuals exhibit both flower forms, whereas in dimorphic enantiostyly, the forms occur on separate plants. Monomorphic enantiostyly is known from at least 10 families, whereas dimorphic enantiostyly is reported in only three. Phylogenetic evidence suggests that monomorphic enantiostyly has evolved from a straight-styled ancestor and that dimorphic enantiostyly is derived from monomorphic enantiostyly. Here, we use theoretical models to investigate the role of pollen transfer in influencing these evolutionary transitions. We used numerical calculations to examine the evolution of monomorphic and dimorphic enantiostyly under different conditions of pollen transfer, inbreeding depression, and pollinator visitation. Our results demonstrate that in comparison to a putative straight-styled ancestor, both monomorphic and dimorphic enantiostyly function to reduce geitonogamous pollen transfer with a concomitant increase in pollen export. Our calculations suggest that the first stage in the evolution of monomorphic enantiostyly involves the deflection of the style only, followed by selection for reciprocity in anther position to promote more precise cross-pollination. Constraints associated with the developmental genetics of left-right asymmetries may account for the low number of evolutionary transitions from monomorphic to dimorphic enantiostyly, despite the evolutionary stability of this condition once it arises.

Biological Evolution↗

Ecological dynamics of mutualist/antagonist communities.

One approach to understanding how mutualisms function in community settings is to model well-studied pairwise interactions in the presence of the few species with which they interact most strongly. In nature, such species are often specialized antagonists of one or both mutualists. Hence, these models can also shed light on the problem of when and how mutualisms are able to persist in the face of exploitation. We used spatial stochastic simulations to model the ecological dynamics of obligate, species-specific mutualisms between plants and pollinating seed parasite insects (e.g., yuccas and yucca moths) in the presence of one of two obligate antagonist species: flower-feeding insects (florivores) or insects that parasitize seeds but fail to pollinate (exploiters). Our results suggest that mutualisms can persist surprisingly well in the presence of highly specialized antagonists but that they exhibit distinctly different temporal and spatial dynamics when antagonists are present. In our models, antagonists tend to induce oscillations in the mutualist populations. As the number of per capita visits by antagonists increase, the system's oscillatory dynamics become more extreme, finally leading to the extinction of one or more of the three species. When the antagonists exhibit high per capita visitation frequencies and long dispersal distances, significant spatial patchiness emerges within these tripartite interactions. We found surprisingly little difference between the ecological effects of florivores and exploiters, although in general florivores tended to drive themselves (and sometimes the mutualists) to extinction at parameter values at which the exploiters were able to persist. These theoretical results suggest several testable hypotheses regarding the ecological and evolutionary persistence of mutualisms. More broadly, they point to the critical importance of studying the dynamics of pairwise interactions in community contexts.

Animals↗

Natural selection on Erysimum mediohispanicum flower shape: insights into the evolution of zygomorphy.

Paleontological and phylogenetic studies have shown that floral zygomorphy (bilateral symmetry) has evolved independently in several plant groups from actinomorphic (radially symmetric) ancestors as a consequence of strong selection exerted by specialized pollinators. Most studies focused on unraveling the developmental genetics of flower symmetry, but little is known about the adaptive significance of intraspecific flower shape variation under natural conditions. We provide the first evidence for natural selection favoring zygomorphy in a wild population of Erysimum mediohispanicum (Brassicaceae), a plant showing extensive continuous variation in flower shape, ranging from actinomorphic to zygomorphic flowers. By using geometric morphometric tools to describe flower shape, we demonstrate that plants bearing zygomorphic flowers received more pollinator visits and had the highest fitness, measured not only by the number of seeds produced per plant but also by the number of seeds surviving to the juvenile stage. This study provides strong evidence for the existence of significant fitness differences associated with floral shape variation in E. mediohispanicum, thus illuminating a pathway for the evolution of zygomorphy in natural populations.

Animals↗

Gender variation of sequential inflorescences in a monoecious plant Sagittaria trifolia (Alismataceae).

In protogynous plants, female flowers of early blooming plants are at a reproductive disadvantage because they cannot set fruit due to the lack of available pollen. To study this phenomenon, gender expression of the monoecious herb Sagittaria trifolia was investigated over the entire flowering season in two field and two cultivated populations in Hubei and Hunan Provinces, China. In racemes of S. trifolia, flowers open sequentially from bottom to top, with female flowers opening first followed by male flowers. This creates a temporal separation of sexes in the species. Under field conditions small plants are often male, with production of both male and female flowers increasing with plant size. Femaleness increased among sequential inflorescences since female flower production increased whereas male flower production did not. Seed production was greater in large inflorescences because they contain more female flowers, and the number of ovules increased in female flowers at basal positions within the raceme. A consistent pattern of high seed set was observed in flowers from both field and cultivated populations. About 1 % of unfertilized ovules resulted from no pollination and 2 % of the seeds produced were only partly developed due to resource limitation. In the first inflorescence of the six experimental populations, 6.7-40.0 % of individuals produced only male flowers, and female flowers of 1.9-6.5 % individuals were aborted. The occurrence of male flowers in early blooming inflorescences could be an adaptive strategy to conserve resources and enhance pollination of female flowers in protogynous S. trifolia.

Analysis of Variance↗

Evidence for ovarian self-incompatibility as a cause of self-sterility in the relictual woody angiosperm, Pseudowintera axillaris (Winteraceae).

Species within the genus Pseudowintera exhibit high rates of self-sterility. Self-sterility in the genus has been previously posited-but not confirmed-to be the result of late-acting ovarian self-incompatibility (OSI) functioning within nucellar tissue of the ovule to prevent self pollen tubes from entering the embryo sac. Structural and functional aspects of pollen-carpel interactions and early seed development following cross- and self-pollination were investigated in P. axillaris to determine the site, timing and possible mechanisms of self-sterility. No significant differences were observed between pollen tube growth, ovule penetration and double fertilization following cross- and self-pollination. Pollen tubes exhibited phasic growth in an extracellular matrix composed of proteins and carbohydrates, as well as arabinogalactans/arabinogalactan proteins. A uniform failure in embryo sac development prior to division of the zygote was apparent within 15 d following double fertilization by self gametes. Results indicate that SI mechanisms in P. axillaris do not prevent double fertilization from occurring. Instead, mechanisms of self-sterility affect post-zygotic development of the embryo sac. Although self-sterility may be attributed to inbreeding depression, given the post-zygotic nature of failure in embryo sac development, the possibility of late-acting OSI is discussed.

Crosses, Genetic↗

Pseudopollen in Dendrobium unicum Seidenf. (Orchidaceae): reward or deception?

BACKGROUND AND AIMS: In 1987, Kjellsson and Rasmussen described the labellar trichomes of Dendrobium unicum Seidenf. and proposed that these hairs function as pseudopollen. Pseudopollen is a mealy material that superficially resembles pollen, is usually laden with food substances and is formed when labellar hairs either fragment into individual cells or become detached from the labellum. However, the trichomes of D. unicum are very different from pseudopollen-forming hairs found in other orchid genera such as Maxillaria and Polystachya. Moreover, Kjellsson and Rasmussen were unable to demonstrate the presence of food substances within these trichomes and argued that even in the absence of food substances, the hairs, in that they superficially resemble pollen, can still attract insects by deceit. The aim of this paper is to investigate whether the labellar trichomes of D. unicum contain food reserves and thus reward potential pollinators or whether they are devoid of foods and attract insects solely by mimicry. METHODS: Light microscopy, histochemistry and transmission electron microscopy. KEY RESULTS: Dendrobium unicum produces pseudopollen. Pseudopollen here, however, differs from that previously described for other orchid genera in that the pseudopollen-forming trichomes consist of a stalk cell and a 'head' of component cells that separate at maturity, in contrast to Maxillaria and some Polystachya spp. where pseudopollen is formed by the fragmentation of moniliform hairs. Moreover, the pseudopollen of Maxillaria and Polystachya largely contains protein, whereas in D. unicum the main food substance is starch. CONCLUSIONS: Flowers of D. unicum, rather than attracting insects solely by deceit may also reward potential pollinators.

Dendrobium↗

Genetic variability and founder effect in the pitcher plant Sarracenia purpurea (Sarraceniaceae) in populations introduced into Switzerland: from inbreeding to invasion.

BACKGROUND AND AIMS: The long-lived and mainly outcrossing species Sarracenia purpurea has been introduced into Switzerland and become invasive. This creates the opportunity to study reactions to founder effect and how a species can circumvent deleterious effects of bottlenecks such as reduced genetic diversity, inbreeding and extinction through mutational meltdown, to emerge as a highly invasive plant. METHODS: A population genetic survey by random amplified polymorphism DNA markers (RAPD) together with historical insights and a field pollination experiment were carried out. KEY RESULTS: At the regional scale, S. purpurea shows low structure (thetast=0.072) due to a recent founder event and important subsequent growth. Nevertheless, multivariate statistical analyses reveal that, because of a bottleneck that shifted allele frequencies, most of the variability is independent among populations. In one population (Tenasses) the species has become invasive and genetic analysis reveals restricted gene flow and family structure (thetast=0.287). Although inbreeding appears to be high (Fis >0.410 from a Bayesian estimation), a field pollination experiment failed to detect significant inbreeding depression upon F1 seed number and seed weight fitness-traits. Furthermore, crosses between unrelated individuals produced F1 seeds with significantly reduced fitness, thus showing local outbreeding depression. CONCLUSIONS: The results suggest that, under restricted gene flow among families, the species may not only have rapidly purged deleterious alleles, but also have undergone some form of selection for inbreeding due to co-adaptation between loci.

Biological Evolution↗