New evidence for the systematic significance of acylated spermidines and flavonoids in pollen of Higher Hamamelidae

New distributional recores of hydroxycinnamoyl spermidines (HCS), including novel trisubstituted HCSs and flavonol glycosides, in pollen of Higher Hamamelidae are presented. The 51 taxa analyzed by HPLC and TLC included members of the families Fagaceae (Castanoideae, Fagoideae, Quercoideae), Betulaceae, Juglandaceae, Myricaceae, Hamamelidaceae, Rosaceae, and Buxaceae (Simmondsia). The results support generic concepts in the Higher Hamamelidae derived from morphological and chloroplast DNA data and support a close evolutionary relationship between the Higher Hamamelidae and the Rosidae.     

Pollen-tube growth and fertilization mode in<Emphasis Type="Italic"> Gymnostoma</Emphasis> (Casuarinaceae): their characteristics and evolution

A unique mode of fertilization called chalazogamy, whereby the pollen tube passes through the chalaza instead of the micropyle, is known in several species of derived genera in Casuarinaceae. In this paper we report the occurrence of chalazogamy in Gymnostoma (G. poissonianum), the most primitive genus in the family. We also show that the pollen tube grows discontinuously from the stigma to ovules in about 3 months. At the time of pollination, the ovules have not yet formed in the ovary, and require a long time to develop. The pollen tube(s) lie in a zigzag line and are branched in the upper region of the style, and their growth is arrested there until the ovary develops further. Studies of the relevant literature further revealed discontinuous pollen-tube growth in relation to a prolonged period between pollination and fertilization, as well as chalazogamy, in Betulaceae, Juglandaceae and/or Fagaceae that are closely related to Casuarinaceae. This feature may have derived early in the evolution of Fagales.     

The best of two worlds: ecology and evolution of ambophilous plants

Ambophily, the mixed mode of wind and insect pollination is still poorly understood, even though it has been known to science for over 130 years. While its presence has been repeatedly inferred, experimental data remain regrettably rare. No specific suite of morphological or ecological characteristics has yet been identified for ambophilous plants and their ecology and evolution remain uncertain. In this review we summarise and evaluate our current understanding of ambophily, primarily based on experimental studies. A total of 128 ambophilous species – including several agriculturally important crops – have been reported from most major habitat types worldwide, but this probably represents only a small subset of ambophilous species. Ambophilous species have evolved both from wind- and insect-pollinated ancestors, with insect-pollinated ancestors mostly representing pollination by small, generalist flower visitors. We compiled floral and reproductive traits for known ambophilous species and compared our results to traits of species pollinated either by wind or by small generalist insects only. Floral traits were found to be heterogeneous and strongly overlap especially with those of species pollinated by small generalist insects, which are also the prominent pollinator group for ambophilous plants. A few ambophilous species are only pollinated by specialised bees or beetles in addition to pollination by wind. The heterogeneity of floral traits and high similarity to generalist small insect-pollinated species lead us to conclude that ambophily is not a separate pollination syndrome but includes species belonging to different insect- as well as wind-pollination syndromes. Ambophily therefore should be regarded as a pollination mode. We found that a number of ecological factors promoted the evolution of ambophily, including avoidance of pollen limitation and self-pollination, spatial flower interference and population density. However, the individual ecological factors favouring the transition to ambophily vary among species depending on species distribution, habitat, population structure and reproductive system. Finally, a number of experimental studies in combination with observations of floral traits of living and fossil species and dated phylogenies may indicate evolutionary stability. In some clades ambophily has likely prevailed for millions of years, for example in the castanoid clade of the Fagaceae.     

The evolution of fertilization modes independent of the micropyle in Fagales and `pseudoporogamy'

In contrast to a majority of angiosperms showing porogamous fertilization, several fagalean families such as Betulaceae and Casuarinaceae are known to show chalazogamy, where fertilization is effected by a pollen tube passing through the chalaza instead of the micropyle. Our developmental study of pollen-tube growth in pistils of Myrica rubra (Myricaceae, Fagales) further shows that pollen tubes reached the nucellus before the micropyle is formed by the integument. Since fertilized ovules appeared as if the pollen tube had passed through the micropyle for fertilization, we propose the new term `pseudoporogamy' to this mode. By mapping diverse modes of fertilization, dependent or independent of the micropyle, onto a phylogenetic tree of Fagales, it appears that fertilization mode evolved from porogamy to chalazogamy and then further from chalazogamy to pseudoporogamy. Possible reasons for the evolution of fertilization modes independent of the micropyle in Fagales are discussed.     

Synteny analysis in Rosids with a walnut physical map reveals slow genome evolution in long-lived woody perennials

Background

Mutations often accompany DNA replication. Since there may be fewer cell cycles per year in the germlines of long-lived than short-lived angiosperms, the genomes of long-lived angiosperms may be diverging more slowly than those of short-lived angiosperms. Here we test this hypothesis.

Results

We first constructed a genetic map for walnut, a woody perennial. All linkage groups were short, and recombination rates were greatly reduced in the centromeric regions. We then used the genetic map to construct a walnut bacterial artificial chromosome (BAC) clone-based physical map, which contained 15,203 exonic BAC-end sequences, and quantified with it synteny between the walnut genome and genomes of three long-lived woody perennials, Vitis vinifera, Populus trichocarpa, and Malus domestica, and three short-lived herbs, Cucumis sativus, Medicago truncatula, and Fragaria vesca. Each measure of synteny we used showed that the genomes of woody perennials were less diverged from the walnut genome than those of herbs. We also estimated the nucleotide substitution rate at silent codon positions in the walnut lineage. It was one-fifth and one-sixth of published nucleotide substitution rates in the Medicago and Arabidopsis lineages, respectively. We uncovered a whole-genome duplication in the walnut lineage, dated it to the neighborhood of the Cretaceous-Tertiary boundary, and allocated the 16 walnut chromosomes into eight homoeologous pairs. We pointed out that during polyploidy-dysploidy cycles, the dominant tendency is to reduce the chromosome number.

Conclusion

Slow rates of nucleotide substitution are accompanied by slow rates of synteny erosion during genome divergence in woody perennials.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1906-5) contains supplementary material, which is available to authorized users.     

Mode of pollen-tube growth in Pistils of Myrica rubra (Myricaceae): a comparison with related families

BACKGROUND AND AIMS: It is generally known that fertilization is delayed for more than a few weeks after pollination in Fagales. Recent studies showed that, during that period, pollen tubes grew in pistils in close association with the development of the ovule in a five-step process in Casuarina (Casuarinaceae) and a four-step process in Alnus (Betulaceae). The number of pollen tubes was reduced from many to one, a fact suggesting that delayed fertilization plays a role for gametophyte selection. Myrica (Myricaceae) also shows delayed fertilization for >2 weeks after pollination, but nothing is known of how pollen tubes grow in the pistil during that period. METHODS: Pollen-tube growth and the development of the ovule in pistils was investigated by fluorescent and scanning electron microscopy and analysis of microtome sections of the pistils. KEY RESULTS: Developmental study of the pollen-tube growth in the pistil of M. rubra showed that the tip of the pollen tube was branched or lay in a zigzag pattern in the upper space of the ovarian locule or near the tip of the integument, and subsequently was swollen on the nucellar surface. Such morphological changes indicate that the pollen-tube growth was temporarily arrested before fertilization. The pollen-tube growth in M. rubra can therefore be summarized as occurring in three steps: (1) from the stigma to the ovarian locule; (2) from the ovarian locule to the nucellar surface; and (3) from the nucellar surface to the embryo sac. CONCLUSION: Myrica differs from other families in that the pollen tubes arrest their growth on the nucellar surface, probably digesting nutrient from nucellar cells. There is little information on five other families of Fagales. An extensive study is needed to better understand the diversity and function of the mode of pollen-tube growth within the order.     

Normapolles flowers of fagalean affinity from the Late Cretaceous of Portugal

 Exquisitely preserved, charcoalified fossil flowers with in situ pollen of the Normapolles-type from the Late Cretaceous of Portugal are described and a new genus and species of Fagales, Normanthus miraensis, are established. Floral organization and structure of floral organs were studied with scanning electron microscopy and microtome sections. Flowers are actinomorphic, epigynous, and pentamerous; the perianth is simple; stamens alternate with tepals; pollen is oblate and vestibulate; the exine is thick and the tectum is scabrate-microgranulate; the gynoecium is bicarpellate and unilocular; the fruits are probably one-seeded. Comparisons with extant taxa demonstrate that N. miraensis shares many similarities with Fagales and in particular with Betulaceae. However, it is not identical with any extant taxon and cannot be included in any extant family. The combination of characters found in the fossil flowers is congruent with wind-pollination syndromes present in many extant angiosperms and clearly indicates wind-pollination of N. miraensis. Received June 13, 2000 Accepted September 28, 2000     

Normapolles plants: a prominent component of the Cretaceous rosid diversification

The Normapolles complex, characterised by its oblate and triaperturate pollen, constitutes an important and diverse element of many Late Cretaceous and Early Cainozoic floras of the Northern Hemisphere. Based on the dispersed pollen record alone it has been difficult to assess systematic affinities, but relationships with Fagales have been proposed. Over the past twenty years several exquisitely preserved Late Cretaceous reproductive structures with Normapolles type pollen in situ have been described. In this study we provide a summary and new information of these floral structures. Further, a new genus, Dahlgrenianthus, is described from the Late Cretaceous of southern Sweden. The genus includes the type species Dahlgrenianthus suecicus, a number of reproductive structures referred to Dahlgrenianthus sp., and Dahlgrenianthus trigonus (Knobloch et Mai) comb. nov. from the Maastrichtian flora of Walbeck, Germany. Dahlgrenianthus comprises small flowers with pentamerous perianth and androecium and a tricarpellate gynoecium. It is distinguished from all other Normapolles floral structures in its hypogynous floral organisation. All Normapolles floral structures described so far are thought to be related to various members of the core Fagales, but the group is obviously not monophyletic. The stratigraphic range of the Normapolles taxa and other fagalean fossils strongly suggests that all major fagalean lineages were present by the Cenomanian or earlier.     

Large-scale phylogenetic analyses reveal fagalean diversification promoted by the interplay of diaspores and environments in the Paleogene

The order Fagales (ca. 1325 species, 32 genera) is one of the most important orders of woody angiosperms in both tropical and temperate forests. Fagalean plants have diverse diaspores and dispersal modes, and have abundant macrofossil records. Here, we present, to our knowledge, the most comprehensive phylogenetic analysis of Fagales to date based on five plastid loci. We reconstructed the phylogenetic relationships within Fagales using parsimony, likelihood and Bayesian approaches. We inferred the evolutionary shifts of diaspore types, dispersal modes, habitats, and pollination syndromes, and estimated divergence times and rates of diversification. Fossil fruit records of Fagales were also reviewed. Our results suggest that fagalean families are all monophyletic and 29 of the 32 genera are monophyletic. The majority of inter-familial and inter-generic relationships were well resolved. Our evolutionary reconstructions indicate that winged diaspore is synapomorphic, and animal dispersal system is symplesiomorphic in Fagales. Within the order, the families diverged in the mid-Cretaceous but mainly diversified after the Cretaceous–Paleogene (K–Pg) boundary. The overwhelming majority of winged and wingless fruited genera diverged or diversified during the Paleogene, reflecting adaptation to wind and animal dispersals, respectively. Correlated evolution analyses strongly supported the correlated transitions between dispersal mode and habitat. The winged fruited groups often inhabit open habitats and are commonly dispersed abiotically, whereas the wingless fruits usually inhabit closed habitats and have a vertebrate-dispersal mode. Environmental changes triggered increased diversification of Fagales in the Paleogene together with evolution of diaspores and dispersal modes. Additionally, our paleobotanically calibrated time-scale for Fagales may be useful for ecological and physiological studies.