Developmental morphology of seedling and shoot and phylogenetic relationship of Diplobryum koyamae (Podostemaceae)

Two new genera of Podostemaceae from northern Central Laos: saltational evolution and enigmatic morphology

Podostemaceae, the river-weeds, are characterized by remarkable differences between species and genera, which resulted from saltational evolution. This paper presents additional cases of such two genera, which are described here from the Phou Khao Khouay National Protected Area in northern Central Laos. Molecular phylogenetic data show that Ctenobryum mangkonense (gen. & sp. nov.) is sister to Hydrodiscus koyamae, while Laosia ramosa (gen. & sp. nov.) is isolated from all Asian genera of subfamily Podostemoideae. Ctenobryum mangkonense is distinct from Hydrodiscus koyamae in the crustose roots (versus rootless in the latter), scattered flowers on the root (versus alternate on the shoot), and pectinate bracts (versus simple, sheath-like). Laosia ramosa is distinct from all the genera in the columnar, endogenously branched axes and single style-stigma complex. The axis is an enigmatic organ with combined characteristics of root, stem and leaf, pending further study. Laos, together with Thailand, is a center of diversity of the Southeast and East Asian Podostemoideae. The three monotypic genera, i.e., Ctenobryum, Hydrodiscus and Laosia, occur in neighboring and closely similar aquatic habitats within the Area. The new taxa are formally described.

Multidisciplinary studies of the diversity and evolution in river-weeds

The moss-like river-weeds or Podostemaceae offer a special opportunity to study the diversity and evolution of plants that are adapted to extreme environments. This paper reviews multidisciplinary studies on this subject. Based on field work in the four continents, we discovered many species and several genera that are new components of biodiversity, and revealed the Podostemaceae floras of East Asia, Southeast Asia, and Australia. The historical biogeography of the family, i.e., the change in distribution in space and time, is characterized by a few dispersals between continents, followed by diversification within each continent. Local species may be derived from parts of separated populations of parental species, which consequently are paraphyletic. The remarkable morphological adaptations of Podostemaceae include the development of the horizontal axis in plant body, with which the plants adhere to rock surfaces under violent current. The vertical axis is reduced or lost and the horizontal axis develops in the embryo and seedling. We also found saltational organ-level variation, such as presence or absence of shoot, shoot apical meristem, root, and root cap; the form of shoot and root; the mode of root branching and leaf production; and the number of cotyledons. Morphological evolution may not be always adaptive to the habitats, which are rocks periodically submerged across the distribution range. Analyses of shoot regulatory gene expression found that, in contrast to the expression pattern in primitive species with ordinary shoots, which is comparable with Arabidopsis, the unique pattern in derived species may result in 'fuzzy' morphology of the shoot and leaf. Finally, problems for future study are pointed out.

Evolution of unusual morphologies in Lentibulariaceae (bladderworts and allies) and Podostemaceae (river-weeds): a pictorial report at the interface of developmental biology and morphological diversification

BACKGROUND

Various groups of flowering plants reveal profound ('saltational') changes of their bauplans (architectural rules) as compared with related taxa. These plants are known as morphological misfits that appear as rather large morphological deviations from the norm. Some of them emerged as morphological key innovations (perhaps 'hopeful monsters') that gave rise to new evolutionary lines of organisms, based on (major) genetic changes.

SCOPE

This pictorial report places emphasis on released bauplans as typical for bladderworts (Utricularia, approx. 230 secies, Lentibulariaceae) and river-weeds (Podostemaceae, three subfamilies, approx. 54 genera, approx. 310 species). Bladderworts (Utricularia) are carnivorous, possessing sucking traps. They live as submerged aquatics (except for their flowers), as humid terrestrials or as epiphytes. Most Podostemaceae are restricted to rocks in tropical river-rapids and waterfalls. They survive as submerged haptophytes in these extreme habitats during the rainy season, emerging with their flowers afterwards. The recent scientific progress in developmental biology and evolutionary history of both Lentibulariaceae and Podostemaceae is summarized.

CONCLUSIONS

Lentibulariaceae and Podostemaceae follow structural rules that are different from but related to those of more typical flowering plants. The roots, stems and leaves - as still distinguishable in related flowering plants - are blurred ('fuzzy'). However, both families have stable floral bauplans. The developmental switches to unusual vegetative morphologies facilitated rather than prevented the evolution of species diversity in both families. The lack of one-to-one correspondence between structural categories and gene expression may have arisen from the re-use of existing genetic resources in novel contexts. Understanding what developmental patterns are followed in Lentibulariaceae and Podostemaceae is a necessary prerequisite to discover the genetic alterations that led to the evolution of these atypical plants. Future molecular genetic work on morphological misfits such as bladderworts and river-weeds will provide insight into developmental and evolutionary aspects of more typical vascular plants.

Origin and development of the cryptic shoot meristem in Zeylanidium lichenoides (Podostemaceae)

PREMISE OF THE STUDY

The shoot apical meristem is the source of aerial shoot systems. In the aquatic eudicot family Podostemaceae, subfamily Podostemoideae, however, shoots develop in the absence of a distinct apical meristem. Previous studies suggest that the cryptic embryonic shoot meristem is involved in primary shoot development in some species (e.g., Zeylanidium lichenoides), although it is unclear whether the meristem maintains meristematic identity. Our aim was to determine how an embryonic shoot meristem is established during embryogenesis and how it is involved in plumular leaf development in the seedling.

METHODS

We observed anatomy and analyzed gene expression in Z. lichenoides, using an ortholog of the shoot meristem marker gene SHOOT MERISTEMLESS (STM).

KEY RESULTS

Expression of the STM ortholog, ZlSTM, began in the apical part of the 16-cell embryo. By the heart-shaped embryo stage, its expression was restricted to the putative organizing center (OC) and the protodermal cells just above them, forming a cryptic embryonic shoot meristem without a typical stem cell (apical initials) layer. During seedling development, expression was not maintained in the meristem, but instead shifted to the adaxial bases of cotyledons where plumular leaves would form.

CONCLUSIONS

ZlSTM expression demonstrated that the meristematic identity is partly retained in the embryonic shoot apex. This cryptic embryonic shoot meristem has a putative OC, but no typical stem cell layer, and it is not maintained during primary shoot development. Modification of the regulatory mechanism between the OC and stem cells might be responsible for this ephemeral shoot meristem in Podostemaceae.

Gene expression analysis of aquatic angiosperms podostemaceae to gain insight into the evolution of their enigmatic morphology

Podostemaceae is a family of aquatic angiosperms growing submerged on rocks in fast-flowing water and called moss-like or alga-like riverweeds. It evolved remarkable innovations to adapt to such an extreme environment, one of which is reduced shoots borne on roots adhering to rock surface. Recent observations revealed that the basal subfamily Tristichoideae, like most other angiosperms, has typical shoot apical meristems (SAMs). In species of the subfamily Podostemoideae, however, shoot apical meristems (SAMs) are not formed during development and new leaves arise from the meristematic basal region of preexisting leaves. The genetic basis of this shoot organogenesis process, e.g., the expression patterns of genes homologous to transcription factors regulating shoot development, is essential to better understand the evolution of Podostemaceae. A gene expression analysis found that the SAM-less Podostemoideae leaf has mixed identity of SAM and leaf, and provided insight into the evolution of the shoot in Podostemaceae.

Comparative anatomy of embryogenesis in three species of Podostemaceae and evolution of the loss of embryonic shoot and root meristems

During embryogenesis in angiosperms, the embryonic shoot and root meristems are created at opposite poles of the embryo, establishing a vertical body plan. However, the aquatic eudicot family Podostemaceae exhibits an unusual horizontal body plan, which is attributed to the loss of embryonic shoot and root meristems. To infer the embryogenetic changes responsible for the loss of these meristems, we examined the embryogenesis of three podostemads with different meristem characters, that is, Terniopsis brevis with distinct shoot and root meristems, Zeylanidium lichenoides with reduced shoot and no root meristems, and Hydrobryum japonicum with no shoot and no root meristems. In T. brevis, as in other eudicots, the putative organizing center (OC) and L1 layer (=the epidermal cell layer) arose to generate a distinct shoot meristem initial, and the hypophysis formed the putative quiescent center (QC) of a root meristem. Z. lichenoides had a morphologically unrecognizable shoot meristem, because a distinct L1 layer did not develop, whereas the putative OC precursor arose normally. In H. japonicum, the vertical divisions of the apical cells of eight-cell embryo prevented putative OC initiation. In Z. lichenoides and H. japonicum, the putative QC failed to initiate because the hypophysis repeated longitudinal divisions during early embryogenesis. Based on their phylogenetic relationships, we infer that the conventional embryonic shoot meristem was lost in Podostemaceae via two steps, that is, the loss of a distinct L1 layer and the loss of the OC, whereas the loss of the embryonic root meristem occurred once by misspecification of the hypophysis.

Female germ unit in Genlisea and Utricularia, with remarks about the evolution of the extra-ovular female gametophyte in members of Lentibulariaceae

Lentibulariaceae is the largest family among carnivorous plants which displays not only an unusual morphology and anatomy but also the special evolution of its embryological characteristics. It has previously been reported by authors that Utricularia species lack a filiform apparatus in the synergids. The main purposes of this study were to determine whether a filiform apparatus occurs in the synergids of Utricularia and its sister genus Genlisea, and to compare the female germ unit in these genera. The present studies clearly show that synergids in both genera possess a filiform apparatus; however, it seems that Utricularia quelchii synergids have a simpler structure compared to Genlisea aurea and other typical angiosperms. The synergids are located at the terminal position in the embryo sacs of Pinguicula, Genlisea and were probably also located in that position in common Utricularia ancestor. This ancestral characteristic still occurs in some species from the Bivalvaria subgenus. An embryo sac, which grows out beyond the limit of the integument and has contact with nutritive tissue, appeared independently in different Utricularia lineages and as a consequence of this, the egg apparatus changes position from apical to lateral.

Phylogeny of the clusioid clade (Malpighiales): evidence from the plastid and mitochondrial genomes

PREMISE OF THE STUDY

The clusioid clade includes five families (i.e., Bonnetiaceae, Calophyllaceae, Clusiaceae s.s., Hypericaceae, and Podostemaceae) represented by 94 genera and ≈1900 species. Species in this clade form a conspicuous element of tropical forests worldwide and are important in horticulture, timber production, and pharmacology. We conducted a taxon-rich multigene phylogenetic analysis of the clusioids to clarify phylogenetic relationships in this clade.

METHODS

We analyzed plastid (matK, ndhF, and rbcL) and mitochondrial (matR) nucleotide sequence data using parsimony, maximum likelihood, and Bayesian inference. Our combined data set included 194 species representing all major clusioid subclades, plus numerous species spanning the taxonomic, morphological, and biogeographic breadth of the clusioid clade.

KEY RESULTS

Our results indicate that Tovomita (Clusiaceae s.s.), Harungana and Hypericum (Hypericaceae), and Ledermanniella s.s. and Zeylanidium (Podostemaceae) are not monophyletic. In addition, we place four genera that have not been included in any previous molecular study: Ceratolacis, Diamantina, and Griffithella (Podostemaceae), and Santomasia (Hypericaceae). Finally, our results indicate that Lianthus, Santomasia, Thornea, and Triadenum can be safely merged into Hypericum (Hypericaceae).

CONCLUSIONS

We present the first well-resolved, taxon-rich phylogeny of the clusioid clade. Taxon sampling and resolution within the clade are greatly improved compared to previous studies and provide a strong basis for improving the classification of the group. In addition, our phylogeny will form the foundation for our future work investigating the biogeography of tropical angiosperms that exhibit Gondwanan distributions.