Development and structure of trichotomous branching in Edgeworthia chrysantha (Thymelaeaceae)

We studied the development and structure of the unusual trichotomous branching of Edgeworthia chrysantha. Three "branch primordia" are formed sequentially on the shoot apex of a main axis and develop into trichotomous branching. The branch primordia are clearly distinguishable from the typical axillary buds of other angiosperms; they develop much more rapidly than axillary buds, and the borders between the branch primordia and shoot apex of the main axis are anatomically unclear. Furthermore, at a later stage, leaves subtending the branch primordia produce typical axillary buds. These results suggest that the trichotomous branching in this species involves the division of the shoot apical meristem. Expression analysis of genes involved in branching or maintenance of the shoot apical meristem in this species should clarify the control mechanism of this novel branching pattern in angiosperms. We also observed the phyllotactic patterns in trichotomous branching and have related these patterns to the shoot system as a whole.     

Towards an ontogenetic understanding of inflorescence diversity

Backgrounds and Aims

Conceptual and terminological conflicts in inflorescence morphology indicate a lack of understanding of the phenotypic diversity of inflorescences. In this study, an ontogeny-based inflorescence concept is presented considering different meristem types and developmental pathways. By going back to the ontogenetic origin, diversity is reduced to a limited number of types and terms.

Methods

Species from 105 genera in 52 angiosperm families are investigated to identify their specific reproductive meristems and developmental pathways. Based on these studies, long-term experience with inflorescences and literature research, a conceptual framework for the understanding of inflorescences is presented.

Key Results

Ontogeny reveals that reproductive systems traditionally called inflorescences fall into three groups, i.e. ‘flowering shoot systems’ (FSS), ‘inflorescences’ sensu stricto and ‘floral units’ (FUs). Our concept is, first, based on the identification of reproductive meristem position and developmental potential. The FSS, defined as a seasonal growth unit, is used as a reference framework. As the FSS is a leafy shoot system bearing reproductive units, foliage and flowering sequence play an important role. Second, the identification of two different flower-producing meristems is essential. While ‘inflorescence meristems’ (IMs) share acropetal primordia production with vegetative meristems, ‘floral unit meristems’ (FUMs) resemble flower meristems in being indeterminate. IMs produce the basic inflorescence types, i.e. compound and simple racemes, panicles and botryoids. FUMs give rise to dense, often flower-like units (e.g. heads). They occur solitarily at the FSS or occupy flower positions in inflorescences, rendering the latter thyrses in the case of cymose branching.

Conclusions

The ontogenetic concept differs from all existing inflorescence concepts in being based on meristems and developmental processes. It includes clear terms and allows homology statements. Transitional forms are an explicit part of the concept, illustrating the ontogenetic potential for character transformation in evolution.     

Significance of the simultaneous growth of vegetative and reproductive organs in the prostrate annual Chamaesyce maculata (L.) Small (Euphorbiaceae)

To elucidate the significance of the simultaneous growth of vegetative and reproductive organs in the prostrate annual Chamaesyce maculata (L.) Small (Euphorbiaceae) from the standpoint of meristem allocation, we investigated plant architecture, meristem allocation, and the spatial and temporal patterns in vegetative growth and reproduction in the reproductive stage. The numbers of secondary and tertiary shoots successively increased by branching in the reproductive stage, and the sum of shoot length was greater in secondary shoots than in primary shoots. The specific shoot length (shoot length per shoot biomass) was greater in lateral shoots than in primary shoots, indicating efficient lateral shoot elongation. The internode length was shorter in secondary shoots than in primary shoots, increasing the number of nodes per shoot length in secondary shoots. Many nodes on a shoot generated two meristems, one of which committed to a flower and one to a lateral shoot. The number of reproductive meristems was greatest in tertiary shoots, and 96% of total reproductive meristems on shoots were generated in lateral shoots. On almost all nodes, the reproductive meristem developed into a flower, and 95–98% of the flowers produced a fruit. Therefore, vegetative growth by branching in the reproductive stage contributed to the increase in reproductive outputs. From the standpoint of meristem allocation, the simultaneous growth of vegetative and reproductive organs in prostrate plant species might be important for increasing the number of growth and reproductive meristems, resulting in the increase in reproductive outputs.     

Developmental morphology of the shoot in Weddellina squamulosa and implications for shoot evolution in the Podostemaceae

BACKGROUND AND AIMS: In angiosperms, the shoot apical meristem produces a shoot system composed of stems, leaves and axillary buds. Podostemoideae, one of three subfamilies of the river-weed family Podostemaceae, have a unique 'shoot' that lacks a shoot apical meristem and is composed only of leaves. Tristichoideae have been interpreted to have a shoot apical meristem, although its branching pattern is uncertain. The shoot developmental pattern in Weddellinoideae has not been investigated with a focus on the meristem. Weddellinoideae are in a phylogenetically key position to reveal the process of shoot evolution in Podostemaceae. METHODS: The shoot development of Weddellina squamulosa, the sole species of Weddellinoideae, was investigated using scanning electron microscopy and semi-thin serial sections. KEY RESULTS: The shoot of W. squamulosa has a tunica-corpus-organized apical meristem. It is determinate and successively initiates a new branch extra-axillarily at the base of an immediately older branch, resulting in a sympodial, approximately plane branching pattern. Large scaly leaves initiate acropetally on the flanks of the apical meristem, as is usual in angiosperms, whereas small scaly leaves scattered on the stem initiate basipetally in association with the elongation of internodes. CONCLUSIONS: Weddellinoideae, like Tristichoideae, have a shoot apical meristem, leading to the hypothesis that the meristem was lost in Podostemoideae. The patterns of leaf formation in Podostemoideae and shoot branching in Weddellinoideae are similar in that these organs arise at the bases of older organs. This similarity leads to another hypothesis that the 'branch' in Weddellinoideae (and possibly Tristichoideae) and the 'leaf' in Podostemoideae are comparable, and that the shoot apical meristem disappeared in the early evolution of Podostemaceae.     

The interplay between inflorescence development and function as the crucible of architectural diversity

Background

Most angiosperms present flowers in inflorescences, which play roles in reproduction, primarily related to pollination, beyond those served by individual flowers alone. An inflorescence''s overall reproductive contribution depends primarily on the three-dimensional arrangement of the floral canopy and its dynamics during its flowering period. These features depend in turn on characteristics of the underlying branching structure (scaffold) that supports and supplies water and nutrients to the floral canopy. This scaffold is produced by developmental algorithms that are genetically specified and hormonally mediated. Thus, the extensive inflorescence diversity evident among angiosperms evolves through changes in the developmental programmes that specify scaffold characteristics, which in turn modify canopy features that promote reproductive performance in a particular pollination and mating environment. Nevertheless, developmental and ecological aspects of inflorescences have typically been studied independently, limiting comprehensive understanding of the relations between inflorescence form, reproductive function, and evolution.

Scope

This review fosters an integrated perspective on inflorescences by summarizing aspects of their development and pollination function that enable and guide inflorescence evolution and diversification.

Conclusions

The architecture of flowering inflorescences comprises three related components: topology (branching patterns, flower number), geometry (phyllotaxis, internode and pedicel lengths, three-dimensional flower arrangement) and phenology (flower opening rate and longevity, dichogamy). Genetic and developmental evidence reveals that these components are largely subject to quantitative control. Consequently, inflorescence evolution proceeds along a multidimensional continuum. Nevertheless, some combinations of topology, geometry and phenology are represented more commonly than others, because they serve reproductive function particularly effectively. For wind-pollinated species, these combinations often represent compromise solutions to the conflicting physical influences on pollen removal, transport and deposition. For animal-pollinated species, dominant selective influences include the conflicting benefits of large displays for attracting pollinators and of small displays for limiting among-flower self-pollination. The variety of architectural components that comprise inflorescences enable diverse resolutions of these conflicts.     

APERTURE EVOLUTION IN THE POLLEN OF PRIMITIVE ANGIOSPERMS

Diversity of pollen apertures in 35 families of the ranalean complex is compared through a series of representative scanning electronmicrographs, and the evolution of pollen aperture types in primitive angiosperms is outlined. A classification of pollen apertures found in the ranalean complex is presented, and ten basic aperture types are recognized: anasulcate, anatrichotomosulcate, zonasulculate, anaulcerate, catasulcate-cataulcerate, inaperturate, disulculate-diulculate, forate, colpate, and porate. Evidence is adduced for the primitive (ancestral) status of anasulcate pollen, and transitional stages in the evolution of other pollen aperture types in the ranalean complex are examined. From an early stock of ranalean angiosperms with anasulcate pollen, there appears to have been development of a number of interesting but evolutionally dead-end lines, represented among others by zonasulculate, anaulcerate, and catasulcate-cataulcerate pollen types. The most important evolutionary trend in early angiosperm pollen seems to have been the development of inaperturate pollen grains in many families of primitive angiosperms, from which there was a second major radiation of aperture types, including evolution of disulculate and forate pollen. Comparative study of pollen apertures observed in living primitive angiosperms suggests a de novo origin of the uniquely angiospermous (dicotyledonous) colpate pollen type from such inaperturate pollen.     

Inflorescence morphology and flower development in Pinguicula alpina and P. vulgaris (Lentibulariaceae: Lamiales): monosymmetric flowers are always lateral and occurrence of early sympetaly

Earlier interpretations of shoot morphology and flower position in Pinguicula are controversial, and data on flower development in Lentibulariaceae are scarce. We present scanning electron microscopy about the vegetative shoot, inflorescence and flower development in Pinguicula alpina and P. vulgaris. Analysis of original data and the available literature leads to the conclusion that the general pattern of shoot branching and inflorescence structure is uniform in all the Pinguicula species studied so far. The inflorescence is a sessile terminal umbel that is sometimes reduced to a solitary pseudoterminal flower. Flower-subtending bracts are either cryptic or present as tiny scales. A next order lateral shoot develops in the axil of the uppermost leaf, below the umbel. It is usually though not always homodromous, i.e., the direction of the phyllotaxy spiral is the same as in the main shoot. Among Pinguicula species that overwinter as a hibernaculum, the initiation of floral organs takes place in the same year as flowering in P. vulgaris, and 1?year earlier in P. alpina. Early congenital petal fusion (??early?? sympetaly) is documented in Pinguicula, though most other members of Lamiales exhibit ??late?? sympetaly. Sporadic occurrence of rudiments of two posterior stamens in Pinguicula is confirmed. A speculation is made that, in angiosperms, monosymmetric flowers cannot be terminal on shoots bearing more than two (or three) phyllomes.     

Reproductive trade-offs in genetically distinct clones ofVaccinium macrocarpon,the American cranberry

Patterns of resource allocation to growth, current reproduction, and potential future reproduction were quantified in six genetically distinct cultivars ofVaccinium macrocarpon. For all cultivars (genotypes), vegetative size is positively correlated with some measures of current reproduction (fruit and flower number) but negatively correlated with others (seed number per fruit, seed weight per fruit). Vegetative growth in the current year is significantly related to the production of reproductive terminal buds, a measure of the potential for reproduction in the following year. Stems with low levels of current reproduction — lower flower number, fruit number, and seed weight — were more likely to form reproductive terminal buds than stems with higher levels of current reproduction. Individual genotypes differed significantly for vegetative size, fruit number, fruit weight, seed number, and seed weight, as well as for the frequency of fruiting stems and reproductive terminal buds produced. Genotypes were segregated in principal component space, indicating overall differences between them in allocation to the suite of variables measured. These results indicate the possibility of fitness differences among cultivars due to genetically determined allocation strategy, which has implications for fitness differences among genotypes within natural populations.     

The ramification of Apinagia riedelii: A key to the understanding of the plant architecture of Podostemaceae,subfamily Podostemoideae

The study of the ramification pattern of Apinagia riedelii results in a new concept of the architecture of this species, with general implications to members of subfamily Podostemoideae with dithecous leaves. The presence of a subtending leaf below the floriferous shoot proves axillary branching also for species with dithecous leaves. Previous opinions of an unusual ramification mode by subfoliar or non-axillary branching or stem bifurcation in combination with dithecous leaves hitherto pleaded for Podostemoideae is refuted. Moreover, the view of the so-called dithecous leaves with one sheath (theca) at the ventral and one at the dorsal side of the leaf, previously regarded as initially connected with branching, has to be changed. The dithecous leaf arises from the branch and not from the mother shoot axis – as previously believed – and represents the addorsed hypsophyll, i.e., the first leaf (prophyll) of the floriferous branch. This finding leads to the conclusion that the lower sheath of the dithecous leaf is the ventral (not dorsal) sheath pointing to the branch and surrounding its flower bud with a ligule or an ochrea and a hood upon the bud. In this way, the branch and its flower bud become seemingly sunk in the leaf base. At the fusion of leaf basis and shoot results this enigmatic common tissue. The wings of the dorsal (upper) sheath of the dithecous leaf point to the mother shoot axis of the branch. Successive floriferous branches along the main stem disclose the shoot axis of A. riedelii as a monopodium (not sympodium) that develops an anthocladial (foliated) inflorescence in the form of a botrys or a compound botrys, respectively. Since it is generally difficult to define cymose or racemose inflorescences if subtending leaves are absent – which occur in most other species of subfamily Podostemoideae with dithecous leaves – the nature of these inflorescences is discussed anew. The findings on A. riedelii have consequences on our comprehension of the shoot architecture of Podostemoideae.     

Molecular evolution of flower development

Flowers, as reproductive structures of the most successful group of land plants, have been a central focus of study for both evolutionists and ecologists. Recent advances in unravelling the genetics of flower development have provided insight into the evolution of floral structures among angiosperms. The study of the evolution of genes that control floral morphogenesis permits us to draw inferences on the diversification of developmental systems, the origin of floral organs and the selective forces that drive evolutionary change among these plant reproductive structures.     

Diversity in obscurity: fossil flowers and the early history of angiosperms

In the second half of the nineteenth century, pioneering discoveries of rich assemblages of fossil plants from the Cretaceous resulted in considerable interest in the first appearance of angiosperms in the geological record. Darwin''s famous comment, which labelled the ‘rapid development’ of angiosperms an ‘abominable mystery’, dates from this time. Darwin and his contemporaries were puzzled by the relatively late, seemingly sudden and geographically widespread appearance of modern-looking angiosperms in Late Cretaceous floras. Today, the early diversification of angiosperms seems much less ‘rapid’. Angiosperms were clearly present in the Early Cretaceous, 20–30 Myr before they attained the level of ecological dominance reflected in some mid-Cretaceous floras, and angiosperm leaves and pollen show a distinct pattern of steadily increasing diversity and complexity through this interval. Early angiosperm fossil flowers show a similar orderly diversification and also provide detailed insights into the changing reproductive biology and phylogenetic diversity of angiosperms from the Early Cretaceous. In addition, newly discovered fossil flowers indicate considerable, previously unrecognized, cryptic diversity among the earliest angiosperms known from the fossil record. Lineages that today have an herbaceous or shrubby habit were well represented. Monocotyledons, which have previously been difficult to recognize among assemblages of early fossil angiosperms, were also diverse and prominent in many Early Cretaceous ecosystems.     

Is the oostegite structure of amphipods determined by their phylogeny or is it an adaptation to their environment?

The oostegites of amphipods attached to peraeopods 2–5 are of two main types — broad with relatively short marginal setae and narrow with long marginal setae. Broad oostegites are found in other peracarids and are considered the primitive type. Amphipods with broad oostegites tend to have smaller eggs than those with narrow oostegites. It is concluded that following the evolution of the major amphipod groups, the oostegites were modified as egg sizes have changed as part of the reproductive strategies of the species within these groups.     

RUST FUNGI AND HOST PLANT COEVOLUTION: DO PRIMITIVE HOSTS HARBOR PRIMITIVE PARASITES?

Abstract— The classical view of rust phylogeny is that rusts found on ferns and conifers are primitive, while rusts that parasitize angiosperms are advanced. This belief was based on the theory that primitive hosts harbor primitive parasites; that is, it assumed coevolution (co-speciation) of hosts and parasites. A cladistic analysis of 30 genera and 28 characters representative of the major patterns of rust fungi diversity is presented. The results of this analysis suggest that tropical short-cycle rusts on angiosperms form the cladistically basal group of rusts, while the rusts on conifers and ferns (Melampsoraceae sensu lato ) form a nested terminal clade. These results suggest that rusts and their hosts have not undergone a long period of parallel cladogenesis (co-speciation); host transfer has probably been at least as frequent as co-speciation. The cladograms indicate evolutionary trends of spore stages and life history: urediniospores evidently preceded the evolution of aeciospores and pycniospores within Uredinales, and heteroecism is a derived condition which evolved at least several times. This study stresses the importance of making use of independent cladistic analyses of both host and parasite in order to test assumptions of coevolution and host transfer.     

Molecular control of early cone development inPinus radiata

Summary A family of genes expressed during early stages of shoot development were isolated fromPinus radiata. A homologue of theLEAFY/FLORICAULA flower meristem-identity genes,NEEDLY (NLY), and three MADS-box genes,PrMADS1, PrMADS2 andPrMADS3 (Pinus radiata MADS-box genes), were expressed at early stages of initiation and differentiation of reproductive (male and female) cone buds, as well as vegetative buds. Expression ofNLY in transgenicArabidopsis thaliana promoted floral fate, demonstrating that it encodes a functional ortholog of theFLORICAUL A/LEAFY genes of angiosperms.Abbreviations DSB dwarf shoot bud - LSTB long-shoot terminal bud - PCB pollen cone bud - SCB seed cone bud - LD long day - SD short day     

EARLY ANGIOSPERM REPRODUCTION: CALODA DELEVORYANA GEN. ET SP. NOV., A NEW FRUCTIFICATION FROM THE DAKOTA FORMATION (CENOMANIAN) OF KANSAS

A new angiosperm fructification, Caloda delevoryana, is described from the Cenomanian age Dakota Formation of central Kansas. It consists of a long, narrow, main axis with numerous secondary axes arranged helically around the main axis. These secondary axes are each terminated in a small receptacle bearing numerous conduplicate carpels. No evidence of a perianth or androecium was found. This fructification bears some similarity to a number of different modern orders, such as the Hamamelidales, Alismatales, Najadales, and Piperales, and families, particularly the Platanaceae and the Aponogetonaceae, but cannot definitely be assigned to any modern taxon within the angiosperms. C. delevoryana exhibits several characters traditionally assumed to be primitive in the angiosperms, and several other features of this fossil are proposed as primitive in the evolution of angiosperms. This floral axis, with its compact mass of numerous secondary axes bearing very small fruits and seeds, may be the product of reduction through diminished growth of internodes and carpels, and elaboration through increased repetition of floral modules. This record adds to the rapidly growing body of paleobotanical data on early angiosperm reproductive structures, which should prove important in the assessment of the extent and direction of angiosperm evolution.     

Optimization theory in plant evolution: An overview of long-term evolutionary prospects in the angiosperms

Examination of literature shows that a number of authors regard outbreeding and heterozygosity as the prevalent factors associated with long-term successful evolution in the angiosperms. A number of plant evolutionists, however, have doubted the truth of such an assumption. Everincreasing reports of the existence of arboreal angiospermous apomixis in tropical forests of the Neotropics and the Far East undermined a thinking which, recently, has rested on optimality. Finding apomixis in trees surprised authors, who held biased opinions about the determinism of outbreeding as the major guiding factor in the evolutionary history of the angiosperms. The thinking that apomixis may turn out to be a regular mating system of the flowering plants met with the approval of some authors, who wondered about the true penetration of the phenomenon among the higher plants. The fact that one-third of all known flowering plants are autogamous has cast further doubt on the deterministic infallibility of outbreeding and successful long-term evolution. Despite claims that the breeding system is directly involved with fitness, while determining the course of optimized evolution, there is comparatively little hard evidence to substantiate a hypothesis which, in the last analysis, has rested principally on common sense. Rather, if continuing field research happens to unveil new cases of woody angiospermous apomixis, a prediction is advanced that the next two biomes to show regular incidence of the phenomenon are Africa’s paleotropical savannas and humid forests. If evolution is partly or wholly dependent on the breeding system to proceed, current knowledge supports views that further enhancement of organic diversification vis-à-vis selection and adaptednesss rests on three major tested mating systems: outbreeding, inbreeding and apomixis.     

Why do lower order branches show greater shoot growth than higher order branches? Considering space availability as a factor affecting shoot growth

Many woody plants show hierarchical shoot growth: annual shoot length decreased with increasing branching order. We hypothesize that plants showing hierarchical shoot growth improve the efficiency in terms of space acquisition and use per invested shoot length. This hypothesis was tested by using a simple geometric simulation model of branch development. In this study, the effective shoot length (EL), the shoot length produced within a growth season without any overlap from other shoots, was used as the index of space availability. We compared EL among shoots on different branching orders of a “simulated” branch system. The EL decreased with an increasing branching order. The results suggested that space availability decreased with increasing branching orders. The results also showed that simulated plants with the hierarchical shoot growth showed higher efficiency in terms of space acquisition per investment than those with the non-hierarchical shoot growth. We concluded that the difference in space availability between the branching orders could be an important ultimate factor causing hierarchical shoot growth.     

Early vessel evolution and the diverisification of wood function: Insights from Malagasy Canellales

Xylem vessels have long been proposed as a key innovation for the ecological diversification of angiosperms by providing a breakthrough in hydraulic efficiency to support high rates of photosynthesis and growth. However, recent studies demonstrated that angiosperm woods with structurally "primitive" vessels did not have greater whole stem hydraulic capacities as compared to vesselless angiosperms. As an alternative to the hydraulic superiority hypothesis, the heteroxylly hypothesis proposes that subtle hydraulic efficiencies of primitive vessels over tracheids enabled new directions of functional specialization in the wood. However, the functional properties of early heteroxyllous wood remain unknown. We selected the two species of Canellales from Madagascar to test the heteroxylly hypothesis because Canellaceae (represented by Cinnamosma madagascariensis) produces wood with vessels of an ancestral form, while Winteraceae, the sister clade (represented by Takhtajania perrieri) is vesselless. We found that heteroxylly correlated with increased wood functional diversity related predominantly to biomechanical specialization. However, vessels were not associated with greater stem hydraulic efficiency or increased shoot hydraulic capacity. Our results support the heteroxylly hypothesis and highlight the importance integrating a broader ecological context to understand the evolution of vessels.