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Homologisierungen: Blatt - Blütenblätter; Sprossknospe - Blüte; Typus - und Grundorgankonzepte - ein Review
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Abstract and Figures
Homology, Type, basic organs, ontogeny, scale leaf, leaf, bud, flower, ontogeny, Angiosperms, methodology. Review.
Die Arbeit untersucht die erkenntnistheoretischen und methodischen Grundlagen der vergleichenden Morphologie. Die Begriffe Homologie und Analogie spielen eine zentrale Rolle. Teil der vergleichenden Morphologie ist die Organogenese. Die wichtigsten Konzepte sind das Homologie‐konzept, das Grundorgankonzept, das Typuskonzept. Die Arbeit enthält ein Review der verschiedenen Grundorgankonzepte und ein Review der Homologisierungen der Blüte mit dem Spross.
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Morphological concepts are used in plant evo-devo (evolutionary developmental biology) 10 and other disciplines of plant biology, and therefore plant morphology is relevant to all of these 11 disciplines. Many plant biologists still rely on classical morphology, according to which there are 12 only three mutually exclusive organ categories in vascular plants such as flowering plants: root, 13 stem (caulome), and leaf (phyllome). Continuum morphology recognizes a continuum between 14 these organ categories. Instead of Aristotelian identity and either/or logic, it is based on fuzzy logic, 15 according to which membership in a category is a matter of degree. Hence, an organ in flowering 16 plants may be a root, stem, or leaf to some degree. Homology then also becomes a matter of degree. 17 Process morphology supersedes structure/process dualism. Hence, structures do not have pro-18 cesses, they are processes, which means they are process combinations. These process combinations 19 may change during ontogeny and phylogeny. Although classical morphology on the one hand and 20 continuum and process morphology on the other use different kinds of logic, they can be considered 21 complementary and thus together they present a more inclusive picture of the diversity of plant 22 form than any one of the three alone. However, continuum and process morphology are more com-23 prehensive than classical morphology. Insights gained from continuum and process morphology 24 can inspire research in plant morphology and plant evo-devo, especially MorphoEvoDevo. 25
By definition, biology is the science of all living beings. However, horizons restricted to either plants or animals have characterized the development of life sciences well beyond the emergence of unified perspectives applying to all forms of life, such as the cell theory or the theory of evolution. Separation between botanical and zoological traditions is not destined to go extinct easily, or shortly. Disciplinary isolation is emphasized by institutional contexts such as scientific societies and their congresses, specialist journals, disciplines recognized as teaching subjects and legitimate and fundable research fields. By shaping the personal agendas of individual scientists, this has a strong impact on the development of biology. In some fields, botanical and zoological contributions have long being effectively intertwined, but in many others plant and animal biology have failed to progress beyond a marginal dialogue. Characteristically, the so-called “general biology” and the philosophy of biology are still zoocentric (and often vertebrato- or even anthropocentric). In this article, I discuss legitimacy and fruitfulness of some old lexical and conceptual exchanges between the two traditions (cell, tissue, and embryo). Finally, moving to recent developments, I compare the contributions of plant vs. animal biology to the establishment of evolutionary developmental biology. We cannot expect that stronger integration between the different strands of life sciences will soon emerge by self-organization, but highlighting this persisting imbalance between plant and animal biology will arguably foster progress.
The previous theory about the origin of microphyllous and megaphyllous leaves can also be applied to seed plants. The existence of a microphyll of archaeophyll type offers new perspectives on the ovule origin. There are various phylogenetic classifications of gymnosperms. This paper brings new morphological insights on these classifications and recognizes three morphological groups of gymnosperms-primary cupulate and primary acupulate megaphyllous lineages and strobilate microphyllous lineage. Evolution from these three groups could continue in various directions, therefore there are many morphological similarities among them leading to many phylogenetic interpretations. The taxonomic groups based on the antophyte theory would be polyphyletic according to new morphological interpretations because all types of gymnosperm strobili could lead to formation of flowers.
Understanding the evolution of biodiversity on Earth is a central aim in biology. Currently, various disciplines of science contribute to unravel evolution at all levels of life, from individual organisms to species and higher ranks, using different approaches and specific terminologies. The search for common origin, traditionally called homology, is a connecting paradigm of all studies related to evolution. However, it is not always sufficiently taken into account that defining homology depends on the hierarchical level studied (organism, population, species), which can cause confusion. Therefore, we propose a framework to define homologies making use of existing terms, which refer to homology in different fields, but restricting them to an unambiguous meaning and a particular hierarchical level. We propose to use the overarching term 'homology' only when 'morphological homology', 'vertical gene transfer' and 'phylogenetic homology' are confirmed. Consequently, neither phylogenetic nor morphological homology are equal to homology. This paper is intended for readers with different research backgrounds. We challenge their traditional approaches, inviting them to consider the proposed framework and offering them a new perspective for their own research.
Background. Specific features of plant development and mechanisms ensuring such processes are expressly linked with the structure of buds and hence the leaf series formed on lateral shoots.Materials and methods. The material of the study was the shoots of the second to fifth orders of nineteen plant samples in their generative age (G1, G2) grown in the well-lighted habitats in the Park of Petrozavodsk and the Park of Komarovo Town (St. Petersburg). All buds on vegetative and vegetative-generative shoots (more than 250) were investigated. The leaf organs of the formed seventy-nine vegetative buds of Ulmus glabra Huds. were consistently disassembled and measured under an MBS-9 binocular. The analysis of the development of bud scales in unfolding buds made it possible to observe structural features unavailable in a static study of buds. Results and conclusions. The growth of the leaf organs in a series was found to occur in different ways. Almost completely suberized external scales represented a thickly grown leaf base fused with poorly developed stipules. The scales from the first to the fourth did not grow at all when the bud was unfolding. The fifth and the sixth scales showed weak intercalary growth during the unfolding. The tips of the seventh up to the tenth scales were weakly suberized and had two blades which formed a cap covering the tip of the bud. The scale which preceded the emergence of a leaf sprout with a leaf blade had on its tip pronounced lobes and a depression between them with a rounded bottom and hairs along the edge. It was followed by a sprout with large separated stipules, an almost unpronounced leaf base and a differentiated leaf blade; hence, there is a threshold in the differentiation of leaf sprouts. In the axil of each growing scale, a small dormant bud was set. Thus, the number of leaf organs on a shoot after unfolding was 10-15, while the leaves of the middle formation did not exceed 3-5, and besides only one growth period was observed in a majority of U. glabra plants in their generative age state. Using U. glabra as an example, the performance of the threshold mechanism within the genetic program of vegetative shoot development was demonstrated.
Recent advances in molecular phylogenetics and a series of important palaeobotanical discoveries have revolutionized our understanding of angiosperm diversification. Yet, the origin and early evolution of their most characteristic feature, the flower, remains poorly understood. In particular, the structure of the ancestral flower of all living angiosperms is still uncertain. Here we report model-based reconstructions for ancestral flowers at the deepest nodes in the phylogeny of angiosperms, using the largest data set of floral traits ever assembled. We reconstruct the ancestral angiosperm flower as bisexual and radially symmetric , with more than two whorls of three separate perianth organs each (undifferentiated tepals), more than two whorls of three separate stamens each, and more than five spirally arranged separate carpels. Although uncertainty remains for some of the characters, our reconstruction allows us to propose a new plausible scenario for the early diversification of flowers, leading to new testable hypotheses for future research on angiosperms.
Zwei unterschiedliche Modelle für die Urpflanze werden vorgestellt. Das erste
ist das bekannte Modell einer krautigen, zweikeimblättrigen Pflanze. Das zweite
Modell ist neu. Es ist das Modell einer mehrjährigen, zweikeimblättrigen Pflanze.
Die Wahl eines Modells gibt die Blickrichtung vor, mit der die Pflanze betrachtet
wird. Der Betrachter wird gelenkt. Beim Modell der krautigen Urpflanze wird
man veranlasst die Seitenorgane in ihrer Reihenfolge an der Sprossachse von unten
nach oben vom Keimblatt bis zum Fruchtblatt zu durchlaufen und miteinander zu
vergleichen. Beim neuen Modell der zweijährigen Urpflanze wird der Betrachter
zusätzlich angeregt, die aufgehende, vegetative Knospe mit der geöffneten Blüte zu
vergleichen. Das alte Modell setzt die Blüte in Bezug zum vegetativen Spross. Beim
neuen Modell ist das auch der Fall, zusätzlich wird die Blüte in Bezug gesetzt zur
aufgehenden vegetativen Knospe. Die Blattmetamorphose der aufgehenden Knospe
zeigt eine klare Gliederung. Drei Bereiche können unterschieden werden: Der
Bereich der äusseren verhärteten Knospenschuppen, der Bereich der inneren sich
schnell streckenden, farbigen und hinfälligen Knospenschuppen und der Bereich der
Stängelblätter. Diese Gliederung findet sich in gesteigerter Form in der Blüte und
korrespondiert mit a) dem Kelchbereich, b) dem Kronenblatt -, Staubblattbereich
und c) dem Fruchtblattbereich.
New morphological and phylogenetic data prompt us to present an updated review of floral morphology and its evolution in the basal ANITA grade of living angiosperms, Chloranthaceae, and Ceratophyllum. Floral phyllotaxis is complex whorled in Nymphaeales and spiral in Amborella and Austrobaileyales. It is unresolved whether phyllotaxis was ancestrally whorled or spiral, but if it was whorled, the whorls were trimerous. The flowers are probably ancestrally bisexual because in most families with unisexual flowers these flowers exhibit rudiments of the opposite sex. Carpels are largely ascidiate and the closure line is short, either transverse or longitudinal. A style is usually absent or, if present, generally short and not plicate. Angiospermy (carpel sealing) is by secretion, rather than postgenital fusion, except in large-flowered Nymphaeales and in Illicium, correlated with unusual fruits. Carpels with a single, median, pendent ovule are probably plesiomorphic. Chloranthaceae and Ceratophyllaceae have an unsettled phylogenetic position, but in some phylogenetic analyses they form a clade, which may be sister to the remaining mesangiosperms (Magnoliidae, monocots, eudicots). This position is supported by their carpel characters, which are similar to those of the ANITA grade and different from those of most other mesangiosperms.
In der vorliegenden Arbeit wird der Knospen-Bildungsprozess bei der Stiel-Eiche(Quercus robur L.) beschrieben. Im Frühjahr braucht es einige wenige Wochen,bis ein junger Trieb vollkommen ausgewachsen ist. Die Knospe, aus der erhervorgegangen ist, hat bis dahin für ihre Entwicklung eineinhalb Jahre gebraucht.
Die Endknospe des jungen Zweiges wird zu diesem Zeitpunkt von den äusserenKnospenschuppen gebildet. Im Laufe der Monate Mai bis Juli entwickelt sichdarin die Anlage für den nächstjährigen Zweig wie auch die ersten Stadien derdazugehörigen Endknospe.
Die Formen-Reihen der Organe an den Übergängen von A) den Laubblätternzur Endknospe (die Metamorphose in die Knospe hinein) und B) der Endknospezu den Laubblättern (die Metamorphose aus der Knospe heraus) werden detailliertbeschrieben. Die Formen-Reihe A) zeigt grosse Ähnlichkeiten mit den Blattreihenkrautiger Pflanzen auf die Blüte hin. In beiden Fällen wird das Oberblatt reduziert,zuerst der Blattstiel, dann die Blattspreite, und übrig bleibt der Blattgrund alsäusserste Schuppen der Winterknospen bei der Stiel-Eiche bzw. als Kelchblätterder Blüte.
Beim Knospen-Austrieb im Frühling (Formen-Reihe B) bzw. beim Aufblühender Bedecktsamer-Blüte öffnet sich ein «Kelch». Bei der Stiel-Eiche bilden dieäusseren Winterknospenschuppen diesen «Kelch», bei der Blüte die Kelchblätter.
Auf die äusseren Knospenschuppen folgen innere, vergängliche, bei zahlreichenGehölzen auch gefärbte Schuppen. In der Blüte stehen innerhalb der Kelchblätterdie Blüten- und Staubblätter. Zentral in der vegetativen Knospe der Gehölze sinddie Laubblätter angeordnet, dicht gedrängt, die Internodien haben sich noch nichtgestreckt. Diese für die Blüte typische Geste verliert sich beim Austreiben des jungenZweiges rasch, bei der Blüte jedoch bleibt sie während der gesamten Blütezeitbestehen. Der Vergleich der in der vegetativen Knospe zentral stehenden Laubblättermit den in der Blüte ebenfalls zentral stehenden Fruchtblättern drängt sich auf.
In der Metamorphosen-Lehre dient die einjährige Pflanze als Modell, mit dessenHilfe man die Metamorphose vom Keim- bis zum Fruchtblatt veranschaulicht.
Die Metamorphose zur Blüte lässt sich aber einfacher nachvollziehen, wenn man die mehrjährige, insbesondere die Gehölz-Pflanze als Modell nimmt. Bei dermehrjährigen Pflanze zeigt sich beim Studium der Knospen-Bildungsprozesse eineenge Verwandtschaft der vegetativen mit der Blüten-Knospe. Bei der Blüte kommt zur Knospen- die Sporenbildung als spezifisches Bildungsprinzip hinzu.
In 1975 HICKEY and WOLFE surveyed leaf architecture across the angiosperms, using the TAKHTAJAN and CRONQUIST systems as a framework for evaluating the systematic value and evolution of leaf characters. Examination of their results in the context of molecular phylogenies reveals many cases in which leaf architecture is more consistent with molecular trees than the TAKHTAJAN-CRONQUIST systems. Rooting of the angiosperms in the "ANITA" grade confirms that the first angiosperms had low rank, pinnate venation, but the blade was ovate, not elliptical-obovate as in Magnoliales. Chloranthoid teeth may be homologous in Chloranthaceae and basal eudicots, but as a symplesiomorpby retained from the first angiosperms, not a synapomorphy. Palmate venation evolved independently in Nymphaeales and other groups, but fewer times than HICKEY & WOLFE assumed, since Aristolochiaceae are associated with Piperales. Molecular trees imply that eudicots originally had palmate venation, as HICKEY & WOLFE postulated for their hamamelid-rosid line, becoming ternately dissected in Ranunculales. However, hamamelids are highly polyphyletic. Some "lower hamamelids" now placed among basal eudicots (Platanus, Tetracentron) illustrate the ancestral palmate venation, which persists into Saxifragales (basal Rosidae), including other former hamamelids (Hamamelidaceae, Cercidiphyllum). As proposed by HICKEY & WOLFE, pinnately compound leaves may be ancestral for most Rosidae. "Higher hamamelids" are nested within Rosidae, as HICKEY & WOLFE argued for Juglandaceae, but they form two separate clades (Fagales, former Urticales in Rosales), implying that many of their leaf similarities, such as urticoid teeth, are convergences. In contrast, the ancestral leaf type in Dilleniaceae, Santalales, Caryophyllales, and Asteridae was probably simple and pinnately veined. Molecular data indicate that Dilleniidae are polyphyletic, a result anticipated by HICKEY & WOLFE, who divided the group into pinnate dilleniids, most of which are related to Asteridae (Ericales), and palmate dilleniids, which form four lines in the Rosidae (Malvales, Brassicales, Cucurbitales, Malpighiales. © E. Schweizerbart'sche Verlagsbuchhandlung (Nägele u. Obermiller), 2007.
“From our acquaintance with this abnormal metamorphosis, we are enabled to unveil the secrets that normal metamorphosis conceals from us, and to see distinctly what, from the regular course of development, we can only infer.”
- J. W. von Goethe (1790)
Dicotyledons are polarly organised in several ways. In plant morphology polarity, a principle allowing comparison of different plant structures has until yet not been studied. A division** of the vegetative plant in shoot and root as polar structures leads to the distinction of four instead of three basic organs: leaf, shoot axis, root axis and root cap together with the root hairs. The flower is also polarly organised, its poles are formed by the carpels and the stamens. The foliage leaves are also polarly organised which is reflected by the morphological relationship of foliage leaf, stamen and carpel. The stamen uses the hypophyll*** as base of construction and the carpel uses the epiphyll**** as base of construction. Hypophyll and epiphyll are the two poles of the foliage leaf. Root and shoot, the polar entities of the vegetative plant and stamen and carpel, the polar entities of the generative plant, are morphologically correlated. Stamen and carpel can be understood as a combination of the basic organs of vegetative and generative parts of the plant. The basic organs of the generative plant are pollen grain and embryo sack with their gametophytes. The quantitative comparison of variable proportions is supplemented by a qualitative comparison of polarities. The result is, that the organisation type of the dicotyledons can yet be understood as constituted of morphological related parts.
* in german Grundorgane
** in german: Gliederung
*** in german: Unterblat
**** in german: Oberblatt
Recent phylogenetic analyses of angiosperms based on molecular and combined molecular and morphological data recognize a grade of basal eudicots situated between the ranunculids and the core eudicots. Buxaceae together with Didymelaceae form a well-supported clade within this grade. Flowers of representatives of this grade are characterized by a considerable variability in organ number and organ differentiation. In particular, perianth evolution of basal eudicots has been difficult to interpret because of a high diversity in number and form of perianth organs. In this investigation, development and structure of inflorescences and flowers of representatives of all buxaceous genera were studied comparatively to gain a better understanding of flower structure in Buxaceae and their relationships to basal eudicots. Inflorescences of Buxus, Notobuxus, and Styloceras kunthianum are most often botryoids with several lateral male flowers and a terminal female flower. Inflorescences of Pachysandra and sometimes Sarcococca are open spikes with lateral male flowers in the upper part of the inflorescence and female flowers below them. In dioecious species of Styloceras, male flowers form long spikes that may be terminated by a peloric flower; female flowers occur singly or in thyrses. In Buxaceae, all phyllomes preceding the reproductive organs on an inflorescence axis or floral axis look superficially similar. They are bractlike, and no obvious differentiation between bracts and tepals can be observed. However, detailed comparative studies of plastochrons, phyllotaxis, size, and shape of organs on the whole reproductive shoot show slight differences between organs. Bractlike phyllomes, if present, preceding the reproductive structures of male flowers of all genera are arranged in a decussate pattern. They are initiated with short plastochrons within each pair, whereas the plastochrons between pairs are longer. The uppermost mainly four bractlike phyllomes show modifications in the direction of tepals. The two pairs of stamens in Buxus are initiated in the same pattern as the bractlike phyllomes, whereas in Sarcococca and Pachysandra the plastochron within each stamen pair is almost zero. In contrast, Notobuxus has six or eight and Styloceras has up to 45 stamens in a more complex arrangement. The remaining floral apex forms a more or less developed pistillode in Sarcococca, Pachysandra, and most Buxus species. The floral center of Notobuxus is less differentiated than in the other genera, whereas a pistillode is even lacking in a few Buxus species and in Styloceras. Carpels of Styloceras, Sarcococca, Pachysandra, Notobuxus, and often Buxus are preceded by an initial pair of transverse phyllomes (prophylls) and several spirally arranged bractlike phyllomes that are only weakly differentiated in the direction of tepals. The nature of these organs is discussed in a broader systematic context.
A new term — the “blastozone” — is proposed to designate regions of the shoot competent for organogenesis. It is argued that the notion of “marginal meristems” is based on the cell theory and thus may not be appropriate to elucidate the process of organ formation. For instance, with respect to the occurrence of initials and of an elevated cell division rate marginal meristems have been shown to be doubtful structures. Furthermore, organogenetic competent regions form only parts of the meristems of the shoot. The study of blastozones from an organismic perspective reveals primary morphogenetic events such as initiation, incorporation, and fusion processes. Loss of morphogenetic competence is associated with histogenetic events, e.g., trichome outgrowth, and indicates the onset of processes leading to maturation. The marginal blastozone of the leaf is then used up although meristem features continue to be expressed. A series of SEM studies in several genera exemplifies the proposed viewpoint, demonstrating some of the morphogenetic potentialities of angiosperm leaf marginal blastozones.
Sequences of nuclear internal transcribed spacers (ITS) and plastid ndhF from 25 representatives of Buxaceae were analyzed separately and in combination to resolve phylogenetic relationships among and within genera; parsimony and maximum likelihood methods were used. The results of the two single-gene analyses were general y congruent, and the trees from the combined analysis were better supported. Two major clades were identified within the Buxaceae family: the clade of Pachysandra, Sarcococca, and Styloceras and the clade of Buxus and Notobuxus. Pachysandra and Sarcococca were two strongly supported monophyletic groups. The sister relationship between Sarcococca and the clade of Pachysandra and Styloceras was also strongly supported. American and Eurasian Buxus each formed a strongly supported clade. The controversial genus Notobuxus was embedded among African members of Buxus; B. hildebrandtii was sister to Notobuxus, with strong support. Relationships within Buxus and Sarcococca were only partially resolved.
Developmental genetic pathways involved in flower formation in model plants such as Arabidopsis and maize enable us to identify genes, gene families, and gene networks that are involved in the regulation of flower initiation, growth and differentiation. These genes can then function as “candidate genes” and their expression, function, and biochemical interactions can be explored in other lineages to determine if they provide a necessary and sufficient toolkit for the development of the flower. Likewise, a view to the fossil record can provide documentation of reproductive innovations occurring within gymnosperms or along the stem lineage leading to angiosperms, elucidating the transitions required for the evolution of the angiosperm flower from an ancestral reproductive strobilus. Here we discuss the origin and subsequent evolution in form of the flower, highlighting recent studies in paleobotany, morphology, evolution, and developmental genetics with the goal of outlining advances in our understanding of...
Shoot apical meristems (SAMs) of seed plants are small groups of pluripotent cells responsible for making leaves, stems and flowers. While the primary SAM forms during embryogenesis, new SAMs, called axillary SAMs, develop later on the body of the plant and give rise to branches. In Arabidopsis plants, axillary SAMs develop in close association with the adaxial leaf base at the junction of the leaf and stem (the leaf axil). We describe the phenotype caused by the Arabidopsis phabulosa-1d (phb-1d) mutation. phb-1d is a dominant mutation that causes altered leaf polarity such that adaxial characters develop in place of abaxial leaf characters. The adaxialized leaves fail to develop leaf blades. This supports a recently proposed model in which the juxtaposition of ad- and abaxial cell fates is required for blade outgrowth. In addition to the alteration in leaf polarity, phb-1d mutants develop ectopic SAMs on the undersides of their leaves. Also, the phb-1d mutation weakly suppresses the shoot meristemless (stm) mutant phenotype. These observations indicate an important role for adaxial cell fate in promoting the development of axiallary SAMs and suggest a cyclical model for shoot development: SAMs make leaves which in turn are responsible for generating new SAMs.
Background and Aims
Dormant resting buds are frequently regarded as static units, with protective cataphylls on the outside and embryonic foliage leaves on the inside. The developmental consequences of cataphyll insertion into the dynamic, cyclical, annually repeating sequence of leaf forms that a resting bud gives rise to have rarely been interrogated. To examine the connection between dormant structure and growing-season development, we compare the complete seasonal heteroblastic sequence of leaf forms of six species of temperate Juglandaceae with distinctly different vegetative resting bud structures. These include buds with cataphylls; buds without cataphylls; and buds with caducous cataphylls that are lost before the onset of winter.
Methods
In a common garden setting over a seven-month growing season, the dimensions of 2249 individual vegetative metamers were tracked from first exposure to abscission along the shoots of saplings and mature trees. The timing of metamer initiation within terminal buds was investigated using micro-CT scanning. Character state transitions of resting bud types were estimated using a phylogenetic tree of Juglandaceae.
Key Results
The presence of cataphylls within a heteroblastic sequence is associated with a single cohort of foliage leaves that flush and abscise synchronously. This growing pattern is highly determinate, with next year’s terminal-bud cataphylls already initiated before spring leaf out. In contrast, in sequences without cataphylls, shorter-lived foliage leaves appear and abscise in a staggered fashion. Despite these differences in leaf demography, all examined heteroblastic sequences produce a series of small, caducous leaf forms that precede terminal bud set.
Conclusions
The ubiquity of caducous leaf forms in Juglandoideae may point to the importance of shoot tip protection far beyond the dormant season. In addition, the presence or absence of cataphylls in resting buds is indicative of distinct shoot ontogenetic patterns, and functional strategies, in summer.
‘Can we doubt … that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind?’ In the Origin of Species (1859) Darwin challenged many of the most deeply held beliefs of the Western world. His insistence on the immense length of the past and on the abundance of life-forms, present and extinct, dislodged man from his central position in creation and called into question the role of the Creator. He showed that new species are achieved by natural selection, and that absence of plan is an inherent part of the evolutionary process. Darwin's prodigious reading, experimentation, and observations on his travels fed into his great work, which draws on material from the Galapagos Islands to rural Staffordshire, from English back gardens to colonial encounters. The present edition provides a detailed and accessible discussion of his theories and adds an account of the immediate responses to the book on publication. The resistances as well as the enthusiasms of the first readers cast light on recent controversies, particularly concerning questions of design and descent.
Carpels, the female reproductive organs of flowering plants, are
of major economic importance since much of our food is
ultimately derived from carpel tissue and they are a defining
innovation for flowering plants. Amazingly, little is known about
the origin and conservation of the developmental program of
the carpel besides the knowledge generated by utilizing
Arabidopsis thaliana. However, in the past few years advances
in ancestral state reconstruction, developmental genetics, and
phylogenetic analyses led to advances in the field of flower
evodevo. Here, I summarize recent work on ancestral state
reconstructions of carpels, and the functions of the major
components of the genetic networks governing carpel
development described for Arabidopsis. Then, I point out how
the stepwise addition of genes during land plant evolution has
generated the A. thaliana carpel’s developmental toolkit. By
merging these observations, I propose a basic version of the
ancestral angiosperm carpel developmental network.
Flower morphology results from the interaction of an established genetic program, the influence of external forces induced by pollination systems, and physical forces acting before, during and after initiation. Floral ontogeny, as the process of development from a meristem to a fully developed flower, can be approached either from a historical perspective, as a "recapitulation of the phylogeny" mainly explained as a process of genetic mutations through time, or from a physico-dynamic perspective, where time, spatial pressures, and growth processes are determining factors in creating the floral morphospace. The first (historical) perspective clarifies how flower morphology is the result of development over time, where evolutionary changes are only possible using building blocks that are available at a certain stage in the developmental history. Flowers are regulated by genetically determined constraints and development clarifies specific transitions between different floral morphs. These constraints are the result of inherent mutations or are induced by the interaction of flowers with pollinators. The second (physico-dynamic) perspective explains how changes in the physical environment of apical meristems create shifts in ontogeny and this is reflected in the morphospace of flowers. Changes in morphology are mainly induced by shifts in space, caused by the time of initiation (heterochrony), pressure of organs, and alterations of the size of the floral meristem, and these operate independently or in parallel with genetic factors. A number of examples demonstrate this interaction and its importance in the establishment of different floral forms. Both perspectives are complementary and should be considered in the understanding of factors regulating floral development. It is suggested that floral evolution is the result of alternating bursts of physical constraints and genetic stabilization processes following each other in succession. Future research needs to combine these different perspectives in understanding the evolution of floral systems and their diversification.
The floral quartet model of floral organ specification poses that different tetramers of MIKC-type MADS-domain transcription factors control gene expression and hence the identity of floral organs during development. Here, we provide a brief history of the floral quartet model and review several lines of recent evidence that support the model. We also describe how the model has been used in contemporary developmental and evolutionary biology to shed light on enigmatic topics such as the origin of land and flowering plants. Finally, we suggest a novel hypothesis describing how floral quartet-like complexes may interact with chromatin during target gene activation and repression.
Root hair cells and pollen tubes, like fungal hyphae, possess a typical tip or polar cell expansion with growth limited to the apical dome. Cell expansion needs to be carefully regulated to produce a correct shape and size. Polar cell growth is sustained by oscillatory feedback loops comprising three main components that together play an important role regulating this process. One of the main components are reactive oxygen species (ROS) that, together with calcium ions (Ca2+) and pH, sustain polar growth over time. Apoplastic ROS homeostasis controlled by NADPH oxidases as well as by secreted type III peroxidases has a great impact on cell wall properties during cell expansion. Polar growth needs to balance a focused secretion of new materials in an extending but still rigid cell wall in order to contain turgor pressure. In this review, we discuss the gaps in our understanding of how ROS impact on the oscillatory Ca2+ and pH signatures that, coordinately, allow root hair cells and pollen tubes to expand in a controlled manner to several hundred times their original size toward specific signals. © 2016 American Society of Plant Biologists. All rights reserved.
The results of developmental and structural studies on three leaf formations of generative shoots of Nandina, Mahonia and Helleborus species of different eco-geographical origin from native area and introduced ones are outlined here. The development of three leaf formations (cataphylls, assimilative leaves, bracts) from different parts of leaf primordia, in our view, is the base of leaf heterogeneity. The results of comparative studies of developmental and anatomical structures show that homological formations (cataphylls and assimilative leaf bases) of shrubs and herbaceous forms are characterized by their considerable similarity. More vigorous development of leaf bases are due to their multifunctionality. Amphistomacy and the presence of homogeneous mesophyll are distinctive features of cataphylls and bracts, unlike assimilative leaf laminas. Cataphylls of herbaceous forms lack many special defensive features which are characteristic of many woody ones. The defensive function of cataphylls is supplied by anatomical features of the epidermal complex and by the presence of anthocyans and tannin substances as well. Great anatomical similarity of bracts and sepals are revealed. It confirms the possibility of bracteous origin of the calyx in Berberidaceae and Ranunculaceae.
The shoot concept of the flower suggests that flowers correspond to vegetative short-shoots except the fact that their lateral appendages are floral and not vegetative leaves. However, in view of the different properties of vegetative and flower meristems, this concept should be questioned. Differential meristem activity resulting in tubes, hypanthia and inferior ovaries, continuous meristem expansion providing space for stamen fascicles and additional structures and the process of (repeated) fractionation using a given space completely, are characteristics of flower meristems hardly explainable with the shoot concept. Linking instead flower development with recent findings in molecular biology and computational modeling widens the view to the fundamental relation between growth and form. Given that the same general principles characterize plant growth at all life stages, the loss of apical growth appears to play the major role in changing geometry, space availability and genetic regulation in flower meristems. The flower, thus, turns out to be the sporangia bearing tip of a shoot.
A survey of the morphogenesis and possible phylogenesis of the foliar organs of ferns and angiosperms including the fertile structures is given. The construction of the sporangia-bearing fern leaf reflects a primitive state of a thallose land plant. The most important morphogenetic processes occur in its marginal meristem which reaches its maximum thickness only a short time after primordium initiation. Dilation of the marginal meristem leads to fractionations which produce the pinnae acroplasticly, periplasticly, or basiplasticly in an alternate, ternate, or binate order. During this period, thickness of the marginal meristem declines until in the last growth period the pinnae blades are modelled. The longitudinal articulation of fern leaves leads to a loose differentiation of a leaf base, petiole, and leaf blade. All these processes are also observed in angiosperms, but diversity is enlarged by the capacity of meristem incorporation and the development of true intercalary growth zones. Gamophylly, interpetiolar and median stipules, peltate blades, and pinnae were possible by incorporation and fusion of marginal meristems.
Shoot development of temperate and tropical members of Berberis s. l. was examined in order to assess: (1) the homology of the spines along the long shoots and the foliage leaves that form on the short shoots; (2) the occurrence of heterophylly and/or heteroblasty in the genus; and (3) the structural correspondence between cataphylls, spines, and foliage leaves. The 1-5-armed spines have been interpreted as modified compound leaves lacking stipules, as a modified lamina (central spine) with stipules (lateral spines), or less often, as transformed branches, or as epidermal outgrowths. On the other hand, the foliage leaves of the short shoots have been interpreted as leaflets of palmately compound leaves. Our results indicate that there are three distinct leaf types per node: (1) Leaves modified in spines spirally arranged in long shoots; (2) foliage, expanded leaves densely arranged in short shoots; and (3) cataphylls protecting axillary buds. The spines are leaf homologs with a clear distinction between the leaf base with stipules, and a laminar portion modified into the 1-5-armed spine; the lateral spines are not stipules as they arise from the marginal meristem of the laminar portion, and not from the leaf base. The foliage leaves also have stipules flanking the leaf base. Both spiny leaves and foliage leaves develop an articulation between the base and the laminar portion. Cataphylls of the short shoots of Berberis s. str. and those of the reproductive short shoots of Mahonia correspond to the entire leaf base, but those of the renewal (vegetative) shoots of Mahonia are spiny and have an odd vestigial pinnately compound lamina. Heterochrony due to ontogenetic truncation caused by the formation of the terminal inflorescence at the apex of the short shoots could be responsible for the lack of petiole/lamina differentiation in the foliage leaves. The spiny long-shoot/foliose short-shoot system of branching in Berberis s. str. appears to be genetically and phylogenetically fixed and not environment-dependent. This represents a clear example of metamorphic heteroblasty sensu Zotz et al. (Botanical Review 77:109–151, 2011) with further occurrence of heterophylly along the short shoots.
Classical plant morphology still provides the conceptual framework for most phytomorphological investigations and highly relevant concepts and data for other botanical disciplines such as plant morphogenesis, molecular genetics, ecology, systematics, evolutionary plant biology, etc. Typical classical morphology is categorical, i.e. the diversity of plant form is reduced to mutually exclusive morphological categories such as root, shoot, stem (caulome), leaf (phyllome), and trichome. In contrast, continuum morphology established a morphological continuum between all these categories. As a consequence, homology becomes a matter of degree. Hence, the difference between continuum morphology and classical morphology is striking. Nonetheless, the two approaches and views need not be seen as opposed to each other. They can be considered complementary: classical morphology emphasizing the difference between typical representatives of morphological categories and continuum morphology stressing the continuum between these fuzzy categories. Furthermore, if the morphological categories are interpreted as extreme types, which by definition are fuzzy and continuous with each other, then classical morphology becomes continuum morphology. If such reinterpretation occurs only to some extent, intermediate positions between typical classical morphology and continuum morphology result. Examples of various intermediate positions indicate that a continuum exists between typical classical morphology and continuum morphology. Hence, there is not only a continuum between morphological categories but also between approaches to and views of the field of plant morphology. Consequences of this reconciliation are briefly discussed.
Recent molecular systematic and developmental genetic findings have drawn attention to plant morphology as a discipline dealing with the phenotypic appearance of plant forms. However, since different terms and conceptual frameworks have evolved over a period of more than 200 years, it is reasonable to survey the history of plant morphology; this is the first of two papers with this aim. The present paper deals with the historic concepts of Troll, Zimmermann and Arber, which are based on Goethe's morphology. Included are contrasting views of ‘unity and diversity’, ‘position and process’, and ‘morphology and phylogeny’, which, in part, are basic views of current plant morphology, phylogenetic systematics and developmental genetics. Wilhelm Troll established the ‘type concept’ and the ‘principle of variable proportions’. He has provided the most comprehensive overview of the positional relations of plant forms. Agnes Arber started from the universal dynamics of life and attempted to describe all structures as processes. She paid attention to ‘repetitive branching’, ‘differential growth’, and ‘parallelism’. As a result she has recently been rediscovered by developmental botanists. Walter Zimmermann rejected any metaphysical influence on plant form and instead called for objective procedures. He was mainly interested in phylogenetic ‘character transformation’ and the ‘reconstruction of genealogical lines’. Guided by the example of flower-like inflorescences, a future paper will deal with functional and developmental constraints influencing plant forms. Recent morphological concepts (‘trialectical’, ‘continuum’/‘fuzzy’, ‘process morphology’) will be discussed and related to current morphological and developmental genetic research.
RÉSUMÉ. — Les vues de Candolle et de Turpin concernant la notion de type des organes appendiculaires et l'organisation métamérique du végétal sont analysées et comparées à celles de Gœthe. La forme donnée à ces idées par Candolle est spécialement importante, puisque c'est sous celle-ci qu'elles devaient se répandre ensuite.
We investigated the early floral development of the monotypic genus Amborella, the sister to all other extant angiosperms. Examination of vegetative shoot development revealed that Amborella possesses both decussate and alternate phyllotaxy; one may simply be a special case of the other as a reaction to meristem size and shape. The transition from bracts to tepals is gradual, making it difficult to determine exactly where a flower begins in this species. Although flowers of Amborella are described as having spiral phyllotaxy, the periphery of the flower could be considered unidirectionally whorled. This new observation, together with observations of both spiral and whorled phyllotaxy in other basal angiosperms (e.g., Nuphar, Drimys, Ceratophyllum), further demonstrates the flexibility of floral development in basal angiosperms; i.e., some basal angiosperms are not fully committed to either spiral or whorled phyllotaxy. The developmental transitions between bracts and perianth, and between stamens and carpels, are continuous. However, the transition between perianth and sporophylls is more distinct, although tepals, stamens, and carpels appear to be structurally homologous. Both stamens and carpels share characteristics of the other sexual organ. We hypothesize that genes correlated with stamen identity (e.g., B-class MADS-box genes) may show a gradual change in expression pattern throughout the floral parts of Amborella and perhaps other basal angiosperms, corresponding to the observed gradual morphological transitions. This "fading borders" model of floral gene expression should be evaluated in Amborella and other basal angiosperms.
The application of homology varies depending on the data being examined. This volume represents a state-of-the-art treatment of the different applications of this unifying concept. Chapters deal with homology on all levels, from molecules to behavior, and are authored by leading contributors to systematics, natural history, and evolutionary, developmental, and comparative biology. This paperback reprint of the original hardbound edition continues to commemorate the 150th anniversary of Sir Richard Owens seminal paper distinguishing homology from analogy. Special features include: * Commemoration of the 150th anniversary of Sir Richard Owens seminal paper distinguishing homology from analogy * Contributors who are renowned leaders in comparative biology * Coverage that is both comprehensive and interdisciplinary.
The length and basal diameter of all lateral and terminal buds of vegetative annual shoots of 7-year-oldJuglans regia trees were measured. All buds were dissected and numbers of cataphylls, embryonic leaves and leaf primordia were recorded.
Each axillary bud was ranked according to the position of its associated leaf from the apex to the base of its parent shoot.
Bud size and content were analysed in relation to bud position and were compared with the size and number of leaves of shoots
in equivalent positions which extended during the following growing season. Length and basal diameter of axillary buds varied
according to their position on the parent shoot. Terminal buds contained more embryonic leaves than any axillary bud. The
number of leaves was smaller for apical and basal axillary buds than for buds in intermediate positions on the parent shoot
only. All new extended shoots were entirely preformed in the buds that gave rise to them. Lateral shoots were formed in the
median part of the parent shoot. These lateral shoots derived from buds which were larger than both apical and basal ones.
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Background and aims – The term ‘petal’ is loosely applied to a variety of showy non-homologous
structures, generally situated in the second whorl of a differentiated perianth. Petaloid organs are extremely
diverse throughout angiosperms with repeated derivations of petals, either by differentiation of a perianth,
or derived from staminodes. The field of evo-devo has prompted re-analysis of concepts of homology
and analogy amongst petals. Here the progress and challenges in understanding the nature of petals are
reviewed in light of an influx of new data from phylogenetics, morphology and molecular genetics.
Method and results – The complex web of homology concepts and criteria is discussed in connection to
the petals, and terminology is subsequently defined. The variation and evolution of the perianth is then
reviewed for the major clades of angiosperms. From this pan-angiosperm variation, we highlight and discuss
several recurrent themes that complicate our ability to discern the evolution of the petal. In particular we
emphasise the importance of developmental constraint, environmental stimuli, interrelationship between
organs and cyclic patterns of loss and secondary gain of organs, and the development of a hypanthium or
corona.
Conclusions – A flexible approach to understanding ‘petals’ is proposed requiring consideration of the
origin of the floral organ (stamen-derived or tepal-derived, or other), the functional role (sepaloid or
petaloid organs), evolutionary history of the organismal lineage, and consideration of developmental forces
acting on the whole flower. The variety and complex evolutionary history of the perianth may necessitate
the exploration of petal development within phylogenetically quite restricted groups, combining data from
morphology with evo-devo.
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The problem of determining affinity among glossopterid gymnosperms is beset by deficiencies in preservation, natural dissociation of parts, and scarcity of features assuredly critical for morphologic comprarison. The glossopterids probably are not a very heterogeneous group of plants, but this is difficult to prove. The Gondwana glacial “hiatus” has resulted in the omission of a critical chapter glossopterid evolution. As a consequence, morphologic features and phyletic probabilities must be evaluated on a much more hypothetical basis than would otherwise be justified.