Morphogens, their identification and regulation

During the course of development, cells of many tissues differentiate according to the positional information that is set by the concentration gradients of morphogens. Morphogens are signaling molecules that emanate from a restricted region of a tissue and spread away from their source to form a concentration gradient. As the fate of each cell in the field depends on the concentration of the morphogen signal, the gradient prefigures the pattern of development. In this article, we describe how morphogens and their functions have been identified and analyzed, focusing on model systems that have been extensively studied.     

Morphogens and endocytosis

During development, secreted signaling proteins of the Wingless/Wnt, Hedgehog and Decapentaplegic (Dpp)/Bone Morphogenic Protein (BMP) families act as morphogens. Previous work had shown that these molecules act directly on distant cells, although until recently nothing was known about how they reach those distant cells. During the past two years, work carried out on Drosophila using genetic and cell biology approaches have revealed that endocytosis plays a central part in the mechanisms that control the spread of morphogens.     

Morphogens and synaptogenesis in Drosophila

During development and adult life synapses are remodeled in response to genetic programs and environmental cues. This synaptic plasticity is thought to be the basis of learning and memory. The larval neuromuscular junction of Drosophila is established during embryogenesis and grows during larval development to accommodate muscle growth and maintain synaptic homeostasis. This growth is dependent on bidirectional communication between the motoneuron and the muscle fiber. The best-characterized retrograde signaling pathway is defined by Glass bottom boat (Gbb), a morphogen of the transforming growth factor-beta (TGF-beta) superfamily. Gbb acts as a muscle-derived retrograde signal that activates the TGF-beta pathway presynaptically. This pathway includes the type II receptor Wishful thinking, type I receptors Thick veins and Saxophone, and the second messenger Smads Mothers against dpp (Mad) and Medea. Mutations that block this pathway result in small synapses that are morphologically aberrant and severely impaired functionally. An emerging anterograde signaling pathway is defined by Wingless, a morphogen of the Wnt family that acts as a motoneuron-derived anterograde signal required for both pre- and postsynaptic development. In the absence of Wingless the neuronal microtubule cytoskeleton regulator Futsch is down-regulated and synaptic growth impaired. Some of these morphogens have conserved roles in mammalian synaptogenesis, and genetic analysis suggests that additional signaling molecules are required for synaptic growth at the Drosophila neuromuscular junction.     

Morphogens and cell survival during development

The notion of "morphogens" is an important one in developmental biology. By definition, a morphogen is a molecule that emanates from a specific set of cells that is present in a concentration gradient and that specifies the fate of each cell along this gradient. The strongest candidate morphogens are members of the transforming growth factor-beta (TGF-beta), Hedgehog (Hh), and Wnt families. While these morphogens have been extensively described as differentiation inducers, some reports also suggest their possible involvement in cell death and cell survival. It is frequently speculated that the cell death induction that is found associated with experimental removal of morphogens is the manifestation of abnormal differentiation signals. However, several recent reports have raised controversy about this death by default, suggesting that cell death regulation is an active process for shaping tissues and organs. In this review, we will present morphogens, with a specific emphasis on Sonic Hedgehog, a mammalian member of the Hh family, not as a positive regulators of cell differentiation but as key regulators of cell survival.     

Problems and paradigms: Morphogens and pattern formation

Morphogen gradient theories have enjoyed considerable popularity since the beginning of this century, but conclusive evidence for a role of morphogens in controlling multicellular development has been elusive. Recently, work on three secreted signalling proteins, Activin in Xenopus, and Wingless and Dpp in Drosophila, has stongly suggested that these proteins function as morphogens. In order to define a factor as a morphogen, it is necessary to show firstly, that it has a direct effect on target cells and secondly, that it affects the development of target cells in a concentration-dependent manner. With these criteria in mind, the evidence available for a variety of proposed morphogens is discussed. While the evidence is not conclusive in most of the cases considered, there is a strong case in favour of the three proteins mentioned above, which suggests that morphogens are potentially of general importance in controlling the development of multicellular organisms.     

Nitrate leaching as a function of plant community richness and composition,and the scaling of soil nutrients,in a restored temperate grassland

Two, two-factor experiments manipulated species and functional form plant richness and the spatial scaling of either nitrogen (N) or phosphorous (P) in restored tallgrass prairie in North Dakota, USA. Nitrate (NO₃ ⁻) leaching was measured in these plots and analyzed for its response to the treatment factors and measured plant community parameters. Nitrate extracted from anion exchange resin was regressed against the first principal component of species and functional form richness, the spatial scaling of N or P, the measured biomass of the functional forms used and the plot values for plant parameters based on weighted averages by species biomass. The treatments applied in the N and P experiments were 1, 2, 5, 10, or 20 plant species taxa, and the application of fertilizer in a random fractal pattern with either fine-scale or coarse-scale heterogeneity. Nitrate leaching decreased with plant diversity and increased by a factor of two going from fine-scale to coarse-scale N. It was also related to a number of plant functional parameters, and was positively correlated with the biomass of late successional C₃ grasses (Koeleria cristata (Lam.) Beauv., Poa pratensis L., Stipa comata Trin. & Rupr., and Stipa viridula Trin.), which are known from previous studies to have negative mycorrhizal responsiveness and are characterized by high root lateral spread per unit of root biomass. Our results show that while plant diversity has a highly significant influence on plant community uptake of NO₃ ⁻, this effect is mediated by the scaling of soil N and the functional traits of the species comprising the plant assemblage.     

Morphogens in motion: growth control of the neural tube

The entire vertebrate nervous system develops from a simple epithelial sheet, the neural plate which, along development, acquires the large number and wide variety of neuronal cell types required for the construction of a functional mature nervous system. These include processes of growth and pattern formation of the neural tube that are achieved through complicated and tightly regulated genetic interactions. Pattern formation, particularly in the vertebrate central nervous system, is one of the best examples of a morphogen-type of function. Cell cycle progression, however, is generally accepted to be dependent on cell-intrinsic factors. Recent studies have demonstrated that proliferation of neural precursors is also somehow controlled by secreted signaling molecules, well-known by their role as morphogens, such as fibroblast growth factor (FGF), vertebrate orthologs of the Drosophila wingless (Wnt), hedgehog (Hh), and transforming growth factor beta (TGF-beta) families, that in turn regulate the activity of factors controlling cell cycle progression. In this review we will summarize the experimental data that support the idea that classical morphogens can be reused to regulate proliferation of neural precursors.     

Ultrastructural and biochemical basis of the digestion of nectar and other nutrients by the moth Erinnyis ello

The midgut epithelium of adult Erinnyis ello L. (Lepidoptera: Sphingidae) forms ramified villus-like folds. The epithelial cells exhibit long microvilli and narrow and sinuous basal channels studded with particles. These morphological features are thought to be involved in the absorption of nutrients present in low concentration in food. Enzymatic assays in E. ello adults showed the presence of amylase in salivary glands, and the occurrence of aminopeptidase, -fructosidase, - and -glucosidase, trehalase and trypsin in the midgut. Calcium differential precipitation of midgut homogenates showed that aminopeptidase and -glucosidase are integral proteins of the microvillar cell membranes, whereas the other enzymes are soluble or partly soluble and partly membrane bound. Detergent-solubilized aminopeptidase (pH optimum 7.5) sediments as a protein with Mr 120000, whereas trehalase (pH optimum 6.5), as a protein with Mr 95000. The activities of amylase (pH optimum 5.5), -glucosidase (pH optimum 5.0) and trypsin (pH optimum 9.5) were not sufficiently high to be further characterized, whereas -fructosidase and membrane-bound -glucosidase were characterized elsewhere. The data suggest that nectar-feeding E. ello adults are able to some extent to digest and absorb starch and proteins, in addition to nectar sugars. Initial digestion is considered to occur in midgut lumen and final digestion on the surface of midgut cells under the action of microvillar enzymes.

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Résumé L'épithélium de l'intestin moyen des adultes de E. ello L; (Lépido., Sphingidae) est replié en villosités. Les cellules épithéliales présentent de longs microvilli et d'étroits canaux sinueux basaux garnis de particules. Ces caractères morphologiques peuvent être interprétés comme impliquant l'absorption d'aliments présents en faible concentration dans la nourriture. Des études enzymatiques ont révélé la présence d'amylase dans les glandes salivaires, et la découverte d'aminopeptidase, de -fructosidase, d'- et -glucosidases, de tréhalase et de trypsine dans l'intestin moyen. La précipitation calcique différentielle d'homogénats de l'intestin moyen a montré que l'aminopeptidase et l'-glucosidase sont des protéines appartenant aux membranes microvillaires cellulaires, tandis que les autres enzymes sont entièrement ou partiellement solubles et partiellement liées aux membranes. L'aminopeptidase solubilisée par un détergent (pH optimum: 7,5) sédimente comme une protéine de Mr 120000, tandis que la tréhalase (pH optimum: 6,5) comme une protéine de Mr 95000. Les activités de l'amylase (pH optimum: 5,5), de la -glucosidase (pH optimum 5,0) et de la trypsine (pH optimum 9,5) n'étaient pas suffisantes pour être caractérisées, tandis que la -fructosidase et l'-glucosidase liée à la membrane ont été caractérisées ailleurs. Les résultats suggèrent que les adultes E. ello consommateurs de nectar sont capables de digérer et d'absorber une certaine quantité d'amidon et de protéines, en plus des sucres du nectar. La digestion doit débuter dans la lumière de l'intestin moyen pour s'achever à la surface des cellules intestinales sous l'action des enzymes des microvilli.

     

Genetic basis for innovations in floral organ identity

Of the many innovations associated with the radiation of the angiosperms, the evolution of a petal identity program is among the best understood from a genetic standpoint. Although the existing data do indicate that similar genetic mechanisms control petal development across diverse taxa, there is also considerable evidence for variability in petal identity programs, likely due to a number of factors. These points are illustrated through a review of our current knowledge on the subject, integrating phylogenetic, morphological, and genetic studies. Comparative studies of petal identity highlight the complex nature of homology in plants and stand as a cautionary tale for the interpretation of gene expression data.     

A common genetic basis to the origin of the leaf economics spectrum and metabolic scaling allometry

Many facets of plant form and function are reflected in general cross‐taxa scaling relationships. Metabolic scaling theory (MST) and the leaf economics spectrum (LES) have each proposed unifying frameworks and organisational principles to understand the origin of botanical diversity. Here, we test the evolutionary assumptions of MST and the LES using a cross of two genetic variants of Arabidopsis thaliana. We show that there is enough genetic variation to generate a large fraction of variation in the LES and MST scaling functions. The progeny sharing the parental, naturally occurring, allelic combinations at two pleiotropic genes exhibited the theorised optimum ¾ allometric scaling of growth rate and intermediate leaf economics. Our findings: (1) imply that a few pleiotropic genes underlie many plant functional traits and life histories; (2) unify MST and LES within a common genetic framework and (3) suggest that observed intermediate size and longevity in natural populations originate from stabilising selection to optimise physiological trade‐offs.     

Morphological and kinematic basis of the hummingbird flight stroke: scaling of flight muscle transmission ratio

Hummingbirds (Trochilidae) are widely known for their insect-like flight strokes characterized by high wing beat frequency, small muscle strains and a highly supinated wing orientation during upstroke that allows for lift production in both halves of the stroke cycle. Here, we show that hummingbirds achieve these functional traits within the limits imposed by a vertebrate endoskeleton and muscle physiology by accentuating a wing inversion mechanism found in other birds and using long-axis rotational movement of the humerus. In hummingbirds, long-axis rotation of the humerus creates additional wing translational movement, supplementing that produced by the humeral elevation and depression movements of a typical avian flight stroke. This adaptation increases the wing-to-muscle-transmission ratio, and is emblematic of a widespread scaling trend among flying animals whereby wing-to-muscle-transmission ratio varies inversely with mass, allowing animals of vastly different sizes to accommodate aerodynamic, biomechanical and physiological constraints on muscle-powered flapping flight.     

Inorganic nutrients,bacteria, and the microbial loop

The realization that natural assemblages of planktonic bacteria may acquire a significant fraction of their nitrogen and phosphorus via the uptake of dissolved inorganic nutrients has modified our traditional view of these microorganisms as nutrient remineralizers in plankton communities. Bacterial uptake of inorganic nitrogen and phosphorus may place bacteria and phytoplankton in competition for growth-limiting nutrients, rather than in their traditional roles as the respective source and sink for these nutrients in the plankton. Bacterial nutrient uptake also implies that bacterivorous protozoa may play a pivotal role in the remineralization of these elements in the microbial loop. The overall contribution of bacterial utilization of inorganic nutrients to total nutrient uptake in the ocean is still poorly understood, but some generalizations are emerging with respect to the geographical areas and community physiological conditions that might elicit this behavior.     

Conservative whole‐organ scaling contrasts with highly labile suborgan scaling differences among compound eyes of closely related Formica ants

Static allometries determine how organ size scales in relation to body mass. The extent to which these allometric relationships are free to evolve, and how they differ among closely related species, has been debated extensively and remains unclear; changes in intercept appear common, but changes in slope are far rarer. Here, we compare the scaling relationships that govern the structure of compound eyes of four closely related ant species from the genus Formica. Comparison among these species revealed changes in intercept but not slope in the allometric scaling relationships governing eye area, facet number, and mean facet diameter. Moreover, the scaling between facet diameter and number was conserved across all four species. In contrast, facet diameters from distinct regions of the compound eye differed in both intercept and slope within a single species and when comparing homologous regions among species. Thus, even when species are conservative in the scaling of whole organs, they can differ substantially in regional scaling within organs. This, at least partly, explains how species can produce organs that adhere to genus wide scaling relationships while still being able to invest differentially in particular regions of organs to produce specific features that match their ecology.     

Cryptic individual scaling relationships and the evolution of morphological scaling

Morphological scaling relationships between organ and body size—also known as allometries—describe the shape of a species, and the evolution of such scaling relationships is central to the generation of morphological diversity. Despite extensive modeling and empirical tests, however, the modes of selection that generate changes in scaling remain largely unknown. Here, we mathematically model the evolution of the group‐level scaling as an emergent property of individual‐level variation in the developmental mechanisms that regulate trait and body size. We show that these mechanisms generate a “cryptic individual scaling relationship” unique to each genotype in a population, which determines body and trait size expressed by each individual, depending on developmental nutrition. We find that populations may have identical population‐level allometries but very different underlying patterns of cryptic individual scaling relationships. Consequently, two populations with apparently the same morphological scaling relationship may respond very differently to the same form of selection. By focusing on the developmental mechanisms that regulate trait size and the patterns of cryptic individual scaling relationships they produce, our approach reveals the forms of selection that should be most effective in altering morphological scaling, and directs researcher attention on the actual, hitherto overlooked, targets of selection.     

Light, nutrients and the growth of herbaceous forest species

The herb layer of forests planted on former agricultural land often differs from that of old-growth forest. This study investigates if the expected increased nutrient availability in the shaded conditions of newly planted forests and the plasticity of the species to adjust their biomass allocation to different levels of light and nutrients help to explain these differences in the herb layers of the two forest types. In a greenhouse experiment biomass distribution and production of two species characteristic for the highly shaded forest floor, Circaea lutetiana and Mercurialis perennis, and two species more common in the forest-edge, Aegopodium podagraria and Impatiens parviflora were studied at different levels of light (2%, 8% and 66% of the full light level) and nutrients (30 and 300 kg N ha^–1 per year). The main factor affecting allocation and biomass production was light availability. Nutrient supply only had a significant effect at the higher light levels. Species were mainly plastic to changes in light and the two species from the forest floor showed to be more rigid in allocation pattern than the species from the forest-edge. So, although the species from the forest-edge were more plastic, they did not profit from the increased nutrient supply because the main factor affecting biomass distribution and production was light availability.     

Inbreeding reduces power-law scaling in the distribution of fluctuating asymmetry: an explanation of the basis of developmental instability

The study of fluctuating asymmetry has been controversial because of conflicting results found in much of the primary literature. It has been suggested that the source of this conflict is the fact that the basis of fluctuating asymmetry is poorly understood and that, as a consequence, methodology of fluctuating asymmetry studies may be flawed. A new model for the phenomenological basis of fluctuating asymmetry, that variation in fluctuating asymmetry is in large part due to the random exponential growth of cell populations (geometric Brownian motion) that are terminated randomly around a genetically programmed development time, is presented here. If termination of development has a genetic component, then scaling effects and kurtosis in the distribution of fluctuating asymmetry should increase with genetic redundancy of the population. This model prediction was tested by comparing the distribution of multivariate size and shape fluctuating asymmetry in large samples collected from both wild populations and four moderately inbred lines of Drosophila simulans. It was found that while wild populations were best described by a lognormal distribution with power-law scaled tails, the inbred lines derived from the wild stock were dramatically normalized (half-normal) in three of four cases. As predicted, the scaling exponent of the upper tail of the distribution of fluctuating asymmetry increased with inbreeding while the kurtosis and mean fluctuating asymmetry decreased with inbreeding. The model suggests an additional explanation of leptokurtosis in fluctuating asymmetry. Kurtosis and scaling of the statistical distribution of fluctuating asymmetry in a population is related directly to genetic differences between individuals and these differences affect their ability to buffer the process of development against random perturbations.