Branching patterns, generating rules, and astogenetic trajectories in arborescent bryozoans

The terminal growth of bryozoans allows a reconstruction of their growth history and generating rules from the developed pattern. Three species of arborescent bryozoans share a bias in growth rate that favors reverser branches (those whose direction of growth is opposite that of their parent branch); this bias produces a common hummocky appearance to the top margin of the colony. Other differences in the growth rules, however, result in markedly different colony forms. For two species, the cue for splitting of branches seems to be the length of the branch; for the other, the cue seems to be the time since the last splitting. Simulations based on the distinction between reversing and nonreversing branches reproduce the hummocky pattern, but offer little insight into the underlying mechanism. Simulations based on occlusion, the line-of-sight blocking of the branches by each other, give similar patterns with fewer assumptions. These simulations also suggest that the pattern is due to a limited availability of nutrients to occluded growing tips, and that nutrients remain relatively localized within colonies. The actual determinants of colony form are likely to combine both nutrition and geometry.     

Evolutionary palaeoecology of symbioses between bryozoans and hermit crabs

Modern hermit crabs form associations with many organisms which encrust, bore into, or cohabit the living chambers of gastropod shells occupied by the crabs. Among these hermit crab symbionts are bryozoan species which develop massive, commonly multilayered, colonies encrusting hermit crab shells. These colonies extend the living chamber of the crab through a characteristic process of helicospiral tubular growth originating from the shell aperture. The scant information available on the ecology of Recent bryozoan‐hermit crab symbioses is reviewed. Symbioses have been recorded from intertidal to upper slope environments, and from tropical to cold temperate zones. None of the hermit crab species are obligatory symbionts of bryozoans, and the majority of the modern bryozoan species involved are also not obligatory symbionts. Fossil examples always lack the hermit crabs, which have a poor fossilization potential; however, the distinctive tubular growth pattern and other features of the bryozoans enable recognition of ancient examples of the symbiosis. The earliest inferred associations between bryozoans and hermit crabs date from the Mid Jurassic, but associations remained uncommon until the Neogene. A remarkably wide taxonomic diversity of Recent and fossil bryozoans are known or inferred symbionts of hermit crabs. The broad evolutionary pattern of the association demonstrates multiple originations of the symbiosis by bryozoans belonging to at least 5 cyclostome and 12 cheilostome families. Only the Miocene‐Recent cheilostome family Hippoporidridae has an evolutionary history closely tied to symbiosis with hermit crabs. There is no evidence for coevolution.     

Effects of two marinas on the composition of fouling assemblages

The composition of fouling assemblages was surveyed inside and near two fully enclosed marinas using settlement plates. The location of a plate, inside or outside the marina, influenced the abundances of four functional groups of fouling organisms (solitary ascidians, arborescent bryozoans, encrusting bryozoans and colonial ascidians). Transplantation of mature assemblages revealed that reductions in the abundance of arborescent bryozoans inside marinas might be explained by increased growth and recruitment of these bryozoans outside the marina. Surveys of settlement revealed decreased recruitment of encrusting bryozoans inside the marinas, a result consistent with patterns of adult abundance. It is proposed that an increased abundance of solitary ascidians inside marinas may be due to decreased competition. A second survey of adult assemblages was performed with multiple ‘Outside’ sites per marina. Effects of location consistent with the first survey were found for arborescent bryozoans, and in one marina area for solitary ascidians and encrusting bryozoans, but not in the other. Although mechanisms can be proposed to explain the effects of the marina (inside or outside) on the abundances of solitary ascidians, arborescent bryozoans and encrusting bryozoans, the second survey revealed that the effects may vary among marinas.     

Inter- and intraspecific variation in gorgonian colony morphology: quantifying branching patterns in arborescent animals

Geomorphological methods for quantifying branching networks are used to describe inter- and intra-specific differences in branching patterns among two species of arborescent Caribbean gorgonian, Plexaura homomalla, P. flexuosa, and a third, undescribed plexaurid from the San Blas Islands, Panama. There were significant differences among species for first, second and third order branch lengths, and for tributary to source ratios for first and second order branches. Extrapolations from the branching parameters successfully predicted differences in the branch structure of naturally generated colony fragments. Within P. homomalla, significant differences with depth in the lengths and tributary to source ratios of first, second, and third order branches provide a measure of the greater bushiness of shallow water colonies. These measures can be used to quantify morphological differences in both ecological and systematic analyses. Data on the branching characters of these three gorgonian species demonstrate that gross colony form can be studied as a quantifiable component of phenotype and that gorgonians display both species level and ecophenotypic variation in colony form.     

The Paxillin-like protein AgPxl1 is required for apical branching and maximal hyphal growth in A.gossypii

The development from young, slowly growing hyphae to fast growing hyphae in filamentous fungi is referred to as hyphal maturation. We have identified the Paxillin-like protein AgPxl1 in Ashbyagossypii as a developmental protein that is specifically required for hyphal maturation. The early development of A.gossypii strains lacking AgPxl1 is indistinguishable from wild-type. However, at later developmental stages the maximal hyphal extension rate is less than half compared to wild-type and apical branching is affected. Apical branching is characterised as the symmetric division of fast growing hyphal tips resulting in two sister hyphae. In Agpxl1Delta strains two thirds of the apical branching events lead to asymmetric sister hyphae where growth of one branch is either completely aborted or slowed down while extension of the other branch is not affected. This suggests that AgPxl1 plays a role in the organisation of growth and efficient division of growth upon apical branching in mature mycelia. The conserved C-terminal LIM domains are necessary for AgPxl1 function and also contribute to tip localisation. AgCLA4, a PAK-like kinase, is epistatic to AgPXL1 and robust localisation of AgPxl1 depends on AgCla4. This suggests that AgCla4 acts upstream of AgPxl1.     

Model for Branch Initiation in Aspergillus nidulans Based on Measurements of Growth Parameters

The rate of hyphal elongation and the number of branches per hypha were measured on short sporelings of Aspergillus nidulans growing at different rates. The rate of elongation was proportional to total length in unbranched and branched hyphae. At each growth rate, the number of branches per hypha increased with increasing length and gave approximately straight-line graphs when plotted against length. The average number of branches per unit of hyphal length was quite different for the various growth rates and increased in direct proportion to the growth rate. The results are interpreted to mean that (i) growing tips have a maximum rate at which they can elongate and which is reached at hyphal lengths characteristic of the particular growth rate and (ii) a new branch is formed when the capacity of the hypha to elongate exceeds that of the existing tips.     

An empirical assessment of tree branching networks and implications for plant allometric scaling models

Several theories predict whole‐tree function on the basis of allometric scaling relationships assumed to emerge from traits of branching networks. To test this key assumption, and more generally, to explore patterns of external architecture within and across trees, we measure branch traits (radii/lengths) and calculate scaling exponents from five functionally divergent species. Consistent with leading theories, including metabolic scaling theory, branching is area preserving and statistically self‐similar within trees. However, differences among scaling exponents calculated at node‐ and whole‐tree levels challenge the assumption of an optimised, symmetrically branching tree. Furthermore, scaling exponents estimated for branch length change across branching orders, and exponents for scaling metabolic rate with plant size (or number of terminal tips) significantly differ from theoretical predictions. These findings, along with variability in the scaling of branch radii being less than for branch lengths, suggest extending current scaling theories to include asymmetrical branching and differential selective pressures in plant architectures.     

Tests of a cellular model for constant branch distribution in the filamentous fungus Neurospora crassa

The growth of mycelial fungi is characterized by the highly polarized extension of hyphal tips and the formation of subapical branches, which themselves extend as new tips. In Neurospora crassa, tip growth and branching are crucial elements for this saprophyte in the colonization and utilization of organic substrates. Much research has focused on the mechanism of tip extension, but a cellular model that fully explains the known phenomenology of branching by N. crassa has not been proposed. We described and tested a model in which the formation of a lateral branch in N. crassa was determined by the accumulation of tip-growth vesicles caused by the excess of the rate of supply over the rate of deposition at the apex. If both rates are proportional to metabolic rate, then the model explains the known lack of dependence of branch interval on growth rate. We tested the model by manipulating the tip extension rate, first by shifting temperature in both the wild type and hyperbranching (colonial) mutants and also by observing the behavior of both tipless colonies and colonyless tips. We found that temperature shifts in either direction result in temporary changes in branching. We found that colonyless tips also pass through a temporary transition phase of branching. The tipless colonies produced a cluster of new tips near the point of damage. We also found that branching in colonial mutants is dependent on growth rate. The results of these tests are consistent with a model of branching in which branch initiation is controlled by the dynamics of tip growth while being independent of the actual rate of this growth.     

Tests of a Cellular Model for Constant Branch Distribution in the Filamentous Fungus Neurospora crassa

The growth of mycelial fungi is characterized by the highly polarized extension of hyphal tips and the formation of subapical branches, which themselves extend as new tips. In Neurospora crassa, tip growth and branching are crucial elements for this saprophyte in the colonization and utilization of organic substrates. Much research has focused on the mechanism of tip extension, but a cellular model that fully explains the known phenomenology of branching by N. crassa has not been proposed. We described and tested a model in which the formation of a lateral branch in N. crassa was determined by the accumulation of tip-growth vesicles caused by the excess of the rate of supply over the rate of deposition at the apex. If both rates are proportional to metabolic rate, then the model explains the known lack of dependence of branch interval on growth rate. We tested the model by manipulating the tip extension rate, first by shifting temperature in both the wild type and hyperbranching (colonial) mutants and also by observing the behavior of both tipless colonies and colonyless tips. We found that temperature shifts in either direction result in temporary changes in branching. We found that colonyless tips also pass through a temporary transition phase of branching. The tipless colonies produced a cluster of new tips near the point of damage. We also found that branching in colonial mutants is dependent on growth rate. The results of these tests are consistent with a model of branching in which branch initiation is controlled by the dynamics of tip growth while being independent of the actual rate of this growth.     

Ontogenetic changes in the capacity of the coral Pocillopora damicornis to originate branches

Most colonial corals vary intraspecifically in growth forms, and the diversity in branching morphology is especially striking. While the effects of environmental factors on growth forms have been studied, the genetic control of coral branching patterns has received little attention. The discovery of ontogenetic changes in the capacity to originate branching would set the stage for studies of how branch formation is genetically controlled. During experiments investigating contact reactions in the coral Pocillopora damicornis, we observed that young colonies derived from settled planulae and colonies regenerated from adult branch tips assumed different growth forms. Young colonies formed at least one branch from the central region of the colony, while colonies regenerated from adult branch tips (3-5 mm long) did not form branches during the 9-month observation period. This pattern was invariable, regardless of the types and outcomes of the contact experiments or the orientation of the branch tips. However, some fragments taken from 1- or 2-year-old colonies formed branches. This suggests that the rate of branch formation in P. damicornis colonies decreases with age. These findings will facilitate investigations of the mechanism of coral branch formation at the molecular level.     

Predation by Patiria miniata (Asteroidea) on bryozoans: Prey diversity may depend on the mechanism of succession

Summary In environments where frequent disturbances interrupt the successional process there will usually be many patches of habitat at intermediate stages of succession. It is then relevant to consider the factors which control local diversity during succession. In offshore kelp forests across the whole Southern Californian Bight settlement panels were rapidly colonised by two species of cyclostome bryozoans (Tubulipora tuba and T. pacifica); but cheilostome bryozoans eventually became dominant during succession because they were able to grow over Tubulipora spp. When abundant, Tubulipora spp. were apparently able to reduce the number of colonies, and hence the number of species, of cheilostome bryozoans settling on the panels. Thus the rapid colonisers may delay the process of succession and reduce the diversity of bryozoans during succession. In field and laboratory experiments we found that the asteroid Patiria miniata preys on Tubulipora spp. but not on cheilostome bryozoans. The predator speeds up the successional process and increases bryozoan diversity by reducing the cover of Tubulipora spp. Other ways in which predators may influence diversity during succession are discussed. The effect of predation may depend on the abundance of the prey and the mechanism of succession.     

Functional morphology and taxonomy of branch dimorphism in the Paleozoic bryozoan genus Rhabdomeson

Silicified maria of the widespread cylindrical genus Rhabdomeson have been discovered that bear conical branches traditionally assignable to Coeloconus . Other, conical stems gave rise to cylindrical branches. That the dimorphic branches are not encrustations is shown by the absence of overgrowths and the regular alignment of apertures between parent stem and branch. A similar relationship is true of the type species of Coeloconus , which is synonymized with Rhabdomeson . Both conical and cylindrical branches were broken but survived to resume growth; this is shown by basal healing patterns and growth reversals. Colony fragmentation has been described in certain Cenozoic and modern bryozoans as a mode of increase, and is inferred to have been important in Rhabdomeson . The presence of fragmentation in taxonomically and stratigraphically widely separated bryozoans suggests that this mode of increase may be more important than realized.     

Reaction-diffusion models of growing plant tips: bifurcations on hemispheres

We study two chemical models for pattern formation in growing plant tips. For hemisphere radius and parameter values together optimal for spherical surface harmonic patterns of index l = 3, the Brusselator model gives an 84% probability of dichotomous branching pattern and 16% of annular pattern, while the hyperchirality model gives 88% probability of dichotomous branching and 12% of annular pattern. The models are two-morphogen reaction-diffusion systems on the surface of a hemispherical shell, with Dirichlet boundary conditions. Bifurcation analysis shows that both models give possible mechanisms for dichotomous branching of the growing tips. Symmetries of the models are used in the analysis.     

Evolution and dynamics of branching colonial form in marine modular cnidarians: gorgonian octocorals

Multi-branched arborescent networks are common patterns for many sessile marine modular organisms but no clear understanding of their development is yet available. This paper reviews new findings in the theoretical and comparative biology of branching modular organisms (e.g. Octocorallia Cnidaria) and new hypotheses on the evolution of form are discussed. A particular characteristic of branching Caribbean gorgonian octocorals is a morphologic integration at two levels of colonial organization based on whether the traits are at the module or colony level. This revealed an emergent level of integration and modularity produced by the branching process itself and not entirely by the module replication. In essence, not just a few changes at the module level could generate changes in colony architecture, suggesting uncoupled developmental patterning for the polyp and branch level traits. Therefore, the evolution of colony form in octocorals seems to be related to the changes affecting the process of branching. Branching in these organisms is sub-apical, coming from mother branches, and the highly self-organized form is the product of a dynamic process maintaining a constant ratio between mother and daughter branches. Colony growth preserves shape but is a logistic growth-like event due to branch interference and/or allometry. The qualitative branching patterns in octocorals (e.g. sea feathers, fans, sausages, and candelabra) occurred multiple times when compared with recent molecular phylogenies, suggesting independence of common ancestry to achieve these forms. A number of species with different colony forms, particularly alternate species (e.g. sea candelabrum), shared the same value for an important branching parameter (the ratio of mother to total branches). According to the way gorgonians branch and achieve form, it is hypothesized that the diversity of alternate species sharing the same narrow variance in that critical parameter for growth might be the product of canalization (or a developmental constraint), where uniform change in growth rates and maximum colony size might explain colony differences among species. If the parameter preserving shape in the colonies is fixed but colonies differ in their growth rates and maximum sizes, heterochrony could be responsible for the evolution among some gorgonian corals with alternate branching.     

Distribution of end-points of a branching network with decaying branch length

The geometry of the human bronchial tree has been described as a network formed by successive dichotomous branching with constant branching angles and geometrically decaying branch lengths. Models having these properties and with randomly distributed branching planes are constructed. The distribution of the end points of the model networks are described by computing the variance of the distributions in the direction of the axis of the network and in the transverse directions. It is found that, for a given decay ratio, there is a branching angle for which the volume filled by the end points is a maximum. The advantages of the network with the decay ratio and branching angle of the human bronchial tree are discussed.     

Cenomanian cheilostome bryozoans from Devon,England

The Cenomanian witnessed a spectacular evolutionary radiation of cheilostome bryozoans, both in terms of species diversity and morphological disparity. However, Cenomanian cheilostome faunas are inadequately known. Twelve species of cheilostome bryozoans are here described from the Cenomanian Beer Head Limestone Formation of SE Devon, England, a nearshore facies of limestones and sands. Two of the cheilostome species are new – Wilbertopora manubriformis sp. nov. and Foratella cervisia sp. nov. – and five cannot be identified beyond genus level. Syntypes of Foratella forata (d’Orbigny, 1853), the Senonian type species of Foratella Canu, 1900, are illustrated using SEM and a lectotype is chosen. All of the species present are neocheilostomes, which were larval brooders. Compared with the non-brooding malacostegans that dominate pre-Cenomanian faunas, most species have avicularia and extensive frontal walls, features probably adaptive against small predators.     

Distribution of Cell Adhesion Molecules (CAM) along the Branching Neurite Surface: Model-Inherited Effects of the Branch Diameters and Mode of CAM Transport

In many cases, an increase in the surface density of cell adhesion molecules (CAM) in the distal parts of a growing neurite is favorable for the neurite elongation. This increase is attained by exocytotic insertion of CAM-containing vesicles into the growth cones with subsequent redistribution of CAM along the cell surface due to lateral diffusion and endocytosis. Using a mathematical model describing these processes, we quantitatively describe conditions providing two qualitatively different profiles in a branching neurite: (i) the CAM surface density increases along both daughter branches, which would be in favor of further outgrowth of both branches, i.e., successful branching, or (ii) the CAM surface density increases along one daughter branch and decreases along another branch, which could lead to the retraction of the latter. The geometric factors and mechanisms underlying the intracellular CAM transport to the daughter growth cones were proved to determine the profile of CAM surface density. A similarity in the diameters of daughter branches, their short lengths, a high value of the lateral transfer constant, and partitioning of CAM transport at the branching point proportionally to the surface areas of daughter branches are in favor of an increase in the CAM surface density along both daughter branches. Asymmetric branching can lead to a decrease in the CAM surface density along the thinner or thicker daughter branch, if CAM trafficking was equally partitioned or was proportional to the branch cross-sectional areas, respectively. The proposed model helps to understand possible relationships between the intracellular CAM trafficking, CAM surface distribution, and geometry of branching of the neurites.     

Age-dependent differential responses ofSaprolegnia hyphal tips to a helical growth-inducing factor in the agar substitute,gellan

Saprolegnia ferax produces more-or-less straight, subapically branched, hyphae when growing in liquid or agar-solidified media, with abundant aerial mycelium on the latter. In Contrast, the same medium solidified with gellan gum induced helical growth with reduced branching and almost no aerial mycelium. Helical growth induction was gellan concentration-dependent, peaking at 0.4–0.6% (w/v), when about 60% of tips were helical. Gellan-induced helices showed concentration-dependent inhibition by agarose and polyethylene glycol. Colonies on gellan-agarose, where helices were inhibited, reverted to having aerial mycelium, whereas those on gellan-polyethylene glycol did not. Branches on helical hyphae were initially linear, but converted to helical growth after about 2 h of extension. This transition was often marked by a branch, thus branch and helix competency appeared to be related. Germinating cysts took twice as long as hyphal inocula before producing helical hyphae, reinforcing the suggestion that helix competence was age-related.Achlya, but notPhytophthora, also showed gellan-induced helical growth and aerial mycelium suppression. These results showed (a) that morphogenic regulators of hyphal growth responded to gelling agents, probably high-molecular-weight polysaccharides, (b) that all growing hyphal tips were not equivalent, and (c) that hyphal tips underwent age-related changes in their response to the environment. The gellan-related differences in aerial mycelium mimic hydrophobin-based mycelium behavior and may thus indicate environmental regulation of hydrophobin production.     

Branch development in Lupinus angustifolius L.#II. Relationship with endogenous ABA, IAA and cytokinins in axillary and main stem buds

The concentrations of indole-3-acetic acid (IAA), cytokinins (CK) and abscisic acid (ABA) were measured in buds of different regions (main stem and lateral branches) of Lupinus angustifolius L. (cv. Merrit) and at different stages in the development of branches. In lupin, branching patterns are the result of discrete regions of axillary branches (upper, middle and basal) which elongate at much different rates. Early in development only the main shoot elongates, followed usually by basal branch growth and then rapid upper branch growth. Branches in the middle of the main stem grow only weakly or fail to develop. Levels of IAA were generally high in the apical buds of slowly growing branches and low in buds from strongly growing branches, whereas CK levels showed the opposite relationship. CK:IAA ratio showed a closer relationship with the rate of growth of a particular branch better than the levels of either CK or IAA alone. During early stages of growth ABA concentration did not follow the rate of branch growth. However, later in development, where growth did not closely match the ratio of CK:IAA, ABA level showed a strong negative relationship with growth. A significant decrease in ABA was associated with continued strong growth of the main stem apex following a decline in CK:IAA ratio. Overall, the best relationship between the level of growth factors in apical buds and branching pattern in lupin was the ratio of CK:IAA, implying that high CK:IAA at a given bud would promote growth. ABA level appeared to play a secondary role, as a growth inhibitor.