Image stacks at a voxel size of 1. Morphological details such as choanocyte chambers were identified, as were the scleres of the highly absorbing silicious skeleton Fig. Developing choanosome and cortex structures are clearly distinguishable due to their characteristic cellular density. The resolution of light and electron microscopic imaging methods is higher than in x-ray microtomography, but only the latter provides us with the necessary 3D image data.
Morphology of a detached T. Since late buds which are already detached from the mother sponge display a typical pattern of morphological structures, we have chosen dataset E see Additional file 2 as an example. Mineral skeleton, aquiferous system and tissue compartment were analyzed.
They were distinguished on the basis of their differing x-ray absorption characteristics Fig. All morphological elements of the sponge could be identified from the volumetric plot. Detailed visualization and analysis of the overall morphology of a stage 4 T. A Stereo pair rendering with segmentation of morphological structures: sponge tissue yellow separated into cortex co and choanoderm cd with developed choanocyte chambers cc , exopinacoderm exp , skeleton red and aquiferous system blue with lacunar system cavities lsc.
B Related volumetric measurements. Main body structures and body extensions ext are marked in grayscale. C - N examples of 1. The variation of these proportions is not random; it represents the morphological arrangement, which can be checked in the sample slices Fig. Body extensions of the sponge are represented in the peripheral regions of the plot outside the cortex region. They are characterized by a reduced aquiferous system and a considerable proportion of skeletal material. As the slim filamentous body extensions are made up of megasclere bundles surrounded by a thin tissue layer, relative values for the skeleton increase significantly in comparison to the body centre.
The same distribution pattern holds for the two additional main axes of the dataset zy-slice and zx-slice stacks. For the cortex and choanoderm the roughly the same pattern is seen as in the former two dataset directions. By finding the average of the proportions along the three main axes xyz-averages, Fig. The resulting plots for all three structures are remarkably symmetric to the centre and standard deviations are relatively low.
This is typical of a globular shaped, almost point-symmetrical sponge body resembling the body architecture of an adult T. Only in the peripheral parts representing the body extensions the structure proportions are more variable and SD increases. Their temporal changes document the development of the sponge cortex and the choanosome into higher level functional morphological units. Depending on the developmental stage of the bud, choanocyte chambers and a network of fine canals are detectable within the choanosome.
Many of these canals have diameters close to the resolution limit. The finest canal structures connecting the incurrent system with the choanocyte chambers are thus not resolved in either of the stages.
The graphs for each bud reveal characteristic spatiotemporal changes in the proportional volumes and quantitative distribution of canals, tissue and skeleton within the buds Fig. The overall graph shape for each structure changes specifically during bud formation and maturation and displays characteristic features at each stage. Newly formed buds display a homogenous distribution of tissue and aquiferous system elements Fig.
Step by step, the morphology changes into the adult phenotype, which is characterized by a high proportion of tissue in the sponge centre the choanosome and a high proportion of peripheral aquiferous system lacunae the cortex , and by mineral scleres in the filamentous body extensions protruding from the sponge surface.
As shown above, in juveniles which are fully functionally developed late bud stages , all structures are represented by distinct peaks or plateaus. However, the cortex and choanosome also leave an imprint on the graphs in their early developmental stages Fig.
Volume analysis of body structures in T. Graphs represent relative volumetric proportions of all main morphological sponge structures: tissue top row , aquiferous system middle row and skeleton bottom row. Graph patterns typical for distinctly developed sponge regions are marked sc - skeleton centre, ext - body extension filaments , st - stalk. Volumetric results are given for the three main axes of the 3D data sets: x-axis dashed , y-axis dotted and z-axis solid.
Buds fall into four categories which we defined by combining volumetrics, graph characteristics and qualitative characters i. The resulting spatiotemporal morphological pattern sequence is typical of bud formation and maturation. Stage 1 buds exhibit a homogenous distribution of tissue and aquiferous system components Figs.
Cortex and choanosome are not differentiated, neither are lacunar cavities and choanocyte chambers Figs 3A , 4A , 5A , and see Additional file 3A. The proportional volume of the skeleton is almost constant at around 1. The megasclere bundle of the stalk defines the main axis of the bud skeleton and consequently the whole bud at this stage. The overall appearance of the bud at this stage is flat and elongated along its axis Fig.
Apart from the stalk, a few megasclere bundles start to form around the future centre of the skeleton. All the bundles are arranged in a plane perpendicular to the stalk see Additional files 3A and 4. SEM images of median section planes of the four bud stages of T. In stage 1 buds A cells are densely packed and evenly distributed. Stage 2 buds B display early developing cortex dco and choanoderm dcd regions as well as few numbers of primordial choanocytes pc and megasclere bundles msb.
Developing lacunar system cavities dlsc are found in stage 3 C as separated developing choanoderm and cortex regions. In stage 4 D clearly developed lacunar system cavities lsc , choanocyte chambers cc and distinguished cortex co and choanoderm cd are present. Stage 2 buds are still connected to the mother sponge through the stalk see Additional file 3B but are more globular in shape and display more advanced skeletal development Fig.
The megasclere bundles radiate into additional directions, but the bundles and the skeletal centre are not as highly organized as in the adult skeleton. The skeleton is more planar star shaped than globular.
The cortex and choanosome regions are not yet differentiated Figs. This process is accompanied by the first occurrence of a small number of choanocyte chambers. The bud still appears flattened as a consequence of the planar arrangement of the skeleton Fig. Stage 3 buds start to develop distinct aquiferous system lacunae in the cortex region and a denser choanosome around the centre Figs.
Accordingly, volumetric graphs show their first peaks for both structures Fig. Visual analysis of 2D image slices and volume renderings indicate the presence of choanocyte chambers in the developing choanosome Fig. The skeleton centre is prominent, focusing on a single point see Additional file 3C. This is also represented in the volumetric graphs by small central peaks Fig.
The skeleton is almost star shaped in 3D, but still retains a certain flatness which also characterizes the outer shape Fig. Stage 3 buds are still connected to the mother sponge through the initial stalk.
Stage 4 buds display an adult-like body structure Figs. The dense choanosome core stands out against the peripheral cortex, which has prominent lacunar system cavities.
Within the choanosome, differentiated choanocyte chambers are present in high numbers Figs. Volumetrics graphs show an almost symmetrical pattern for the two body compartments Fig. This almost globular graph pattern and outer shape Figs. The number of megascleres has increased, and the bundles are arranged homogenously, forming a spherical star see Additional file 3D-E and Additional file 4. We found two different cases which we regard as subtypes of the same stage represented by D and E in Figs.
In both cases the buds were detached from the mother sponge. The main difference is the total volume, which is affected by the proportion of tissue and skeleton see Additional file 6. It is not possible in either case to identify the site of the former connecting stalk, as its megascleres have been integrated into the main skeleton see Additional files 3D-E and Additional file 5. Stage 4b, the more advanced of the two, is characterized by a higher cell mass, a higher number of megascleres per bundle and a number of finer megascleres which are apparently synthesized within the bud Fig.
This particular stage represents a juvenile sponge and seems to be fully functional. In contrast, the earlier stage 4a displays lower biomass and less prominent megasclere bundles Fig.
On the other hand, the aquiferous system is equal to the one in stage 4b, and thus in the adult sponge Figs 3D-E , 4D-E. Interestingly, the diameter of stage 4a buds is smaller and the biomass is lower than in the examples for stages 1 to 3. The developmental stage of a bud, then, cannot be inferred directly from its diameter.
A number of morphological methods are available for studying temporal morphological developmental series. In the case of sponges, morphological structures such as skeletal elements spicules can be viewed in their undisturbed context [ 23 ]. Classical histological semi-thin sections and light microscopic analysis give higher resolutions, but the sectioning itself destroys spicules if they have not had to be dissolved beforehand.
It allows morphological patterns to be identified and quantified using 3D-morphometrics [ 24 ]. This turned out to be an important prerequisite in defining discrete bud stages of T.
Poriferan budding is a key element in understanding the evolution of asexual reproduction in metazoans. The morphogenesis that takes place during bud development in T. Thus, sponge asexual developmental processes are spatiotemporally patterned, comparable to budding in cnidarians [e.
What conclusions can we draw from our results in terms of a general interpretation of sponge budding? And in a wider context, what can we learn about the early evolution of asexual reproduction and its regulation in Metazoa? Assuming development is continuous, the distinct stages in bud development in T. The morphological changes are schematically summarized in Figure 6. Budding starts with the migration of cells and the transportation of the first megascleres into the early bud [not investigated here, for details see [ 31 ]].
Stage 1 buds are dominated by the stalk connecting the bud and the mother sponge, which also represents a first symmetry axis Fig. Cells migrating into the emerging bud from the mother sponge arrange axisymmetrically around the tip of the stalk, forming a characteristic small bulb. The future skeletal centre is formed within the centre of this cellular bulb Fig.
The overall spatiotemporal pattern of bud morphological development is characterized by several temporally overlapping processes: 1. Rearrangement of megascleres from the primary axis via a planar star to a spherical star shape.
Formation of the aquiferous system, constituted by choanocyte chamber differentiation compare Fig. Differentiation of a choanosomal centre and a cortical region, which seems not to start before the skeleton merges from the planar star to the spherical star shape Fig.
This process coincides in most cases with the release from the mother sponge, and we regard it as the onset of the full functionality of the aquiferous system. A comparison between bud sizes and the proportion of skeletal elements to tissue reveals no correlation see Additional file 2 , so bud size does not seem to be significant in the spatiotemporal pattern of bud development. Scheme of bud development in T.
A Four bud stages are characteristic, with the first three connected to the mother sponge by a stalk: Skeletal elements in red megasclere bundles and aster spheres ; megasclere bundles partly simplified as cylinders; Tissue in grey, separated into cortex light grey and choanoderm dark grey.
B Details of bud stages left and schematic graphs of morphological functional unit distribution. There are indications of rotational symmetry along the initial connecting stalk st in stages one to three compare Additional file 4.
Stage 4 buds display an adult-like body morphology with point symmetry to the skeleton centre sc; see Additional file 5. Choanoderm development starts in stage 2, accompanied by the development of the megaster spheres in stage 3.
Differentiation into a cortex co and choanoderm cd is characterized by the development of the aquiferous system larger canals in stage 2; lacunae in stage 3.
Body extensions ext filaments are found in stage 4 buds. For further details see text. Our results demonstrate that choanosome and cortex develop in correlation with the differentiation of the star shaped skeleton. This is documented from stage 3 onwards and is crucial in overall bud morphogenesis. The importance of skeletal development in bud formation has been hypothesized previously [ 18 ], but the present 3D morphometric analysis makes it possible to quantify the link between skeleton arrangement and cortex differentiation see Fig.
In the detached juvenile sponges stage 4 buds lacunae and a wide canal system are already highly developed, as in adult specimens. A high number of choanocyte chambers are also found.
These juvenile sponges can be regarded as morphological and functional equivalents of adult T. By this stage, the series of events that play a role in bud development has been completed. Apart from general body growth, which takes place by means of spicule synthesis, cell division, differentiation and extracellular matrix production, no further changes in morphological patterning occur.
From a structural point of view a steady state has been reached which indicates that the juvenile sponge has developed adult-like body architecture. From now on it will only increase in size. This may involve a substantial volume increase of up to 7. Larger adults with a diameter of approximately 2 cm typically have a volume of around 4. We assume that in stage 4 the sponge reaches the state of 'constant morphogenesis' [ 12 ]. In this species the development of the skeleton is seen as a key element in the budding process.
Just as in T. Soon spicule production starts in the emerging bud too. For early buds a fan-shaped configuration of the skeleton has been described which later changes to a radial star shaped architecture like in our case.
Maas for the first time differentiated stages in bud development. We regard his 'fan-shaped bud stage' of T. In contrast, we found the similar organizational level in stage 3 buds of T.
For buds just about to be released Maas observed the onset of choanoderm and cortex separation. This corresponds to our definition of a stage 2 bud. In direct comparison, distinct morphological structures develop earlier in T. Budding also occurs in certain invertebrates, e. Hydra sponge , corals , echinoderm larvae, and some acoel flatworms. The bud breaks off to become a new individual Hydra. Budding in plants is a form of vegetative reproduction. It occurs naturally.
However, it can also be induced artificially, by horticulture. In this regard, the propagative technique is referred to as grafting wherein the bud of one plant is inserted onto another plant so as both plants can continue growing together. In most cases, a bud of a plant is inserted at the bark of the stem of another plant. Roses are an example of a plant that is commonly bud grafted.
Human intelligence provided the means to utilize abstract ideas and implement reasoning. This tutorial takes a further l.. Darwin's Finches are an example of natural selection in action. They are an excellent example of the way species' gene p.. This tutorial elaborates on how the nervous system works, particularly at the tissue level of the brain. There are three.. Freshwater ecosystem is comprised of four major constituents, namely elements and compounds, plants, consumers, and deco..
This tutorial describes the role of gibberellin family in plants. Find out the effects of gibberellin on plant growth an.. A single individual can produce offspring asexually and large numbers of offspring can be produced quickly. In a stable or predictable environment, asexual reproduction is an effective means of reproduction because all the offspring will be adapted to that environment.
In an unstable or unpredictable environment asexually-reproducing species may be at a disadvantage because all the offspring are genetically identical and may not have the genetic variation to survive in new or different conditions. On the other hand, the rapid rates of asexual reproduction may allow for a speedy response to environmental changes if individuals have mutations. An additional advantage of asexual reproduction is that colonization of new habitats may be easier when an individual does not need to find a mate to reproduce.
There are a number of ways that animals reproduce asexually. Fission , also called binary fission, occurs in prokaryotic microorganisms and in some invertebrate, multi-celled organisms.
After a period of growth, an organism splits into two separate organisms. Some unicellular eukaryotic organisms undergo binary fission by mitosis. In other organisms, part of the individual separates and forms a second individual. This process occurs, for example, in many asteroid echinoderms through splitting of the central disk. Some sea anemones and some coral polyps Figure 1a also reproduce through fission. Budding is a form of asexual reproduction that results from the outgrowth of a part of a cell or body region leading to a separation from the original organism into two individuals.
Budding occurs commonly in some invertebrate animals such as corals and hydras. In hydras, a bud forms that develops into an adult and breaks away from the main body, as illustrated in Figure 1b, whereas in coral budding, the bud does not detach and multiplies as part of a new colony.
Figure 1.
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