Why does volvox exist as a sphere

Volvox do not eat in a traditional sense. They do not have a mouth to eat from and do not have an anus to excrete from. Instead, the Volvox eats mainly through photosynthesis. This means that Volvox are capable of converting sunlight into energy as primary producers. Because of their photosynthetic tendencies, the Volvox is rarely observed living in heavily shaded areas. Their mobility allows them to seek out sunlight. Likewise, they are not often found in deep waters where the sunlight cannot reach them.

There are two types of photosynthesis that may occur: oxygenic photosynthesis and anoxygenic photosynthesis. Volvox convert sunlight into usable energy mainly through oxygenic photosynthesis. During this process, sunlight transfers electrons within water and carbon dioxide to produce sugars or carbohydrates.

Carbon dioxide is released in the process while oxygen is created. Humans breathe oxygen, while plants breathe carbon dioxide. Each part of the Volvox structure is explained below:. All life on earth is classified into unique groups depending upon distinguishing characteristics.

Each group can further be subdivided into smaller groups. The classification system for organisms can be broken down into seven different levels: kingdom, phylum, class, order, family, genus, and species in that order. Kingdoms are the most basic classification of living things. There are five kingdoms in total. A phylum is the first attempt at narrowing down the list of organisms based on a physical similarity, which suggests that there is a common ancestry among similar organisms.

Classes, orders, families, and genus are all narrowed down even further based on similar traits until we finally get to a single species. Since Volvox is a genus, they can further be broken down into species.

Each classification is further explained below as it related to the Volvox:. Volvox are commonly found within deep ponds, lagoons, puddles, ditches, swales, and more. They tend to thrive in areas that receive a large amount of rainwater.

They choose to live within nutrient-rich water and grow rapidly in the warmth. Volvox are commonly observed in pond scum. Volvox are widely regarded in the scientific community as a model species thanks to their unique reproductive tendencies.

They have the ability to reproduce both sexually and asexually. Asexual reproduction is the most common means of reproduction amongst the Volvox. Favorable or unfavorable environmental conditions will result in either asexual reproduction or sexual reproduction. In the wild, it is unknown what the ratio is between asexually reproduced Volvox and sexually reproduced Volvox.

But there is still much to learn. What new gene functions evolved to permit the evolution of asymmetric division and inversion? How did the other novel developmental traits of Volvox evolve? And are there similarities between the way multicellularity evolved in the volvocine algae and the way it evolved in other kinds of organisms?

With the rate of recent progress in this field, answers to these questions, and more, should be on their way soon. Cheng, Q. The role of GlsA in the evolution of asymmetric cell division in the green alga Volvox carteri.

Development Genes and Evolution , — Duncan, L. Journal of Molecular Evolution 65 , 1—11 Herron, M. Triassic origin and early radiation of multicellular volvocine algae. PNAS , — Evolution of complexity in the volvocine algae: Transitions in individuality through Darwin's eye. Evolution 62 , — King, N. The unicellular ancestry of animal development.

Developmental Cell 7 , — Kirk, D. Germ-soma differentiation in Volvox. Developmental Biology , — A twelve-step program for evolving multicellularity and a division of labor. BioEssays 27 , — Cambridge: Cambridge University Press, Volvox carteri as a model for studying the genetic and cytological control of morphogenesis. Miller, S. Development , — Nedelcu, A. Environmentally induced responses co-opted for reproductive altruism. Biology Letters 5 , — Nishii, I. A kinesin, invA , plays an essential role in Volvox morphogenesis.

Cell , — Peterson, K. Origin of the Eumetazoa: Testing ecological predictions of molecular clocks against the Proterozoic fossil record. Prochnik, S. Genomic analysis of organismal complexity in the multicellular green alga Volvox carteri.

Science , — Sanderson, M. Molecular data from 27 proteins do not support a Precambrian origin of land plants. American Journal of Botany 90 , — Ueki, N. Controlled enlargement of the glycoprotein vesicle surrounding a Volvox embryo requires the InvB nucleotide-sugar transporter and is required for normal morphogenesis. Plant Cell 21 , — Idaten is a new cold-inducible transposon of Volvox carteri that can be used for tagging developmentally important genes. Genetics , — What Is a Cell?

Eukaryotic Cells. Cell Energy and Cell Functions. Photosynthetic Cells. Cell Metabolism. The Origin of Mitochondria. Mitochondrial Fusion and Division. The Origin of Plastids. The Origins of Viruses.

Discovery of the Giant Mimivirus. Volvox, Chlamydomonas, and the Evolution of Multicellularity. Yeast Fermentation and the Making of Beer and Wine. Dynamic Adaptation of Nutrient Utilization in Humans.

Nutrient Utilization in Humans: Metabolism Pathways. An Evolutionary Perspective on Amino Acids. Mitochondria and the Immune Response. Stem Cells in Plants and Animals.

Promising Biofuel Resources: Lignocellulose and Algae. The Discovery of Lysosomes and Autophagy. The Mystery of Vitamin C. Miller, Ph. Citation: Miller, S. Nature Education 3 9 How does multicellularity evolve? Scientists who study a family of green algae that includes unicellular Chlamydomonas and multicellular Volvox are beginning to find answers to this question.

Aa Aa Aa. What Is Multicellularity? Figure 2: Volvox carteri and Chlamydomonas reinhardtii. A Young Volvox adult, with about 2, small somatic cells in a monolayer at the surface, and nineteen large gonidia embedded in the extracellular matrix ECM , just under the somatic cell layer.

Multicellularity in the Volvocine Algae. Strategies for Investigating the Evolution of Multicellularity. Figure 3: Gene and pathway co-option and the origins of asymmetric cell division and cellular differentiation in Volvox. A The function of glsA appears to have been co-opted without change from an unknown function in the unicellular ancestor of Volvox, so that it is now part of a pathway shaded green that is required for asymmetric cell division.

References and Recommended Reading Buss, L. The Evolution of Individuality. Article History Close. Share Cancel. Revoke Cancel. Keywords Keywords for this Article. Save Cancel. Flag Inappropriate The Content is: Objectionable. Flag Content Cancel. Email your Friend. Submit Cancel. This content is currently under construction. Explore This Subject. Topic rooms within Cell Origins and Metabolism Close.

No topic rooms are there. Or Browse Visually. Student Voices. Creature Cast. Simply Science. Green Screen. Green Science. Bio 2. J After the short period of denting, the embryos remain spherical. During cleavage, all cells are roughly spherical or, particularly immediately before each cleavage division, somewhat elongated Figure 3A.

The cells exhibit a length of 4. Between the end of cleavage and the beginning of inversion, all cells within the spherical cellular monolayer develop into what we refer to as teardrop-shaped cells Figures 3B , 4A , 5A1, A3 , and 6B, C, D, E, F. Each of these approximately radially symmetrical cells is cone-shaped at the cell end that faces the interior Figure 6B, D, F ; we refer to the cone-shaped apical end of the cell as the flagellar end because the flagella will develop on this side of the cell.

The opposite end of the teardrop-shaped cells, facing the exterior, shows a roughly hexagonal cross-section Figure 6E ; we call this end of the cell the chloroplast end because the single chloroplast largely localizes on this side of the cell. For all optical sections of embryos, we used phalloidin-tetramethylrhodamine B isothiocyanate TRITC as a cytoplasmic stain because it stains filamentous actin F-actin in embryonic cells [ 72 ], and F-actin was more or less uniformly distributed in the cytoplasm both in this and in a previous study [ 72 ], the resolution of the cLSM was insufficient to see individual actin filaments.

However, there was one exception: in teardrop-shaped cells, F-actin was mainly localized at the flagellar apical ends of the cells Figure 6B, C. At the beginning of inversion, the cells of the anterior hemisphere remain teardrop shaped Figure 4B, C , but the cells of the posterior hemisphere become elongated along their chloroplast-end-to-flagellar-end axis.

The conversion of teardrop-shaped cells into spindle-shaped cells in the posterior hemisphere occurs in conjunction with the formation of the bend region somewhat below the equator of the embryo Figures 1B1, B2 , 2A, B , 3C , 4B, C and 7H. In the emerging bend region, the shapes of the just developed spindle cells changes again and increasingly reflects the maximum curvature of the cell sheet in this area.

The elongated cells of this region stay pointed and radially symmetrical only at their flagellar ends, whereas they become increasingly wedge-shaped at their chloroplast ends; we refer to these cells as paddle-shaped cells.

At the outermost chloroplast ends, the major axis of the cell's cross-section is 1. The bases of the cells at the chloroplast ends frequently appear as elongated hexagons. The minor axis of the outermost chloroplast end of paddle-shaped cells is perpendicular to the circular line around the embryo and is oriented along a line that more or less forms a half circle in the bend region when a midsagittal cross-section of an embryo is observed Figures 4C and 7H.

While invagination of the posterior hemisphere proceeds and the transition from spindle to paddle shapes progresses from the equator toward the posterior pole, the teardrop-shaped cells of the anterior hemisphere become increasingly elongated flat cells that partially overlap each other.

The conversion from teardrop-shaped to elongated progresses from the rim of the anterior cap of the embryo toward its anterior pole. We refer to the elongated flat cells as disc shaped Figures 4B, C and 7A upper right part of the image and 7G. However, these cells are not uniformly shaped, and quite a few are buckled. The bases of the paddle-shaped cells at the chloroplast ends still have the appearance of elongated hexagons Figure 8A.

In the mid-inversion stage, the transition from the remaining spindle to paddle cells seems to progress from the equator of the embryo toward its posterior pole and, as mentioned above, the radius of the curvature in the bend region increases considerably. The greater that the radius of the curvature in the bend region is, the thicker the chloroplast ends are on their narrow side Figures 4D, E and 8C, D.

With a further increase of this radius, the paddle-shaped cells change into cells that are radially symmetrical throughout their length Figures 4E, F and 8D ; these cells still have pointed flagellar ends but the bases of the cells at the flattened chloroplast ends appear as elongated hexagons Figures 4E, F and 8A, D, F , and we refer to these cells as pencil shaped.

Pencil-shaped cells have a length of During most of the mid-inversion stage, cells close to the not yet inverted posterior pole are still spindle-shaped Figure 4D, E.

When these cells have their turn in inversion, the radius of the curvature in the bend region reaches its maximum. These spindle-shaped cells do not seem to become paddle shaped but directly change into the pencil shape Figure 4D, E, F. Before inversion of the posterior hemisphere is complete, disc-shaped cells at the rim of the anterior cap become increasingly pencil shaped also without passing through the paddle stage and, at the same time, seem to move over the rim of the anterior cap toward the posterior pole Figures 3E, F, G , 4C, D, E and 5C1, C3.

This process appears to be the initiation of inversion of the anterior hemisphere. Shortly after the beginning of the change from disc to pencil shape at the rim of the anterior cap, the phialopore appears at the anterior pole and the opening widens continuously. The transition from disc to pencil shape seems to progress toward the phialopore. Disc-shaped cells around the phialopore alone become increasingly elongated along a circular line described by the edge of the opening Figures 3F and 8B.

During this last stage of inversion, the remaining disc-shaped cells of the not yet inverted portion of the anterior hemisphere become pencil-shaped Figures 3G , 4G , 5D1, D3 and 9A, C, D, F and simultaneously seem to move over the rim of the remaining portion of the anterior cap.

In the late inversion stage, pencil-shaped cells have a length of Once all cells changed into pencil shape, the phialopore closes and inversion is completed. Right after inversion is complete, the cells of the embryo change their shape again, but this time all cells do it concertedly. The pencil-shaped cells become shorter and end up at a length of 4.

We refer to these cells as column shaped. The column-shaped cells present a hexagonal cross-section along their entire length, except for at the slightly rounded flagellar ends, while the chloroplast ends are flattened Figure 10C, D.

Approximately 1 h after the completion of inversion, the cell shapes change for the last time: all the column-shaped cells gradually develop into what we refer to as gemstone-shaped cells Figures 5F1, F3 and 11A. At the flagellar end, these cells exhibit shapes similar to flat, radially symmetrical cones but have more or less hexagonal bases at their chloroplast ends. The cells with the final gemstone shape have a diameter of 4.

Details of juveniles approximately 1 h after the completion of inversion late post-inversion stage. C SEM view of the cell monolayer from outside the spheroid; the conspicuous zones of the ECM are the BZ and FZ3, as described in [ 77 ] and [ 78 ]; and at regions where flagella partially broke off during preparation, double-headed arrows highlight the regular distance and orientation of flagellar pairs.

At the pre-inversion stage, the CB system connects all teardrop-shaped cells at their chloroplast ends Figures 6E ; there are no CBs at the flagellar ends Figure 6D, F. In early inversion, the teardrop-shaped cells of the anterior hemisphere are still connected at their chloroplast ends, but the cells of the posterior hemisphere with the just developed spindle shape have their CBs in the equatorial region of the cells Figure 7C and inset in 7C.

At the same time that the bend region forms between the two hemispheres, the spindle-shaped cells in the equatorial region of the embryo seem to move concertedly relative to the CB system until the CBs localize at their chloroplast ends; this transition seems to progress from the equator of the embryo toward its posterior pole.

Cells in the bend region with the just developed paddle shape exhibit CBs only at their outermost chloroplast ends Figure 7A, B, D, E, H , which allows for the smallest possible radius of the curvature of the cell monolayer. In disc-shaped cells, which result from the conversion of teardrop-shaped cells in the anterior hemisphere, the cytoplasmic connections to other cells are found at the edges of the discs Figure 7A upper right part of the image and 7G.

Despite beginning the inversion process, the V. In the V. Inversion progresses during the mid-inversion stage, and the CBs localize in spindle-, paddle- and disc-shaped cells just as described for the early inversion stage.

The CBs of the just developed pencil-shaped cells of the already inverted posterior hemisphere localize at their outermost chloroplast ends Figure 8F , just as in paddle-shaped cells. When the phialopore opens in the mid-inversion stage, the CBs between cells at the anterior pole of the embryo and somatic cells of its parent break off mechanically.

While the phialopore widens continuously, the CBs around the phialopore become elongated along a circular line described by the edge of the opening Figure 8B. During the last part of the inversion process, the CBs localize in disc- and pencil-shaped cells Figure 9H, I just as described above. After inversion, CBs connect the just developed column-shaped cells which result from the conversion of pencil-shaped cells at the chloroplast ends Figure 10F.

All cells remain interconnected by CBs after inversion and throughout their lifetime in contrast to cells of V. During inversion of a V. The outgrowth of the flagella starts around the beginning of embryonic inversion. During inversion, the flagella of a V. However, these growing flagella have already begun to beat; therefore, the juveniles rotate slowly within the parent spheroid. The synthesis of extracellular matrix ECM begins at approximately the end of inversion.

In particular, the characteristic ECM subzone cellular zone 3 CZ3 becomes apparent; CZ3 consists of coherent fibrous material that creates honeycomb-like chambers at a significant distance around individual cells Figure 10G.

After inversion, the juveniles continuously grow in size by depositing increasing amounts of ECM. The rapid growth of juveniles after inversion is only affected by the secretion of ECM material, which causes each cell to move away from its neighbors Figure Based on these results, we developed 3D models with consensus cell shapes for the teardrop-, spindle-, disc-, paddle-, pencil-, column- and gemstone-shaped cells Figure 12A, B, C, D, E, F, G.

The localization of the different cell shapes within the embryo is indicated in Figure A1 to A4 Teardrop-shaped cells: radially symmetrical cells that are cone shaped at the flagellar ends but have hexagonal cross-sections at the chloroplast ends Figure 13A, B, C, D. D1 to D4 Paddle-shaped cells: elongated cells that are pointed and radially symmetrical only at the flagellar ends but wedge shaped at the chloroplast ends Figure 13C, D ; D1a broad and D1b narrow side of the cell.

E1 to E4 Pencil-shaped cells: radially symmetrical, elongated cells that are pointed at the flagellar ends and flattened at the chloroplast ends Figure 13E, F, G, H. F1 to F4 Column-shaped cells: radially symmetrical, elongated cells that are flattened at the chloroplast ends and rounded at the flagellar ends Figure 13I. G1 to G4 Gemstone-shaped cells: radially symmetrical cells that are shaped like flat cones at the flagellar ends but have hexagonal cross-sections at the chloroplast ends.

A1 to G1 Wire frame models; cell sizes and the flagellar fla and chloroplast chl ends of the cells are indicated. All other parts of the figure depict the in vivo arrangement of the cells with a green surface texture from different viewing directions. The positions of CBs are indicated by red connections between cells. A2 to G2 Frontal side view. A3 to G3 View of the chloroplast ends.

A4 to G4 Slanted side view. Arrows indicate the viewing directions in other parts of the figure. CB: cytoplasmic bridge. Based on all of our findings, we developed a summary model for the complete process of embryonic type B inversion in V. The model includes the localization of the observed cell shapes; the directions of cell layer movements; the relative position of the CB system; the length of flagella; and the localization and shapes of nuclei during the pre-, early, mid-, late and post-inversion stages.

The summary model is discussed below. Model of the inversion process in V. Cell content is shown in green; red lines indicate the position of the CB system; and nuclei are shown in blue. Frames show examples of the location of the cell shapes represented in 3D in Figure The directions of the cell layer movements are indicated by black arrows. A Pre-inversion stage. B , C Early inversion stage; inversion begins with the contraction of the posterior hemisphere and the formation of the bend region.

D to F Mid-inversion stage; the posterior hemisphere continuously moves into the anterior hemisphere and inverts at the same time; the phialopore opens at the anterior pole and widens; and the anterior hemisphere begins to move over the already inverted posterior hemisphere and inverts at the same time.

G , H Late inversion stage; the anterior hemisphere continues to move over the already inverted cell monolayer; and inversion ends with the closure of the phialopore. I Early post-inversion stage. When the type B inversion of V. The sequence of the major movement patterns of the cellular monolayer is quite different in both species.

The presence of CBs between the cells of the embryo's cell monolayer and the coordinated movements of cells relative to the CB system appear to be essential for inversion in both V. Likewise, cell shapes and the order of coordinated cell shape changes are important for a successful inversion of embryos in both species. However, the order of the coordinated shape changes is different between the species, and the shapes also show differences. The presence of flask-shaped cells in the bend region during inversion is characteristic of V.

The long 'bottlenecks' of these cells, which localize at the chloroplast ends of the cells, are radially symmetrical. The paddle-shaped cells in the bend region of inverting V. However, paddle-shaped cells are not radially symmetrical at their chloroplast ends but instead are wedge shaped Figure 12D and Table 1 , and they present no bottlenecks or stalks. Cells that have passed the bend region in V. In the posterior hemisphere, embryonic cells change from teardrop to spindle to paddle except for the cells close to the posterior pole to pencil and then to column shaped.

In contrast, cells from the anterior hemisphere change from teardrop to disc to pencil and then to column shaped Figure Changes in volume might be achieved by emptying or filling contractile vacuoles; alternatively, cells might change their volume by exporting or importing cytoplasm to or from their neighbor cells through the CB system.

In contrast, the cell volume is constant throughout inversion in V. In contrast, a V. Changes in cell shape and coordinated movements of cells relative to the CBs start somewhat below the equator and proceed to the posterior pole; then a second process with changes in cell shape and coordinated movements of cells relative to the CBs begins at the former equator and proceeds to the anterior pole.

In contrast to the V. The phialopores in V. In contrast, the opening widens, while the anterior hemisphere moves over the posterior hemisphere Figures 3F and 8B and Table 1.

Denting may be caused by non-simultaneous cell shape changes of individual cells or a patch of a few cells.

In summary, the comparison of type A and type B inverters suggests that both species follow the same basic principle: concerted, spatially and temporally coordinated changes in cellular shapes and concerted migration of cells relative to the CBs connecting them.

However, differences exist in almost all details of the inversion process. When the A and B types of inversion are mapped on the evolutionary tree of volvocine algae see Additional File 1 [ 25 , 29 , 58 , 59 , 66 , 73 , 79 — 83 ], type B inversion seems to have been the earlier form of Volvox inversion because it appears on a deeper branch than type A inversion.

However, if this hypothesis is true, type A inversion must have evolved independently in four different lineages from a type B ancestor [ 25 , 66 ]. If type A evolved before type B, it would mean that type B inversion evolved independently in three different lineages from a type A ancestor: in the section Volvox lineage, including V. Even if the repeated, independent evolution of type B from type A or the other way round is possible, the number of differences observed between type A and type B inverters makes it more reasonable that type A and type B inversion evolved in parallel.

However, the true course of evolutionary events remains unclear. The different types of inversion and other differences observed during inversion described above substantiate a polyphyletic origin of the genus Volvox ; this conclusion was also reached in earlier studies based on phylogenetic analyses of internal transcribed spacer sequences [ 63 ], sequences of five chloroplast genes [ 58 , 73 ] and comparison of flagellar beating patterns [ 83 , 84 ].

Thus, reclassification of this genus is required. The developmental and morphological differences of the species within the section Volvox compared to other Volvox species outside this section support the creation of a new genus for the species of the section Volvox. Epithelial folding during inversion is basically a biomechanical process whereby individual cells and cohorts of cells produce and respond to forces to generate the complex form of a developing multicellular organism [ 85 ].

Based on our results, which are summarized in Figures 12 and 13 , and due to biomechanical possibilities and implications about the involved tensile and compressive forces, we developed the following global mechanistic scenario that predicts how a spherical cellular monolayer can turn itself inside out. Before inversion in the spheroidal V. This shape suggests that anisotropic cytoskeletal events along their chloroplast-end-to-flagellar-end axes formed these cells Figure 12A1.

The trigger for the beginning of the inversion process seems to be a so-far unknown signaling molecule or morphogen that does not appear to be uniformly distributed throughout the pre-inversion embryo Figure 13A but appears to be localized only, or at least mainly, in the posterior hemisphere of the embryo.

Therefore, this putative trigger seems to operate only on the teardrop-shaped cells of the posterior hemisphere, which become increasingly spindle shaped Figures 12B and 13B , while the cells of the anterior hemisphere remain teardrop shaped Figures 12A and 13B.

The cross-section of spindle-shaped cells is smaller than the cross-section of teardrop-shaped cells compare Figures 12A1 and 12B1 , which may explain the contraction of the posterior hemisphere of the embryo Figure 13B. This contraction occurs in conjunction with the formation of the bend region somewhat below the equator of the embryo Figure 13B. Thus, the spherical embryo begins epithelial folding without having a cell sheet with a free edge in contrast to the V.

In the bend region Figure 13C , the spindle-shaped cells Figure 12B change more and more into paddle-shaped cells Figure 12D. Simultaneously, these cells seem to move relative to the CBs so that the CBs of paddle-shaped cells eventually are at their chloroplast ends, which face the inner side of the curve Figures 12D and 13C. The network of CBs appears to be the substrate upon which the coordinated cell shape changes operate to deform the cell sheet. The cell shape conversions together with the movements of cells relative to the CBs seem to progress from the equator of the embryo toward its posterior pole compare Figure 13B to 13E , and appear to drive inversion of the posterior hemisphere and its simultaneous movement into the anterior hemisphere Figure 13B, C, D, E, F.

However, how is the signal to change shape and CB positions forwarded to neighboring cells, and how is it coordinated? Once epithelial folding is initiated in the circular bend region, a radially symmetrical wave of a trigger molecule or simply of a mechanical force transmission might perpetuate and forward the folding command from cells in the bend region of the embryo toward the posterior pole of the embryo by inducing cell shape changes and movements of cells relative to the CBs in neighboring cells.

What shapes the paddle cells of the bend region? Fundamentally, the paddle-shaped cells Figure 12D in the bend region seem to be shaped by anisotropic cytoskeletal events along the chloroplast-end-to-flagellar-end axis Figure 12D1 , which produce strongly elongated cells that get thinner toward the chloroplast end Figures 12D and 13C. Basically, these cells seem to be radially symmetrical throughout the length of the cell; thus, the cytoskeletal forces in radial directions of the cell seem to be isotropic.

However, due to the increasing curvature of the bend region especially in early inversion , the chloroplast ends of the cells come very close together, and a lack of space seems to arise at these ends of the cells.

Finally, the cells appear to get compressed at the chloroplast end; thus, the minor axes of the cross-sections at these ends are only one-third or one-fourth the length of the major axes Figure 12D3, D4.

The smaller the radius of the bend region, the stronger the compression of the chloroplast end of the paddle-shaped cells appears to be. Therefore, the shape of the paddle cells at the chloroplast end Figure 12D3, D4 seems to reflect an isotropic cytoskeletal activity in radial directions in the context of an anisotropic mechanical environment. Once inversion of the posterior hemisphere is well advanced and the radius of the curvature in the bend region increases Figure 13E , the paddle-shaped cells Figure 12D change into pencil-shaped cells Figure 12E.

The remaining spindle-shaped cells Figure 12B close to the not yet inverted posterior pole never become paddle shaped but directly change into the pencil shape Figure 13E. The invagination of the posterior cell sheet seems to generate circumferential tension along the rim of the anterior cap Figure 13C, D.

Therefore, the teardrop-shaped cells Figure 12A at the rim of the anterior cap Figure 13C, D become increasingly disc shaped Figure 12C. What shapes the disc cells? Basically, the disc-shaped cells Figure 12C might show a more or less isotropic cytoskeletal activity but, due to tensions on the cell sheet in the anterior-posterior direction of the embryo in response to pulling at the site of bending Figure 13D , the cells get elongated and flattened Figures 12C and 13D.

Thus, disc-shaped cells seem to be shaped by an anisotropic mechanical environment. This anisotropic mechanical environment has a second consequence: the disc-shaped cells appear to begin to move over the rim of the anterior cap Figure 13C, D , which seems to be the beginning of the inversion of the anterior hemisphere. The conversion of teardrop-shaped into elongated cells appears to progress in a radially symmetrical wave from the rim of the anterior cap of the embryo toward its anterior pole compare Figure 13D and 13E.

Eventually, tension in the anterior-posterior direction of the embryo produces thinning and elongation of the whole anterior cell layer Figure 13D, E. It remains unclear whether the movement of the CBs to opposite ends of the disc-shaped cells Figures 12C2 and 13E is a passive response to pulling at the site of bending or whether there is an active remodeling of the cytoskeleton including active movement of the CBs; the latter seems more likely because embryonic cells can actively move the CBs [ 24 ].

Disc-shaped cells Figure 12C that moved over the rim of the anterior cap Figure 13D, E appear to condense into a smaller area beyond the rim, compared to the area they occupied before they rolled over the rim, that is, they reduce their cross-sections at the chloroplast ends and become pencil-shaped cells; simultaneously, these cells also seem to move relative to the CBs so that the CBs eventually are at their chloroplast ends Figures 12E and 13E.

Again, the network of CBs appears to be the substrate upon which the coordinated cell shape changes operate to act on the cell sheet. The conversions from disc to pencil shapes together with the concerted movements of the cells with respect to the CB system seems to maintain the circumferential tension on the rim of the remaining portion of the anterior cap and, as a consequence, on the not yet inverted cell sheet of the anterior hemisphere. Because more and more disc-shaped cells Figure 12C move over the rim, the circumferential tension on the cell sheet of the anterior hemisphere increases continuously.

When the remaining disc-shaped cells of the anterior hemisphere and their CBs are stretched to their maximum length, further tension on the anterior hemisphere results in the opening of the phialopore Figure 13E. The appearance of a circular opening means that the dominant tensions on the anterior hemisphere are likely to be circumferential. While the phialopore widens Figure 13E, F, G , both the cells and the CBs around the phialopore as well as submarginal cells in the sheet and their CBs appear to become passively stretched along a circular line described by the edge of the opening Figures 3F and 8B.

However, the phialopore widens dramatically until the diameter of the opening is even greater than the outer diameter of the inverted posterior hemisphere Figure 13F, G. This expansion of the phialopore cannot be explained only by stretching of the cell sheet; there also seems to be a rearrangement of cells at the phialopore. The movement of disc-shaped cells Figure 12C over the rim of the remaining anterior cap and the conversion of disc-shaped cells into pencil-shaped cells Figure 12E continues; thus, the remaining anterior hemisphere appears to glide over the already inverted posterior hemisphere until the anterior cap is completely inverted compare Figure 13E to 13H.

This in-turning of the cell sheet over the surface of the outer layer is comparable to involution, one of the major morphogenetic movements that occur during gastrulation; involution can be observed, for example, in mesoderm migration in the Amphibian gastrula [ 86 ]. At the end of inversion, the phialopore gets smaller and smaller Figure 13H. Relaxation of the previous circumferential tension that occurred during opening of the phialopore might be one reason but is probably not the only reason for this process, there also seems to be a rearrangement of cells at the opening.

Finally, the phialopore closes without leaving any visible trace of the previous opening behind Figure 13H, I. Right after the completion of inversion, all cells of the embryo change concertedly from pencil Figure 12E to column shapes Figures 12F and 13I. Somewhat later, they develop gradually into gemstone-shaped cells Figure 12G.

Following inversion, the juvenile organism resembles a miniature adult; it will increase in size without further cell division by depositing large quantities of ECM. Both type A and type B inverters must solve the same developmental problem: they begin their lives inside out and must turn their spherical cell monolayer outside in to achieve their adult configuration.

Inverters of both types follow the same basic principle: concerted, spatially and temporally coordinated changes in cellular shapes act together with concerted migration of the cells relative to the CB system. However, differences exist in almost all details of the inversion process, suggesting analogous inversion processes that arose through parallel evolution.