Type B Embryo Inversion in Volvox Globator Stephanie Höhn and Armin Hallmann*

Höhn and Hallmann BMC Biology 2011, 9:89 http://www.biomedcentral.com/1741-7007/9/89 RESEARCH ARTICLE Open Access There is more than one way to turn a spherical cellular monolayer inside out: type B embryo inversion in Volvox globator Stephanie Höhn and Armin Hallmann* Abstract Background: Epithelial folding is a common morphogenetic process during the development of multicellular organisms. In metazoans, the biological and biomechanical processes that underlie such three-dimensional (3D) developmental events are usually complex and difficult to investigate. Spheroidal green algae of the genus Volvox are uniquely suited as model systems for studying the basic principles of epithelial folding. Volvox embryos begin life inside out and then must turn their spherical cell monolayer outside in to achieve their adult configuration; this process is called ‘inversion.’ There are two fundamentally different sequences of inversion processes in Volvocaceae: type A and type B. Type A inversion is well studied, but not much is known about type B inversion. How does the embryo of a typical type B inverter, V. globator, turn itself inside out? Results: In this study, we investigated the type B inversion of V. globator embryos and focused on the major movement patterns of the cellular monolayer, cell shape changes and changes in the localization of cytoplasmic bridges (CBs) connecting the cells. Isolated intact, sectioned and fragmented embryos were analyzed throughout the inversion process using light microscopy, confocal laser scanning microscopy, scanning electron microscopy and transmission electron microscopy techniques. We generated 3D models of the identified cell shapes, including the localizations of CBs. We show how concerted cell-shape changes and concerted changes in the position of cells relative to the CB system cause cell layer movements and turn the spherical cell monolayer inside out. The type B inversion of V. globator is compared to the type A inversion in V. carteri. Conclusions: Concerted, spatially and temporally coordinated changes in cellular shapes in conjunction with concerted migration of cells relative to the CB system are the causes of type B inversion in V. globator. Despite significant similarities between type A and type B inverters, differences exist in almost all details of the inversion process, suggesting analogous inversion processes that arose through parallel evolution. Based on our results and due to the cellular biomechanical implications of the involved tensile and compressive forces, we developed a global mechanistic scenario that predicts epithelial folding during embryonic inversion in V. globator. Background developmental stages including gastrulation, neurulation One of the most fascinating problems in developmental and organogenesis. Epithelia are composed of closely biology is how embryonic cells work together to pro- arranged epithelial cells that adhere together at intercel- duce highly organized multicellular organisms. Although lular junctions that give the epithelial sheet mechanical embryonic tissues may ultimately be shaped in a spe- strength. This adherence of the cells is thought to be cies-specific manner, there are basic remodeling events the key for the transmission of forces between neighbor- that are shared, even among distantly related species. ing cells through the links between the actomyosin Epithelial folding is one of these common morphoge- cytoskeleton and the adherens junctions [1]. In netic processes and is involved in metazoan metazoan development, folding or bending of the cell sheet is achieved by cells at the bend points that change * Correspondence: [email protected] from a cuboidal to a wedge shape. This cell wedging is Department of Cellular and Developmental Biology of Plants, University of observed in several invaginations, including blastopore Bielefeld, Universitätsstrasse 25, 33615 Bielefeld, Germany © 2011 Höhn and Hallmann; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Höhn and Hallmann BMC Biology 2011, 9:89 Page 2 of 26 http://www.biomedcentral.com/1741-7007/9/89 groove formation and neurulation in amphibians and exhibit large reproductive cells before inversion (for the formation of the primitive groove in birds [1-7]. It example, V. carteri), the reproductive cells point out- also exists in other cases, such as gastrulation in some ward. This inside-out orientation is corrected as the echinoderms [8], but is less obvious in these cases, likely embryo turns itself right-side out during inversion. because mechanisms other than cell wedging are The process of inversion was discovered in V. carteri f. involved [1]. The bending and folding of a sheet of cells weismannia approximately 100 years ago, but it was is fundamentally a biomechanical, supercellular process initially misinterpreted as a pathological phenomenon involving coordinated changes in the shapes of many [51]. The correct interpretation of inversion was pro- neighboring cells that interact; however, the biological vided by Kuschakewitsch approximately a decade later and biomechanical processes that underlie such three- [52,53]. Subsequent work revealed that inversion in Vol- dimensional (3D) morphogenetic events in metazoans vox involves two cellular events: dramatic changes in are usually complex and difficult to investigate cell shapes and coordinated movements of cells relative [2-7,9-19]. to the CBs [20-24,26-29,31,54-57]. However, there are In the simple multicellular green algae Volvox,amor- two fundamentally different sequences through which phogenetic process known as inversion occurs during embryos of the genus Volvox turn right-side out: type A embryogenesis. Volvox inversion bears significant simi- and type B (Figure 1) [25,58]. In type A inversion, which larity to metazoan gastrulation and has been suggested occurs in V. carteri and species such as V. obversus, V. as a model system for studying the curling of an epithe- africanus, V. gigas and V. tertius,anopening,calledthe lium [20-25]. The similarities between the morphoge- phialopore, appears at the anterior pole; the anterior netic process of inversion in Volvox carteri and hemisphere inverts through the phialopore; the posterior epithelial folding events in metazoans include cell adher- hemisphere then inverts; and finally, the phialopore ence, cell shape changes, cell movements, transmission closes (Figure 1A) [22,32-34,54]. (It should be noted of forces between neighboring cells and the involvement that by convention, the anterior pole of a pre-inversion of cytoskeletal elements [20,22-24,26-29]. However, embryo becomes the posterior pole of the inverted embryonic Volvox cells have no adherens junctions; embryo [22,25].) In type B inversion (Figure 1B), as instead, they adhere together through a network of observed in species such as V. globator, V. capensis, V. numerous and quite robust cytoplasmic bridges (CBs) dissipatrix, V. rousseletii and V. barberi, the posterior that are the result of incomplete cytokinesis [22] and hemisphere inverts before the phialopore opens, fol- allow for force transmission. As in metazoans, folding or lowed by inversion of the anterior hemisphere; finally, bending of the embryonic cell sheet in V. carteri is the phialopore closes [26,38-40]. Thus, in type B inver- achieved by cells at the bend points, which undergo a sion, the opening of the phialopore does not indicate distinct transition in cell shape [20,22-24,26-29]. More- the beginning of inversion, and the hemispheres invert over, concerted movements of cells with respect to the in reverse order compared to type A inversion. CB system are crucial for the process of inversion in V. In terms of evolution, inversion must have arisen quite carteri [20,22-24,26-29]. recently because phylogenetic analyses indicate that mul- Most previous research on inversion was performed in ticellularity evolved in volvocine green algae ‘only’ V. carteri [20-25]; however, this process also occurs, but approximately 200 million years ago [59], which is much has been studied in much less detail, in other species of more recently than in any other group, and inversion the genus Volvox such as V. tertius [30-32], V. obversus must have evolved in parallel with the transition to multi- [33,34], V. aureus [35,36], V. globator [37,38], V. rousse- cellularity or even later. Phylogenetic analyses have also letii [38-40] and V. capensis [38]. Inversion also exists in indicated a polyphyletic origin of the genus Volvox more basal volvocine relatives of Volvox [25,41], such as [22,60-66] (see Additional File 1) suggesting that it is not Pleodorina californica [42], Eudorina elegans [43-45], E. a true genus. A few species of this genus, including V. indica [46], Pandorina morum [47], Platydorina caudata globator, form a small but robust monophyletic group [48] and Gonium pectorale [44,49,50], even if the pro- that is referred to as the section Volvox [58,62,64,67,68]. cess is less distinct in these species. All Volvox species within the section Volvox exhibit type Inversion is the solution to the awkward inside-out B inversion (see Additional File 1). The other species of situation of the embryo in multicellular volvocine algae. the genus Volvox and the genera Eudorina and Pleodor- After completion of embryonic

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