Endosymbionts: factors on egg pattern formation

Elimination of the intracellular symbionts of Euscelis plebejus either by X-ray irradiation of the posterior pole of the freshly laid egg or by interruption of egg infection by application of tetracycline or lysozyme to female leafhoppers leads to the production of embryos without abdomens, ‘head-embryos’.Homogenates of symbiont-free eggs and symbiont-containing eggs in the state of invagination have a pH of 7·5±0·2 and 7·0±0·2 and an osmotic pressure (pO) of 8·3±0·2 and 7·8±0·2, respectively. The presence of symbionts leads to a decrease of both the pH and pO.These data indicate that the correct formation of the posterior gradient, necessary for normal abdomen development, is dependent on the presence of endosymbionts at the posterior pole. It is possible that the symbionts change the pH and pO of the posterior gradient. These results are consistent with a hypothetical model of early differentiation of the Euscelis egg.     

The egg came first, of course! Anterior-posterior pattern formation in Drosophila embryogenesis and oogenesis

The anterior-posterior body pattern of the Drosophila embryo is initiated through the action of maternal gene products. In particular, three groups of maternally acting genes (the anterior, posterior and terminal groups) have been shown to direct the synthesis and spatial restriction of the three major organizing activities in the egg. The initial spatial localizations of the maternal organizing activities are established during oogenesis. After fertilization these activities regulate zygotic gene activity along the anterior-posterior axis of the egg.     

Early pattern formation in the developing Drosophila eye

The formation of complex cellular arrays from unpatterned epithelia is a widespread developmental phenomenon. Insights into the mechanisms regulating this transformation have come from studying the development of the Drosophila compound eye. Pattern formation in the eye primordium is a highly ordered process in which the onset of differentiation is coordinated with synchronization of cell cycle progression. Recent studies have identified a number of genes that are required for early patterning events, and provide a link between the regulation of proliferation and pattern formation.     

The cleavage pattern of the axolotl egg studied by cinematography and cell counting

Summary The temporal pattern of cleavage in the egg of the axolotl,Ambystoma mexicanum, was studied 1. by time-lapse microcinematography, and 2. by counting the total number of blastomeres dissociated at successive stages.Eggs were filmed from the one-cell stage till the early gastrula either (A) simultaneously from above and below with a double-camera assembly, or (B) from the side with a single camera.The animal blastomeres divide synchronously from the 2nd up to and including the 10th cleavage. The cycle length is roughly constant from the 3rd till the 10th cleavage. The cycle from the 2nd to the 3rd cleavage is slightly longer, while that from the 1st to the 2nd cleavage is about 20% longer. After the 10th cleavage the synchrony of divisions is lost owing to variable lengthening of cell cycles in individual blastomeres. Gastrulation starts around the onset of the 15th cleavage in the animal blastomeres.The analysis of films taken in side view reveals seven recurring cleavage waves, from the 5th till the 11th cleavage. Cells in the animal, equatorial and vegetative regions in sequence repeatedly pass through the three successive phases of the cleavage cycle—rounding-up, division, and relaxation—but with a shift in phase. The start of the 10th cleavage division of the slowest vegetative cells more or less coincides with that of the 11th division of the animal cells; from then on the cleavage waves become increasingly obscured.Morulae and blastulae were dissociated by placing them in 1/15 M phosphate buffer (pH 7.8) for the duration of 2–3 cleavage cycles and then removing the vitelline membrane. In this solution cell divisions continued without disturbance of the temporal cleavage pattern. The dissociated cells were fixed either just prior to the onset of the next cleavage (up to the 10th cleavage) or at those times when cleavageswould have been expected, had there been no lenthening of cleavage cycles (beyond the 10th cleavage). The total cell number was counted, dividing cells being scored as two.Prior to the 11th cleavage the total cell number increased exponentially. Beyond the 10th cleavage the rate of increase was considerably lower. At the time when gastrulation would have started if the egg had not been dissociated, the total cell counts were 13,000–15,000, whereas the number anticipated without lengthening of cleavage cycles would be of the order of 130,000 (2¹⁷).The application of Balfour's rule to amphibian eggs is criticized.     

Divide and conquer: pattern formation in Drosophila embryonic epidermis

The pattern of differentiated cell types within tissues and organs is often established by organizers, the localized sources of secreted ligands. Although the mechanisms underlying organizer function have been extensively studied, only in a few cases is it clear how an organizer ultimately controls each individual cell's fate across a field of progenitor cells. One of these cases involves the establishment of a precise pattern of cell differentiation across the embryonic epidermis in Drosophila. Here, we review several recent reports that help to elucidate the regulatory principles used to control this pattern. Because organizers are conserved, the same fundamental principles might operate in other organizers.     

hedgehog and engrailed: pattern formation and polarity in the Drosophila abdomen

Like the Drosophila embryo, the abdomen of the adult consists of alternating anterior (A) and posterior (P) compartments. However the wing is made by only part of one A and part of one P compartment. The abdomen therefore offers an opportunity to compare two compartment borders (A/P is within the segment and P/A intervenes between two segments), and ask if they act differently in pattern formation. In the embryo, abdomen and wing P compartment cells express the selector gene engrailed and secrete Hedgehog protein whilst A compartment cells need the patched and smoothened genes in order to respond to Hedgehog. We made clones of cells with altered activities of the engrailed, patched and smoothened genes. Our results confirm (1) that the state of engrailed, whether 'off' or 'on', determines whether a cell is of A or P type and (2) that Hedgehog signalling, coming from the adjacent P compartments across both A/P and P/A boundaries, organises the pattern of all the A cells. We have uncovered four new aspects of compartments and engrailed in the abdomen. First, we show that engrailed acts in the A compartment: Hedgehog leaves the P cells and crosses the A/P boundary where it induces engrailed in a narrow band of A cells. engrailed causes these cells to form a special type of cuticle. No similar effect occurs when Hedgehog crosses the P/A border. Second, we look at the polarity changes induced by the clones, and build a working hypothesis that polarity is organised, in both compartments, by molecule(s) emanating from the A/P but not the P/A boundaries. Third, we show that both the A and P compartments are each divided into anterior and posterior subdomains. This additional stratification makes the A/P and the P/A boundaries fundamentally distinct from each other. Finally, we find that when engrailed is removed from P cells (of, say, segment A5) they transform not into A cells of the same segment, but into A cells of the same parasegment (segment A6).     

Dorsal ventral polarity and pattern formation in the Drosophila embryo

The establishment of polarity along the dorsal-ventral axis of the Drosophila embryo requires the graded distribution of the dorsal morphogen. Several maternal genes are responsible for the formation of the gradient and their products act in an ordered series of events that begins during oogenesis and involves two different cell types, the oocyte and the follicle cells. The last step in the series results in selective nuclear localization of dorsal proteins, dorsal is thought to regulate the expression of zygotic genes in a concentration dependent way. The zygotic genes determine cell fates in specific regions of the embryo and direct other genes involved in the processes of differentiation.     

Modeling pattern formation in hydra: a route to understanding essential steps in development

Modeling of pattern formation in hydra has revealed basic mechanisms that underlie the reproducible generation of complex and self-regulating patterns. Organizing regions can be generated by a local self-enhancing reaction that is coupled with an inhibitory effect of longer range. Such reactions enable pattern formation even in an initially almost homogeneous assembly of cells. A long-ranging feedback of the organizer onto the competence to perform the pattern-forming reaction stabilizes the polar axial pattern during growth and allows for regeneration with preserved polarity. Hypostome formation is assumed to be under the control of two positive feedback loops in which Wnt3 is a common element. In addition to the well-established loop employing beta-catenin, a second cell-local loop is involved, possibly with Brachyury as an additional component. This model accounts for the different expression patterns of beta-catenin and Wnt3. Wnt molecules are proposed to play a dual role, functioning as activators and, after processing, as inhibitors. Since Wnt genes code for complete pattern-forming systems, gene duplication and diversification lead to a family of genes whose expression regions have a precise relation to each other. Tentacle formation is an example of positioning a second pattern-forming system by medium-ranging activation and local exclusion exerted by the primary system. A model for bud formation suggests that a transient pre-bud signal is involved that initiates the formation of the foot of the bud, close to the normal foot, as well as close to the bud tip. Many dynamic regulations, as observed in classical and molecular observations, are reproduced in computer simulations. A case is made that hydra can be regarded as a living fossil, documenting an evolutionary early axis formation before trunk formation and bilaterality were invented. Animated simulations are available in the supplementary information accompanying this paper.     

Modeling plant growth and pattern formation

Plants continue to grow and generate new organs in symmetric patterns throughout their lives. This development requires an interconnected regulation of genes, hormones, and anisotropic growth, which in part is guided by environmental cues. Recently, several studies have used a combination of experiments and mathematical modeling to elucidate the mechanisms behind different growth and molecular patterns in plants. The computational models were used to investigate the often non-intuitive consequences of different hypotheses, and the in silico simulations of the models inspired further experimentation.     

A pattern formation mechanism to control spatial organization in the embryo of Drosophila melanogaster

It is known that cells are already committed to a particular segment at the cellular blastoderm stage during embryogenesis of Drosophila melanogaster. Recently, several segmentation genes have been observed to be expressed in a sequence of banded spatial patterns in the syncytial blastoderm, prior to the formation of the cellular blastoderm. It is demonstrated in this paper that a two component reaction-diffusion (RD) system with net production functions which are antisymmetric with respect to the uniform steady-state values, is capable of producing a sequence of seven spatial patterns in the syncytial blastoderm. The sequence of patterns obtained exhibit a strong preference for banded or striped patterns. The first pattern is a simple anteroposterior gradient while the second is a gradient in the dorsoventral direction. The next five patterns are a sequence of banded patterns which exhibit frequency doubling, i.e. the number of bands in each pattern tend to be double the number in the previous pattern. The predicted pattern sequence is comparable to that observed in the expression of some segmentation genes. It is suggested that a pattern formation mechanism based on such an RD system may exist in the embryo where it produces a sequence of prepatterns to regulate the expression of various segmentation genes leading ultimately to a segmented embryo. There is sufficient spatial information in the sequence of banded prepatterns for the segments to be unique.     

The ovarian structure and mode of egg production in two polygamous pipefishes: a link to mating pattern

In this study, the ovarian structure and mode of egg production were examined in two pipefishes, the broad-nosed pipefish Syngnathus typhle and the straight-nosed pipefish Nerophis ophidion, which show different types of polygamous mating patterns. Syngnathus typhle showed an ovary with one germinal ridge and asynchronous egg production, corresponding to previous findings in other polygamous Syngnathus pipefishes. In contrast, the ovary of N. ophidion had two germinal ridges and eggs were produced synchronously in groups, similar to what has been observed in monogamous syngnathids. The egg production of N. ophidion, however, is clearly distinguished from that of monogamous syngnathids by the additional egg production after an ovulation. It is suggested that the differences in female mating strategies result from the difference in egg production process and that this is related to the difference in mating pattern between these two polygamous species.