Tweedle proteins form extracellular two-dimensional structures defining body and cell shape in Drosophila melanogaster

Tissue function and shape rely on the organization of the extracellular matrix (ECM) produced by the respective cells. Our understanding of the underlying molecular mechanisms is limited. Here, we show that extracellular Tweedle (Twdl) proteins in the fruit fly Drosophila melanogaster form two adjacent two-dimensional sheets underneath the cuticle surface and above a distinct layer of dityrosinylated and probably elastic proteins enwrapping the whole body. Dominant mutations in twdl genes cause ectopic spherical aggregation of Twdl proteins that recruit dityrosinylated proteins at their periphery within lower cuticle regions. These aggregates perturb parallel ridges at the surface of epidermal cells that have been demonstrated to be crucial for body shaping. In one scenario, hence, this disorientation of epidermal ridges may explain the squatty phenotype of Twdl mutant larvae. In an alternative scenario, this phenotype may be due to the depletion of the dityrosinylated and elastic layer, and the consequent weakening of cuticle resistance against the internal hydrostatic pressure. According to Barlow's formula describing the distribution of internal pressure forces in pipes in dependence of pipe wall material properties, it follows that this reduction in turn causes lateral expansion at the expense of the antero-posterior elongation of the body.

Secondary neurulation-fated cells in the tail bud undergo self-renewal and tubulogenesis regulated by a Sox2 gradient

During amniote development, anterior and posterior components of the neural tube form by primary neurulation (PN) and secondary neurulation (SN), respectively. Unlike PN, SN proceeds by the mesenchymal-to-epithelial transition of SN precursors in the tail bud, a critical structure for the axial elongation. Our direct cell labeling delineates non-overlapping territories of SN- and mesodermal precursors in the chicken tail bud. SN-fated precursors are further divided into self-renewing and differentiating cells, a decision regulated by graded expression levels of Sox2. Whereas Sox2 is confined to SN precursors, Brachyury (T) is widely and uniformly distributed in the tail bud, indicating that Sox2+/Brachyury+ cells are neural-fated and not mesodermal. These results uncover multiple steps during the neural posterior elongation, including precocious segregation of SN precursors, their self-renewal, and regulation by graded Sox 2.

Elongation during segmentation shows axial variability, low mitotic rates, and synchronized cell cycle domains in the crustacean, Thamnocephalus platyurus

Segmentation in arthropods typically occurs by sequential addition of segments from a posterior growth zone, but cell behaviors producing posterior elongation are not well known. Using precisely staged larvae of the crustacean, Thamnocephalus platyurus, we systematically examined cell division patterns and morphometric changes associated with posterior elongation during segmentation. We show that cell division is required for normal elongation but that cells in the growth zone need only divide ~1.5 times to meet that requirement; correspondingly, direct measures of cell division in the growth zone are low. Morphometric measurements of the growth zone and of newly formed segments suggest tagma-specific features of segment generation. Using methods for detecting two different phases in the cell cycle, we show distinct domains of synchronized cells in the posterior. Borders of cell cycle domains correlate with domains of segmental gene expression, suggesting an intimate link between segment generation and cell cycle regulation.Summary StatementPosterior growth zone has synchronized cell cycle domains but shows little cell division during segment addition in a crustacean. Dimensions of the shrinking posterior growth zone change at tagma boundaries.

Embryology and gonad development in Oscheius shamimi sp. n. (Nematoda: Rhabditida)

A new species of Rhabditinae collected from compost, is described and illustrated together with observations on embryonic and post-embryonic development. Oscheius shamimi sp. n. is characterised by a medium-sized body (female: L = 0.76-1.52 mm, a = 12-20, b = 4.2-6.3, c = 6.8-13.8, c′ = 4.1-5.8, V = 45-51), finely annulated cuticle; slightly demarcated lip region; lips fused to form three doublets; six lines in lateral field; small stoma with isomorphic metastegostom; cylindrical pharyngeal corpus; slightly protruded vulval lips with subventral cuticular flap; long, proximally dilated, rectum; 53-61 μm long, robust spicules with long capitula having strongly cuticularised ventral walls; pseudopeloderan bursa with nine pairs of genital papillae in 1 + 1 + 1/3 + 3 + ph configuration and six to seven pairs of copulatory muscle bands. Oscheius shamimi sp. n. is amphimictic with a 1:1 sex ratio. The eggs are ovoid and smooth-shelled and measure 48-78 × 37-48 μm. Most eggs are laid in the late stages of embryonation. The embryonation time was 14-15 h at 25 ± 2°C. The genital primordium was orientated obliquely to the longitudinal axis and did not show division of primordial nuclei during the first moult. The didelphic female reproductive system was formed as a result of anterior and posterior elongation of the primordium while the monorchic reproductive system of the male developed from an anterior elongation of the primordium. The life cycle from egg to adult was completed in 3-3.5 days at 25 ± 2°C.

The Genetics and Development of an Eyespot Pattern in the Butterfly Bicyclus anynana: Response to Selection for Eyespot Shape

The normally circular eyespots on the wing of the butterfly Bicyclus anynana were selected to become elliptical in two divergent lines, with antero-posterior elongation of the eyespots in one line and proximodistal elongation in the other. Selection was continued for nine generations, and symmetrical realized heritabilities of ∼15% were achieved initially. The elliptical eyespot shapes characteristic of each line were still produced when the signaling center of the eyespot (the focus) was surgically rotated by 90 or 180° or when an eyespot was induced ectopically by localized damage. We conclude that selection changed general properties of the epidermis that responds to signals emanating from the eyespot focus but did not affect the mechanism of focal signaling.

Mitotic Pattern in the Chick Pronephric Duct

By means of a variety of ingenious experiments it has been shown that the pronephric duct originates anteriorly and grows caudally to join the cloaca (for review, see Burns, 1955). It has been concluded on the basis of histological study (Lillie, 1919), and from operative experiments in which the pronephric duct path was blocked by grafts (Waddington, 1938), that in the chick the pronephric duct extends posteriorly by proliferation of its own cells. In the present study, the proliferative pattern in the outgrowing duct of the chick has been examined in normal and in colchicine-treated embryos. Results indicate that proliferation is particularly marked towards the posterior duct tip. It was also noted that tensions of the blastoderm (see Spratt, 1956) which are disrupted by colchicine treatment appear to play some part in maintenance of normal diameter and posterior elongation of the duct.

Form and habit in Pinna carnea Gmelin

The shell of the Pinnidae consists of characteristic fan-shaped valves united for their entire length dorsally by some form of ligament. The posterior and postero-ventral extensions of the valves are formed exclusively by the outer lobe of the m antle edge and so consist of the outer calcareous (prismatic) layer only. In the Pinnidae this layer has an exceptionally high organic content and so is flexible. The inner (nacreous) calcareous layer is thin and confined to the region occupied by the body, i.e. between the two adductors. The ligament is composed of three layers (disregarding the outermost, but vestigial, periostracum ). Anteriorly it consists of inner and outer layers; posterior to this for a short distance of outer layer, and for the remaining, and greatest, length it represents a fusion layer. This also continues forward over the middle region and traces persist still farther forward. The inner ligament layer is secreted by the m antle isthmus, i.e. it corresponds to the inner calcareous layer of the valves; the outer ligament layer is secreted by the outer lobes of the m antle edge, i.e. it corresponds to the outer calcareous layer of the valves. The fusion layer represents the result of union posteriorly of the outer lobes of the m antle edge. T h at part of the ligament which is formed in the same way as the valves (all being constituents of the shell) is here termed the prim ary ligament, that part formed by fusion of the outer lobes of the m antle edge constituting the secondary ligament. The mantle epithelium is divisible into proximal and distal regions, both containing mucous glands. The former, continuous with the m antle isthmus, secretes the inner calcareous layer; the latter is pigmented. A part from the adductors, the m antle is attached only by the large anterior and posterior pallial retractors which subdivide within the m antle folds. Posterior elongation of the m antle involves corresponding extension of the eulamellibranchiate ctenidia. All connexions between the two ctenidia and between ctenidia and m antle are by ciliary junctions. The ctenidia are very m uscular; observations of Atkins on collection and sorting of particles are confirmed. The unique gutter-like waste canals, originally described by Stenta, ensure that pseudofaeces and other waste from the inhalant cham ber are continuously removed. A preoral, unpaired racemose gland opens into the inhalant chamber. Its most probable function is that of excretion; anything discharged from it will be removed by way of the waste canals. The pallia! organ in the exhalant chamber is composed of a stalk and a more swollen head. It can be greatly distended and probably serves to clear away shell fragments. The projecting valves are subject to frequent damage. The Pinnidae live vertically embedded in soft substrata into which they cannot withdraw. The animals burrow as they grow but only to the extent that the portion of the shell occupied by the body (i.e. as far as the position of the posterior adductor) is buried. The wide posterior region of the shell is always exposed. Water can thus be drawn in from well above the surface of the substratum. The waste canals in the inhalant chamber and the powerful exhalant current keep the cavity clear. The projecting valves can be rapidly repaired since they are composed exclusively of the outer calcareous layer. Such repair strengthens the valves. During repair new inner and outer ligament layers may be laid down beneath the previously formed primary ligament. In the Lamellibranchia change in form and proportions of the body on the one hand, and of the mantle and shell on the other, are best discussed by reference to the two major axes in the sagittal plane, i.e. the antero-posterior and median axes of the body and the hinge and normal axes of the mantle and shell. In their evolution the Pinnidae probably passed through a ‘Modiolus stage’ with the large posterior adductor close to the m argin of the shell and little secondary extension of the ligament. Subsequently posterior extension of the mantle and so of the shell doubled the length of the animal and was accompanied by secondary extension of the ligament. Such extensions of the mantle occur throughout the Anisomyaria (including the Pinnidae). They involve loss of the primitive pallial attachments apart from the adductors (the anterior of which is always reduced and often lost). The mantle becomes re-attached to the shell by secondary pallial retractors in the Pinnidae and also in Malleus , but along a new line peripherally in the Pectinacea and Ostreacea. Existing data on development in the Pinnidae show that, with the formation of the dissoconch, new shell, secreted by the outer lobe of the mantle edge, is added to the posterior margin of the almost equilateral prodissoconch. The adult form is probably quickly acquired by the post-larva, the proportions of the different regions then remaining constant although with continual reduction anteriorly. Success in the Pinnidae is due partly to characteristics shared with related families, partly to unique features. The former include great extension of mantle lobes without peripheral attachment, the latter include waste canals, pallial organ and pallial retractors. The rigid ligament only unites the valves; it has no opening thrust. The Pinnidae can survive loss of the anterior adductor or fusion of the ventral margins of the valves. It is only essential that the posterior adductor should be able when necessary to pull together the flexible posterior margins of the valves.