Hydrocoel Morphogenesis Forming the Pentaradial Body Plan in a Sea Cucumber, Apostichopus Japonicus

Echinoderms constitute an animal phylum characterized by the pentaradial body plan. During the development from bilateral larvae to pentaradial adults, the formation of the multiple of five hydrocoel lobes, i.e., the buddings from the mesodermal coelom, is the firstly emerging pentameral character. The developmental mechanism underlying the hydrocoel-lobe formation should be revealed to understand the evolutionary process of this unique and highly derived body plan of echinoderms, although the morphogenetic mechanisms of hydrocoel lobes is largely uninvestigated. In this study, using the sea cucumber Apostichopus japonicus, in which the hydrocoel is easily observable, the developmental process of hydrocoel lobes was described in detail, focusing on the cell proliferation and rearrangement. Cell proliferation was not specifically distributed in the growing tips of the hydrocoel lobes and inhibition of the cell proliferation did not affect the lobe formation. During lobe formation, epithelium of the hydrocoel lobes were firstly stratified and then transformed into single-layered, suggesting that radial cell intercalation contributes to hydrocoel-lobe formation.

Canalized morphogenesis driven by inherited tissue asymmetries in Hydra regeneration

The emergence and stabilization of a body axis is a major step in animal morphogenesis, determining the symmetry of the body plan as well as its polarity. To advance our understanding of the emergence of body-axis polarity we study regenerating Hydra. Axis polarity is strongly memorized in Hydra regeneration even in small tissue segments. What type of processes confer this memory? To gain insight into the emerging polarity, we utilize frustrating initial conditions by studying regenerating tissue strips which fold into hollow spheroids by adhering their distal ends, of opposite original polarities. Despite the convoluted folding process and the tissue rearrangements during regeneration, these tissue strips develop a new organizer in a reproducible location preserving the original polarity and yielding an ordered body plan. These observations suggest that the integration of mechanical and biochemical processes supported by their mutual feedback attracts the tissue dynamics towards a well-defined developmental trajectory biased by weak inherited cues from the parent animal. Hydra thus provide an example of dynamic canalization in which the dynamic rules themselves are inherited, in contrast to the classical picture where a detailed developmental trajectory is pre-determined.

Differences in the Structural Components Influence the Pumping Capacity of Marine Sponges

Marine sponges are important sessile, benthic filter feeders with a body plan designed to pump water efficiently. The sponge body plan generally consists of mineral spicules, gelatinous mesohyl, and the pores and canals of the aquiferous system. These structural components have stark differences in compressibility, mass, and volume; therefore, their proportion and distribution are likely to affect sponge morphology, anatomy, contraction, and finally the pumping capacity. We examined seven demosponge species (from high spicule skeleton contents to no spicules) commonly found along the central west coast of India for structural components, such as total inorganic contents (spicule skeleton and foreign inclusions), body density, porosity, and mesohyl TEM for the high microbial abundance/low microbial abundance status. Additionally, we estimated the sponge pumping rate by measuring the excurrent velocity, the abundance of individual pumping units and cells, i.e., choanocyte chambers and choanocytes, and also carried out a morphometric analysis of aquiferous structures. The excurrent velocity and the oscular flow rates showed a positive relationship with the oscular crosssectional area for all the study species. The inorganic spicule contents by their weight as well as volume formed a major component of tissue density and higher proportions of spicules were associated with reduced aquiferous structures and lower pumping rate. The ash mass% and the ash free dry weight (AFDW %) in the sponge dry mass showed separate and distinct associations with aquiferous system variables. For example, the number of choanocytes per chamber showed a wide difference between the studied species ranging from 35.02 ± 2.44 (C. cf. cavernosa) to 120.35 ± 8.98 (I. fusca) and had a significant positive relationship with AFDW% and a negative relationship with ash mass%. This study indicates that the differences in the proportions of structural components are closely related to sponge gross morphology, anatomy, and probably body contractions, factors that influence the sponge pumping capacity.

Hex Hox: De Novo Transcriptome Assembly and Embryonic Hox Gene Expression in the Burrowing Mayfly Hexagenia Limbata (Ephemeridae)

Background: Hox genes are key regulators of appendage development in the insect body plan. The body plan of Mayfly (Ephemeroptera) nymphs differs due to the presence of evolutionarily significant abdominal appendages called gills. Despite mayflies’ basal phylogenetic position and novel morphology amongst insects, little is known of their developmental genetics. Here we present an annotated transcriptome for the mayfly Hexagenia limbata, with annotated sequences for putative Hox peptides and embryonic expression profiles for the Hox genes Antp and Ubx/abd-A. Results: Transcriptomic sequencing of early instar H. limbata nymphs yielded a high-quality assembly of 83,795 contigs, of which 22,975 were annotated against Folsomia candida, Nilaparvata lugens, Zootermopsis nevadensis and UniRef90 protein databases. Peptide annotations included eight of the ten canonical Hox genes (lab, pb, Dfd, Scr, Antp, Ubx, abd-A and Abd-B), most of which contained all functional domains and motifs conserved in insects. Expression patterns of Antp and Ubx/abd-A in H. limbata were visualized from early to late embryogenesis, and are also highly conserved with patterns reported for other non-holometabolous insects.Conclusions: We present evidence that both H. limbata Hox peptide sequences and embryonic expression patterns for Antp and Ubx/abd-A are extensively conserved with other insects. These findings suggest mayfly Antp and Ubx/abd-A play similar appendage promoting and repressing roles in the thorax and abdomen, respectively. The identified expression of Ubx and abd-A in early instar nymphs further suggests that mayfly gill development is not subject to Ubx or abd-A repression. Previous studies have shown that insect Ubx and abd-A repress appendages by inhibiting their distal structures, which can permit the development of proximal appendage types. In line with past morphology-based work, we propose that mayfly gills are proximal appendage structures, possibly homologous to the proximal appendage structures of crustaceans.

Calcium fluctuations drive morphological patterning at the onset of Hydra morphogenesis

Morphogenesis in animal development involves significant morphological transitions leading to the emerging body plan of a mature animal. Understanding how the collective physical processes drive robust morphological patterning requires a coarse-grained description of the dynamics and the characterization of the underlying fields. Here I show that calcium spatial fluctuations serve as an integrator field of the electrical-mechanical processes of morphogenesis in whole-body Hydra regeneration and drive the morphological dynamics. We utilize external electric fields to control the developmental process and study a critical transition in morphogenesis, from the initial spheroidal shape of the tissue to an elongated cylindrical shape defining the body plan of a mature animal. Morphogenesis paused under external voltage is associated with a significant increase of the calcium activity compared with the activity supporting normal development. The enhanced calcium activity is characterized by intensified spatial fluctuations, extended spatial correlations across the tissue and faster temporal fluctuations. In contrast, the normal morphogenesis process is characterized by relatively moderate calcium fluctuation activity and restrained spatial correlations. Long-range communication however, is essential for development. Blocking gap-junctions halts morphogenesis by suppressing the long-range electrical communication, severely reducing the overall calcium activity and enhancing its localization in the tissue. Normal calcium activity is resumed following the wash of the blocker drug, leading to a morphological transition characterizing a normal regeneration process and the emergence of a mature animal. Our methodology of controlling morphogenesis by a physical electric field allows us to gain a global statistical view of the dynamics. It shows that the normalized calcium spatial fluctuations exhibit a universal shape distribution, across tissue samples and conditions, suggesting the existence of a global constrain over these fluctuations. Studying the correlations in space and time of the calcium fluctuation field at the onset of morphogenesis opens a new vista on this process and paints a picture of development analogous to a dynamical phase transition.

An arachnid′s guide to be an ant: morphological and behavioural mimicry in ant-mimicking spiders

Batesian mimicry imposes several challenges to mimics and evokes adaptations in multiple sensory modalities. Myrmecomorphy, morphological and behavioral resemblance to ants, is seen in over 2000 arthropod species. Ant-like resemblance is observed in at least 13 spider families despite spiders having a distinct body plan compared to ants. Quantifying the extent to which spider's shape, size, and behavior resemble model ants will allow us to comprehend the evolutionary pressures that have facilitated myrmecomorphy. Myrmaplata plataleoides are 'accurate' mimics of the weaver ants, Oecophylla smaragdina. In this study, we quantify the speed of movement of model, mimic, and non-mimetic jumping spiders. We use traditional and geometric morphometrics to quantify traits such as foreleg and hindleg size, body shape between the model ant, mimic, and non-mimics. Our results suggest that while the mimics closely resemble the model ants in speed of movement, they occupy an intermediate morphological space compared to the model ants and non-mimics. We suggest that ant-mimicking spiders are better at mimicking ant's locomotory movement than morphology and overall body shape. Our study provides a framework to understand the multimodal nature of mimicry and helps discern the relative contributions of such traits that drive mimetic accuracy in ant-mimicking spiders.

Revisiting kinorhynch segmentation: variation of segmental patterns in the nervous system of three aberrant species

Background Kinorhynch segmentation differs from the patterns found in Chordata, Arthropoda and Annelida which have coeloms and circulatory systems. Due to these differences and their obsolete status as ‘Aschelminthes’, the microscopic kinorhynchs are often not acknowledged as segmented bilaterians. Yet, morphological studies have shown a conserved segmental arrangement of ectodermal and mesodermal organ systems with spatial correspondence along the anterior-posterior axis. However, a few aberrant kinorhynch lineages present a worm-like body plan with thin cuticle and less distinct segmentation, and thus their study may aid to shed new light on the evolution of segmental patterns within Kinorhyncha. Results Here we found the nervous system in the aberrant Cateria styx and Franciscideres kalenesos to be clearly segmental, and similar to those of non-aberrant kinorhynchs; hereby not mirroring their otherwise aberrant and posteriorly shifted myoanatomy. In Zelinkaderes yong, however, the segmental arrangement of the nervous system is also shifted posteriorly and misaligned with respect to the cuticular segmentation. Conclusions The morphological disparity together with the distant phylogenetic positions of F. kalenesos, C. styx and Z. yong support a convergent origin of aberrant appearances and segmental mismatches within Kinorhyncha.

Organ- and Site-Specific HOX Gene Expression in Stromal Cells

HOX genes are a group of evolutionarily conserved genes that encode a family of transcription factors that regulate early developmental morphogenetic processes and continue to be expressed into adulthood. These highly conserved HOX factors play an unquestioned crucial role as master regulators during embryonic vertebrate development and morphogenesis by controlling the three dimensional body plan organization. HOX genes specify regions of the body plan of an embryo along the head-tail axis. They encode proteins that specify the characteristics of ‘position’, ensuring that the correct structures form in the correct places of the body. Expression of HOX is known to persist in many tissues in the postnatal period suggesting the role of these genes not only during development but also for the functioning of tissues throughout life. The tissue-specific pattern of HOX gene expression is inherent in stromal/stem cells of mesenchymal origin, such as mesenchymal stromal cells, fibroblasts, smooth muscle cells, and preadipocytes, enabling them to memorize their topographic location in the form of their HOX code and to fulfill their location-specific functions. In this chapter, we focus on the expression and potential role of HOX genes in adult tissues. We review evidence that site-specific expression of HOX genes is connected to location-specific disease susceptibility and review studies showing that dysregulated expression of HOX genes can be associated with various diseases. By recognizing the importance of site-specific molecular mechanisms in the organ stroma, we gain new insights into the processes underlying the site-specific manifestation of disease.