Different temporal requirements for the LRR transmembrane receptors Tartan and Toll-2 in the formation of contractile interfaces at compartmental boundaries

Compartmental boundaries physically separate groups of epithelial cells, a property fundamental for the organization of the body plan in both insects and vertebrates. In many examples, this physical separation has been shown to be the consequence of a regulated increase in contractility of the actomyosin cortex at boundary cell-cell interfaces, a property important in developmental morphogenesis beyond compartmental boundary formation. In this study, we took an unbiased screening approach to identify cell surface receptors required for actomyosin enrichment and polarisation at parasegmental boundaries (PSBs) in early Drosophila embryos, leading us to uncover different temporal requirements for two LRR receptors, Tartan and Toll-2. First, we find that Tartan is required during germband extension for actomyosin enrichment at PSBs, confirming an earlier report. Next, by following in real time the dynamics of loss of boundary straightness in tartan mutant embryos compared to wildtype and ftz mutant embryos, we show that Tartan is not required beyond germband extension. At this stage, actomyosin enrichment at PSBs becomes regulated by Wingless signalling. We find that Wingless signalling regulates Toll-2 expression and we show that Toll-2 is required for planar polarization of actomyosin after the completion of germ-band extension. Thus the formation of contractile interfaces at PSBs depends on a dynamic set of LRR receptors cues. Our study also suggests that the number of receptor cues is small and that the receptors are interchangeable.

Dynamics of maternal gene expression in Rhodnius prolixus

The study of the developmental processes in Rhodnius prolixus has recently advanced with the sequencing of the genome. In this work, we study maternal gene expression driving oogenesis and early embryogenesis in R. prolixus. We analyze the transcriptional profile of mRNAs to establish the genes expressed across the ovary, unfertilized eggs and different embryonic stages of R. prolixus until the formation of the germ band anlage (0, 12, 24, and 48 hours post egg laying). We identified 81 putative maternal and ovary-related genes and validated their expression by qRT-PCR. Consistent with a role in oogenesis and early development of R. prolixus, we show that parental RNAi against Rp-BicD results in embryos that did not show any distinguishable embryonic structure. In this framework we propose three hierarchies of maternal genes that affect early and late oogenesis, and embryonic patterning.

From a Single Cell to Segmental Structures

Beginning with Aristotle 2400 years ago, research on crustacean embryology has a long tradition. Rathke’s 1829 landmark study on the noble crayfish initiated modern approaches. Crustaceans in general—and most of their large taxa—show a great diversity in all stages of their developmental pathways from the zygote up to the adult animal. This chapter describes the various modes of cleavage, gastrulation, germ band formation, and segmentation found in crustacean taxa. Cleavage is either total, partial, or mixed. Total cleavage can be indeterminate, without predictable cell lineage; or determinate, with a stereotyped cell division pattern. Gastrulation modes can also vary to a high degree. One finds invagination, epiboly, immigration, delamination, and a mix of some of these. Likewise, the stages of germ layer separation and the number of cells that initiate gastrulation differ. In yolk-rich eggs, a germ disk forms at the future ventral side of the embryo, and the axes and orientation of the germ are recognizable. Through elongation in the anteroposterior direction by a posterior growth zone and intercalary cell divisions, the germ disk is transformed into the germ band. As a result of a unique, stereotyped cell division pattern in the germ band of malacostracans, germ band growth and the segmentation process up to the differentiation of neuronal precursors and early limb anlagen can be analyzed at the level of individual cells. Recent morphological and molecular techniques allow a very detailed spatiotemporal resolution of developmental processes and they offer new perspectives on long-standing morphological questions.

The early Cambrian fossil embryo Pseudooides is a direct-developing cnidarian, not an early ecdysozoan

Early Cambrian Pseudooides prima has been described from embryonic and post-embryonic stages of development, exhibiting long germ-band development. There has been some debate about the pattern of segmentation, but this interpretation, as among the earliest records of ecdysozoans, has been generally accepted. Here, we show that the ‘germ band’ of P. prima embryos separates along its mid axis during development, with the transverse furrows between the ‘somites’ unfolding into the polar aperture of the ten-sided theca of Hexaconularia sichuanensis , conventionally interpreted as a scyphozoan cnidarian; co-occurring post-embryonic remains of ecdysozoans are unrelated. We recognize H. sichuanensis as a junior synonym of P. prima as a consequence of identifying these two form-taxa as distinct developmental stages of the same organism. Direct development in P. prima parallels the co-occuring olivooids Olivooides, and Quadrapyrgites and Bayesian phylogenetic analysis of a novel phenotype dataset indicates that, despite differences in their tetra-, penta- and pseudo-hexa-radial symmetry, these hexangulaconulariids comprise a clade of scyphozoan medusozoans, with Arthrochites and conulariids, that all exhibit direct development from embryo to thecate polyp. The affinity of hexangulaconulariids and olivooids to extant scyphozoan medusozoans indicates that the prevalence of tetraradial symmetry and indirect development are a vestige of a broader spectrum of body-plan symmetries and developmental modes that was manifest in their early Phanerozoic counterparts.