Pieplow, C. | Search bioRxiv

bioRxiv2022

Molecular mechanisms of tubulogenesis revealed in the sea star hydro-vascular organ

A fundamental goal in the organogenesis field is to understand how cells organize into tubular shapes. Toward this aim, we have established the hydro-vascular organ in the sea star Patiria miniata as a model for tubulogenesis. In this animal, bilateral tubes grow out from the tip of the developing gut, and precisely extend to specific sites in the larva. This growth requires cell migration coupled with mitosis in distinct zones. Cell proliferation requires FGF signaling, whereas the three-dimensional orientation of the organ depends on Wnt signaling. Specification and maintenance of tube cell fate requires Delta/Notch signaling. Moreover, we identify target genes of the FGF pathway that contribute to tube morphology, revealing molecular mechanisms for tube outgrowth. Finally, we report that FGF activates the Six1/2 transcription factor, which serves as an evolutionarily ancient regulator of branching morphogenesis. This study uncovers novel mechanisms of tubulogenesis in vivo and we propose that cellular dynamics in the sea star hydro-vascular organ represents a key comparison for understanding the evolution of vertebrate organs. Highlights The hydro-vascular organ of the sea star presents a valuable model of tubulogenesis In this organ tube extension is driven by cell migration coupled with cell proliferation at specific growth zones The Wnt pathway controls directional outgrowth The FGF pathway promotes regionalized cell proliferation The Notch/Delta pathway is essential in cell fate repression in tubulogenesis A screen of FGF function revealed essential target gene expression, including the transcription factor Six1/2 Within a sister group to chordates, the sea star will reveal ancient mechanisms of tubulogenesis

developmental biology↗

bioRxiv2022

Sex-Specific Genes Identified in Sea Urchin Gonads are Expressed Prior to Metamorphosis

A great collaboration between the germline and somatic cells of an organism is the creation of a functional gonad. Here we begin to test this mechanism in a sea urchin by use of RNA-seq, and quantitative mRNA measurements throughout the life cycle of the sea urchin, Lytechinus variegatus (Lv). We found through de novo analyses that the ovary and testis of this echinoderm contains the many transcripts predicted for gamete morphology and function, but we also discovered many uncharacterized gene products unique to each gonad. We found that transcripts involved in glycolysis are highly enriched in the ovary compared to the testis, and that Lv has an expansion of nanos genes not observed in other sequenced echinoderms. We developed a pipeline integrating timepoint RNA-seq data throughout development to identify hallmark gene expression in gonads. We found activation of meiotic genes surprisingly early in development, and observed that candidate genes involved in sex determination are first expressed during larval growth, well before metamorphosis. We further discovered that individual larvae express varying levels of male- or female-hallmark genes before metamorphosis, including factors for the germ line, oocyte, sperm, and meiosis related genes. Together these data support the hypothesis that sex determination in the sea urchin is initiated prior to metamorphosis, from a shared profile of male and female factors, and that the many uncharacterized genes unique to the gonads may reveal unique pathways and mechanisms in echinoderm reproduction. HighlightO_LITestis and Ovary of the sea urchin, Lytechinus variegatus, have distinct transcriptome profiles C_LIO_LIL. variegatus gonad RNA-seq reveals the expression of 9 nanos genes C_LIO_LIThe ovary in the sea urchin uniquely expresses genes of the glycolytic pathway, including an ovary-specific transcript of GAPDH C_LIO_LIGenes involved in meiosis and in sex determination are expressed early in larval development C_LIO_LIFrom gene expression profiles we conclude here that sex determination is initiated prior to metamorphosis in this echinoderm C_LI

developmental biology↗