Endostyle cell recruitment as a frame of reference for development and growth in the Urochordate Oikopleura dioica

Nkx2-1 and FoxE regionalize glandular (mucus-producing) and thyroid-equivalent traits in the endostyle of the chordate Oikopleura dioica

The endostyle is a ventral pharyngeal organ used for internal filter feeding of basal chordates and is considered homologous to the follicular thyroid of vertebrates. It contains mucus-producing (glandular) and thyroid-equivalent regions organized along the dorsoventral (DV) axis. Although thyroid-related genes (Nkx2-1, FoxE, and thyroid peroxidase (TPO)) are known to be expressed in the endostyle, their roles in establishing regionalization within the organ have not been demonstrated. We report that Nkx2-1 and FoxE are essential for establishing DV axial identity in the endostyle of Oikopleura dioica. Genome and expression analyses showed von Willebrand factor-like (vWFL) and TPO/dual oxidase (Duox)/Nkx2-1/FoxE as orthologs of glandular and thyroid-related genes, respectively. Knockdown experiments showed that Nkx2-1 is necessary for the expression of glandular and thyroid-related genes, whereas FoxE is necessary only for thyroid-related genes. Moreover, Nkx2-1 expression is necessary for FoxE expression in larvae during organogenesis. The results demonstrate the essential roles of Nkx2-1 and FoxE in establishing regionalization in the endostyle, including (1) the Nkx2-1-dependent glandular region, and (2) the Nkx2-1/FoxE-dependent thyroid-equivalent region. DV axial regionalization may be responsible for organizing glandular and thyroid-equivalent traits of the pharynx along the DV axis.

3D reconstruction of structures of hatched larva and young juvenile of the larvacean Oikopleura dioica using SBF-SEM

The larvacean Oikopleura dioica is a planktonic chordate and an emerging model organism with a short life cycle of 5 days that belongs toTunicata (Urochordata), the sister clade of vertebrates. It is characterized by the rapid development of a tadpole-shaped body. Organ formation in the trunk proceeds within 7 h after the hatching of the tailbud larvae at 3 h after fertilization (hpf) and is completed at 10 hpf, giving rise to fully functional juveniles as miniature adult form. Serial block face scanning electron microscopy was used to acquire ~ 2000 serial transverse section images of a 3 hpf larva and a 10 hpf juvenile to characterize the structures and cellular composition of the trunk and organs using 3D images and movies. Germ cells were found to fuse and establish a central syncytial cell in the gonad as early as 10 hpf. Larval development gave rise to functional organs after several rounds of cell division through trunk morphogenesis. The feature would make O. dioica ideal for analyzing cellular behaviors during morphogenetic processes using live imaging. The detailed descriptions of the larvae and juveniles provided in this study can be utilized as the start and end points of organ morphogenesis in this rapidly developing organism.

A chordate species lacking Nodal utilizes calcium oscillation and Bmp for left-right patterning

Larvaceans are chordates with a tadpole-like morphology. In contrast to most chordates of which early embryonic morphology is bilaterally symmetric and the left-right (L-R) axis is specified by the Nodal pathway later on, invariant L-R asymmetry emerges in four-cell embryos of larvaceans. The asymmetric cell arrangements exist through development of the tailbud. The tail thus twists 90° in a counterclockwise direction relative to the trunk, and the tail nerve cord localizes on the left side. Here, we demonstrate that larvacean embryos have nonconventional L-R asymmetries: 1) L- and R-cells of the two-cell embryo had remarkably asymmetric cell fates; 2) Ca²⁺ oscillation occurred through embryogenesis; 3) Nodal, an evolutionarily conserved left-determining gene, was absent in the genome; and 4) bone morphogenetic protein gene (Bmp) homolog Bmp.a showed right-sided expression in the tailbud and larvae. We also showed that Ca²⁺ oscillation is required for Bmp.a expression, and that BMP signaling suppresses ectopic expression of neural genes. These results indicate that there is a chordate species lacking Nodal that utilizes Ca²⁺ oscillation and Bmp.a for embryonic L-R patterning. The right-side Bmp.a expression may have arisen via cooption of conventional BMP signaling in order to restrict neural gene expression on the left side.

Functional specialization of chordate CDK1 paralogs during oogenic meiosis

Cyclin-dependent kinases (CDKs) are central regulators of eukaryotic cell cycle progression. In contrast to interphase CDKs, the mitotic phase CDK1 is the only CDK capable of driving the entire cell cycle and it can do so from yeast to mammals. Interestingly, plants and the marine chordate, Oikopleura dioica, possess paralogs of the highly conserved CDK1 regulator. However, whereas in plants the 2 CDK1 paralogs replace interphase CDK functions, O. dioica has a full complement of interphase CDKs in addition to its 5 odCDK1 paralogs. Here we show specific sub-functionalization of odCDK1 paralogs during oogenesis. Differential spatiotemporal dynamics of the odCDK1a, d and e paralogs and the meiotic polo-like kinase 1 (Plk1) and aurora kinase determine the subset of meiotic nuclei in prophase I arrest that will seed growing oocytes and complete meiosis. Whereas we find odCDK1e to be non-essential, knockdown of the odCDK1a paralog resulted in the spawning of non-viable oocytes of reduced size. Knockdown of odCDK1d also resulted in the spawning of non-viable oocytes. In this case, the oocytes were of normal size, but were unable to extrude polar bodies upon exposure to sperm, because they were unable to resume meiosis from prophase I arrest, a classical function of the sole CDK1 during meiosis in other organisms. Thus, we reveal specific sub-functionalization of CDK1 paralogs, during the meiotic oogenic program.

Pleiotropy as the Mechanism for Evolving Novelty: Same Signal, Different Result

In contrast to the probabilistic way of thinking about pleiotropy as the random expression of a single gene that generates two or more distinct phenotypic traits, it is actually a deterministic consequence of the evolution of complex physiology from the unicellular state. Pleiotropic novelties emerge through recombinations and permutations of cell-cell signaling exercised during reproduction based on both past and present physical and physiologic conditions, in service to the future needs of the organism for its continued survival. Functional homologies ranging from the lung to the kidney, skin, brain, thyroid and pituitary exemplify the evolutionary mechanistic strategy of pleiotropy. The power of this perspective is exemplified by the resolution of evolutionary gradualism and punctuated equilibrium in much the same way that Niels Bohr resolved the paradoxical duality of light as Complementarity.