The Degenerate Tale of Ascidian Tails

Development and circuitry of the tunicate larval Motor Ganglion, a putative hindbrain/spinal cord homolog

The Motor Ganglion (MG) is a small collection of neurons that control the swimming movements of the tunicate tadpole larva. Situated at the base of the tail, molecular and functional comparisons suggest that may be a homolog of the spinal cord and/or hindbrain ("rhombospinal" region) of vertebrates. Here we review the most current knowledge of the development, connectivity, functions, and unique identities of the neurons that comprise the MG, drawn mostly from studies in Ciona spp. The simple cell lineages, minimal cellular composition, and comprehensively mapped "connectome" of the Ciona MG all make this an excellent model for studying the development and physiology of motor control in aquatic larvae.

Loss of collagen gene expression in the notochord of the tailless tunicate Molgula occulta

In tunicates, several species in the Molgulidae family have convergently lost the tailed, swimming larval body plan, including the morphogenesis of the notochord, a major chordate-defining trait. Through the comparison of tailless M. occulta and a close relative, the tailed species M. oculata, we show that notochord-specific expression of the Collagen Type I/II Alpha (Col1/2a) gene appears to have been lost specifically in the tailless species. Using CRISPR/Cas9-mediated mutagenesis in the tailed laboratory model tunicate Ciona robusta, we demonstrate that Col1/2a plays a crucial role in the convergent extension of notochord cells during tail elongation. Our results suggest that the expression of Col1/2a in the notochord, although necessary for its morphogenesis in tailed species, is dispensable for tailless species. This loss is likely a result of the accumulation of cis-regulatory mutations in the absence of purifying selective pressure. More importantly, the gene itself is not lost, likely due to its roles in other developmental processes, including during the adult stage. Our study further confirms the Molgulidae as an interesting family in which to study the evolutionary loss of tissue-specific expression of indispensable genes.

Developmental Table and Three-Dimensional Embryological Image Resource of the Ascidian Ascidiella aspersa

Ascidiella aspersa is an ascidian in the class of chordates—the closest relatives of vertebrates. A. aspersa is a potential model organism for bio-imaging studies due to its extremely transparent embryos as well as is a globally distributed cosmopolitan species. However, there is no standard developmental table for this organism. Here, as a first step to establish A. aspersa as a model organism, we report a standard developmental table as a web-based digital image resource. This resource used confocal laser scanning microscopy to scan more than 3,000 cross-sectional images and 3D-reconstructed images of A. aspersa embryos during embryogenesis. With reference to the standardized developmental table of Ciona intestinalis type A, 26 different developmental stages (Stages 1–26) from fertilized eggs to hatched larvae were redefined for A. aspersa. Cell lineages up to the cleavage period were annotated: The cleavage patterns, the embryonic morphology, and the developmental time were then compared with Ciona. We found that the cleavage patterns and developmental time up to the neurula period in A. aspersa were extremely conserved versus. Ciona. The ratio of the trunk and tail length in the tailbud period were smaller than Ciona indicating a relatively short tail. In addition, the timing of the bending of the tail is earlier than Ciona. This A. aspersa standard 3D digital resource is essential for connecting different omics data to different spatiotemporal hierarchies and is useful for a system-level understanding of chordate development and evolution.

Future Tail Tales: A Forward-Looking, Integrative Perspective on Tail Research

Synopsis Tails are a defining characteristic of chordates and show enormous diversity in function and shape. Although chordate tails share a common evolutionary and genetic-developmental origin, tails are extremely versatile in morphology and function. For example, tails can be short or long, thin or thick, and feathered or spiked, and they can be used for propulsion, communication, or balancing, and they mediate in predator-prey outcomes. Depending on the species of animal the tail is attached to, it can have extraordinarily multi-functional purposes. Despite its morphological diversity and broad functional roles, tails have not received similar scientific attention as, for example, the paired appendages such as legs or fins. This forward-looking review article is a first step toward interdisciplinary scientific synthesis in tail research. We discuss the importance of tail research in relation to five topics: (1) evolution and development, (2) regeneration, (3) functional morphology, (4) sensorimotor control, and (5) computational and physical models. Within each of these areas, we highlight areas of research and combinations of long-standing and new experimental approaches to move the field of tail research forward. To best advance a holistic understanding of tail evolution and function, it is imperative to embrace an interdisciplinary approach, re-integrating traditionally siloed fields around discussions on tail-related research.

An introduction to an evolutionary tail: EvoDevo, structure and function of post-anal appendages

Although tails are common and versatile appendages that contribute to evolutionary success of animals in a broad range of ways, a scientific synthesis on the topic had yet to be initiated. For our Society for Integrative and Comparative Biology (SICB) symposium we brought together researchers from different areas of expertise (e.g., robotosists, biomechanists, functional morphologists, and evolutionary and developmental biologists), to highlight their research but also to emphasize the interdisciplinary nature of this topic. The four main themes that emerged based on the research presented in this symposium are: 1) How do we define a tail? 2) Development and regeneration inform evolutionary origins of tails, 3) Identifying key characteristics highlights functional morphology of tails, 4) Tail multi-functionality leads to the development of bioinspired technology. We discuss the research provided within this symposium, in light of these four themes. We showcase the broad diversity of current tail research and lay an important foundational framework for future interdisciplinary research on tails with this timely symposium.