The Amphioxus Architect
How Professor T.C. Tung Unlocked Secrets of Vertebrate Origins
Introduction: A Thought Experiment
Imagine you could rewind the evolutionary clock by 550 million years and witness the very first creatures that would eventually give rise to all vertebrates, including humans. What would they look like? How would they develop from a single cell into a complex organism? This isn't just speculative fantasy—scientists have found a living window into this distant past in the form of a small, unassuming marine creature called amphioxus (also known as lancelet). For decades, the secrets of this animal's development remained largely mysterious until Professor T.C. Tung, a pioneering Chinese embryologist, dedicated his career to unraveling its mysteries 1 .
At a time when China was largely isolated from the international scientific community, Professor Tung established innovative techniques and made fundamental discoveries about how life develops, using this humble organism to answer profound questions about our own evolutionary history. His work, conducted mostly in the 1950s-1970s, laid the foundation for understanding how the vertebrate body plan evolved from simpler chordate ancestors 1 4 .
Evolutionary Significance
Amphioxus represents a crucial evolutionary link between invertebrates and vertebrates, providing insights into our own developmental origins.
Experimental Innovation
Professor Tung developed pioneering microsurgical techniques to manipulate amphioxus embryos at various developmental stages.
Why the Amphioxus? A Living Fossil Holds the Key
Amphioxus represents an evolutionary missing link—a modern representative of the most basally divergent group of chordates, the lineage that would eventually give rise to vertebrates. These small, fish-like filter feeders possess the characteristic chordate features including a dorsal hollow nerve cord, notochord (a precursor to our backbone), pharyngeal slits, and segmented muscles 4 .
What makes amphioxus so valuable to science is its unique position in the tree of life. As the earliest diverging chordate lineage, it provides the best proxy for what the ancestor of all chordates, including vertebrates, might have looked like approximately 520 million years ago 4 . Amphioxus shares with vertebrates a similar body plan but lacks many complex characteristics such as paired sensory organs (image-forming eyes or ears), paired appendages, and migrating neural crest cells 4 .
The genomic simplicity of amphioxus is equally important. Unlike vertebrates, whose genomes underwent two complete rounds of duplication, the amphioxus genome has remained largely intact, maintaining a single copy of many genes that have multiple counterparts in vertebrates 4 5 . This makes it an ideal model for tracing the evolutionary history of gene families and regulatory networks.
Amphioxus (Branchiostoma lanceolatum), a key model organism for understanding vertebrate evolution.
Key Chordate Features in Amphioxus:
- Dorsal hollow nerve cord
- Notochord
- Pharyngeal slits
- Segmented muscles
Missing Vertebrate Complexities:
- Paired sensory organs
- Paired appendages
- Migrating neural crest cells
- Complex brain regions
The Scientist Behind the Legacy: Professor T.C. Tung
Professor T.C. Tung (1902-1979) was a prominent experimental embryologist whose career spanned a transformative period in biology. Born in Jin County, Zhejiang Province, China, he obtained his Bachelor's degree from the Department of Biology at Fudan University in Shanghai in 1927 before continuing his education in Europe 1 . He earned his Doctor of Science degree in 1934 from the Université Libre de Bruxelles in Belgium, where he studied under Professors A. Brachet and A.M. Dalcq 1 .
Education Timeline
1927
Bachelor's degree from Fudan University
1934
Doctor of Science from Université Libre de Bruxelles
1934
Returned to China as Full Professor
Tung's international training continued with working visits to the Institute of Marine Biology in France and a training course at Cambridge University in the UK before he returned to China in 1934 as a Full Professor 1 . Despite taking on significant administrative roles throughout his career—including Vice-President of Shandong University, Director of the Institute of Oceanology, and Vice-President of the Chinese Academy of Sciences—he remained dedicated to hands-on research, spending most of his life conducting bench work in his laboratories 1 .
Research Interests
- Determination of the egg axis and symmetry planes
- Early differentiation and organizing substances
- Induction between embryonic cells
- Immunological studies on nuclear transplanted eggs
- Cell fusion in various animal models
- Amphioxus embryology 1
The Pivotal Experiment: Isolated Blastomeres and Developmental Potential
One of Professor Tung's most significant contributions to amphioxus embryology was his elegant series of experiments investigating the developmental potential of isolated blastomeres (early embryonic cells). These studies addressed a fundamental question in developmental biology: When do embryonic cells become committed to specific fates? 7
Methodology: Surgical Precision on a Microscopic Scale
Professor Tung and his colleagues employed incredibly skillful microsurgical techniques to manipulate amphioxus embryos at various early stages of development 1 . The general approach involved:
Microsurgical separation
Using fine glass needles and micro-pipettes, Tung carefully separated individual blastomeres from embryos at specific developmental stages (2-cell, 4-cell, 8-cell, up to 32-cell stages) 7 .
Observation and analysis
The development of these isolated cells was meticulously observed and compared to normally developing control embryos 7 .
Results and Analysis: A Window into Developmental Flexibility
Professor Tung's experiments yielded fascinating insights into the developmental capabilities of amphioxus embryos:
| Stage of Isolation | Developmental Outcome | Key Observations |
|---|---|---|
| 2-cell stage | Each blastomere developed into a complete, though smaller, larva | Demonstrated high regulatory capacity early in development |
| 4-cell stage | Isolated blastomeres frequently formed complete larvae | Regulatory ability remained strong but began to diminish |
| 8-cell stage | Isolated blastomeres typically formed partial structures | Onset of restriction in developmental potential |
| 32-cell stage | Isolated blastomeres formed only specific tissue types | Clear evidence of established cell fate determination |
These findings revealed that amphioxus embryos initially possess a strong regulative capacity—the ability to adjust their development when parts are removed or rearranged. This regulative ability gradually decreases as development proceeds, with cells becoming increasingly committed to specific developmental pathways 7 .
Comparison of Developmental Patterns in Different Organisms
| Organism | Early Development Pattern | Key Characteristics |
|---|---|---|
| Amphioxus | Initially regulative, becoming mosaic | Flexible early development with progressive fate restriction |
| Frog | Mosaic development | Earlier commitment to specific cell fates |
| Sea urchin | Highly regulative | Isolated blastomeres can form complete larvae even at later stages |
Tung's work with amphioxus provided crucial evidence that all chordates likely share a common developmental strategy wherein regulative patterns of development gradually transition to mosaic patterns as embryogenesis proceeds 3 . This helped resolve earlier controversies between the "mosaic" theory of development (associated with Wilhelm Roux) which proposed that cells separate hereditary materials in different amounts to daughter cells, and the "regulative" theory (supported by Hans Driesch) which emphasized the ability of cells to adjust their developmental fate based on context 3 .
The Scientist's Toolkit: Essential Materials and Methods in Amphioxus Research
Professor Tung's groundbreaking work required the development and mastery of specialized techniques and materials. The following table summarizes key components of the amphioxus researcher's toolkit, both in Tung's time and in modern applications:
| Item | Function/Application | Example from Tung's Research |
|---|---|---|
| Microsurgical tools | Fine glass needles and pipettes for embryo manipulation | Skillful separation of individual blastomeres at specific developmental stages 1 |
| Seawater-based media | Maintenance of embryos and larvae in physiological conditions | Filtered seawater, sometimes with dilution (7:1 seawater:distilled water) 6 |
| Agarose-coated dishes | Prevention of embryo adhesion to container surfaces | Petri dishes coated with 1% agarose to prevent sticking of eggs 6 |
| Fixation solutions | Preservation of embryonic structures for detailed analysis | 4% MOPS-PFA buffer, pH 7.5; methanol series (70%, 50%, 25%) 6 |
| Clearing reagents | Rendering opaque specimens transparent for deep imaging | CUBIC1 (urea, 4NTEA, Triton X-100) and CUBIC2 (urea, TEA, sucrose) solutions 6 |
| Immunohistochemistry reagents | Specific labeling of cellular structures and proteins | Antibodies against acetylated-α-tubulin (neuronal scaffold) and melanopsin (photoreceptors) 6 |
Lasting Impact and Modern Applications
Professor Tung's contributions extend far beyond the specific findings of his experiments. His work established amphioxus as a viable and valuable model system for experimental embryology, developing practical methods for collecting and maintaining these animals in laboratory settings 1 . At a time when scientific communication between China and the rest of the world was limited, he built a robust research program that would inspire future generations of evolutionary developmental biologists.
The questions that drove Tung's research continue to resonate in modern science. Contemporary studies have confirmed and expanded upon his findings, showing that:
- The Nodal-Activin signaling pathway functions as a conserved neural induction signal in chordates, including amphioxus 9
- The amphioxus dorsal blastopore lip is homologous to the vertebrate "Spemann organizer" - a key region responsible for triggering neural tissue formation 9
- Single-cell analyses reveal three developmental origins for the vertebrate nervous system, with an anterior FoxQ2-dependent mechanism deeply conserved in invertebrates 8
The "amphioxus polymorphism paradox" that intrigued Tung and his contemporaries—the surprising contrast between morphological similarity and high genetic diversity among amphioxus populations—continues to be investigated with modern genomic tools 5 . Recent whole-genome resequencing has revealed that while amphioxus possesses an exceptionally high number of genetic polymorphisms (approximately one single-nucleotide polymorphism every 30-34 bases), these variations have minimal impact on protein structure and function, explaining how morphological conservation persists despite genetic diversity 5 .
Research Impact Timeline
1930s-1970s
Tung establishes amphioxus as experimental model
1980s-1990s
Molecular techniques confirm Tung's findings
2000s-Present
Genomic era expands on Tung's foundational work
Conclusion: An Enduring Legacy
Professor T.C. Tung's work on amphioxus embryology represents a remarkable convergence of skilled experimentation, biological insight, and evolutionary perspective. At a time when molecular biology was beginning to transform our understanding of life, his careful microscopic manipulations revealed fundamental principles of chordate development that continue to inform evolutionary biology today.
His research philosophy—emphasizing direct observation, meticulous experimentation, and evolutionary context—provides a timeless model for scientific inquiry. The techniques he pioneered for studying amphioxus development have evolved into sophisticated modern methodologies, but the fundamental questions he addressed remain central to evolutionary developmental biology.
As we continue to unravel the genetic and developmental mysteries of vertebrate origins, we stand on the foundation built by Professor Tung and other pioneering embryologists who recognized the extraordinary value in studying seemingly ordinary creatures. In the simple, elegant development of amphioxus, we find echoes of our own embryonic beginnings and glimpses into the deep evolutionary past that shaped all chordates, including humans.
The story of Professor Tung's work with amphioxus reminds us that profound insights often come from studying nature's simplest examples, and that dedication to fundamental questions can yield understanding that transcends generations and scientific paradigms.