Can you change a chicken into a frog, a fish or a chameleon?

Gastrulation is one of the most important phases in early embryonic development. Before gastrulation, vertebrate embryos are simple two-dimensional sheets of cells. By the end of gastrulation, an embryo will have begun to differentiate distinct cell types, set up the basic axes of the body and internalize some of the precursors for organs in a three-dimensional structure. Amniotes, like chickens and humans, will have developed a primitive streak, the precursor to the brain and skin, while fish and amphibians will have developed a spherical-shaped blastopore.

Gastrulation is a feat of self-organization, requiring the ballet-like coordinated movements of hundreds to tens of thousands of cells.

But, despite its importance in development, scientists only partially understand the underlying mechanisms that coordinate this large-scale movement of cells.

Now a team of researchers from the Harvard, the University of California San Diego and the University of Dundee in the U.K., have developed a theoretical framework that can reproduce and predict the patterns associated with gastrulation in a chicken embryo.

Building on their experimental findings published earlier this year in Science Advances, and using a combination of theory and experiment, the researchers demonstrated that small changes in cell parameters and behavior can have a dramatic impact on the resulting gastrulation patterns.

The new research was also published in Science Advances.

"Linking the developmental processes underlying morphogenesis in an organism to the variations of these same processes across evolution is an old question in biology," said L. Mahadevan, the Lola England de Valpine Professor of Applied Mathematics at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), and Professor of Organismic and Evolutionary Biology, and of Physics in the Faculty of Arts and Sciences (FAS) and senior author of the paper.

"Moving away from the gene-centric molecular focus on this question, we asked if there are emergent biophysical principles that operate on the multi-cellular level and can help explain the self-organized gastrulation patterns from both a developmental and an evolutionary perspective."

Working closely with experimentalists, the researchers developed a theoretical and computational model that could recreate the movement of the epithelial layer of cells in chick embryos during gastrulation.

The team then identified two parameters -- one related to the initial distribution of cells in an embryo and the other related to cell behavior -- to tweak during gastrulation.

"When we changed these two parameters in the model, which was only informed by the mechanisms of chick development, it remarkably showed gastrulation patterns naturally seen in other species," said Mattia Serra, first author of the paper and former Schmidt Science fellow in the Mahadevan group.

Serra is an Assistant Professor in Physics at UC San Diego.

Consistent with the computational results, experiments show that perturbing the same parameters in vivo in a chick embryo caused the chick to form a disk-like blastopore, seen in frogs, a ring-like circular primitive streak, like those seen in fish, or an elongated elliptical canal-like streak seen in chameleon.

"Our work suggests that the general biophysical principles underlying active self-organized flows and forces during embryogenesis have the power to explain developmental processes and their evolutionary variations across different species of vertebrates," said Mahadevan.

"We were surprised to see how simple mechanochemical rules could predict highly distinct coherent flows of thousands of cells measured in a living chick embryo," said Serra.

This research not only sheds light on the principles for self- organization in early development but could also help researchers understand the evolutionary history of developmental processes and suggest ways to control the development of synthetic organoids.

This research looked at the early stages of development, when the embryo is just transforming from two-dimensions into three.

Next, Mahadevan wants to understand what happens when the embryo starts to fold and buckle to create the more complex shapes of organs and eventually whole organisms.

"Using our knowledge of developmental processes at the molecular and cellular level, we eventually hope to provide an integrated framework for how cells form into tissues and tissues into organs, towards a better understanding of morphogenesis," said Mahadevan.

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Chicken breeding in Japan dates back to fourth century BCE

Conclusive evidence of chicken breeding in the Yayoi period of Japan has been discovered from the Karako-Kagi site.

The chicken is one of the most common domesticated animals, with a current estimated population of over 33 billion individuals. They are reared for their meat and eggs, and may be kept as pets.

The chicken is believed to have been domesticated in Southeast Asia about 3500 years ago, following which they were carried to all corners of the world. The exact date of introduction of chicken breeding to Japan is under debate, as there are no historical records and archeological evidence is inconclusive.

Professor Masaki Eda at the Hokkaido University Museum led a team to uncover the earliest conclusive evidence of chicken breeding in Japan. The findings, which show chickens were bred in the Karako-Kagi site, a settlement from the Yayoi period [5th century BCE to around 2nd century BCE], were published in the journal Frontiers in Earth Sciences.

"Chickens and their wild relatives belong to a family of birds called Phasianids, which includes pheasants, turkeys and quail," explains Eda. "Bones of juvenile phasianids recovered from archeological sites could not indisputably be identified as belonging to chickens or to similarly sized wild pheasants. Identification of juveniles is important, as it would indicate that breeding of chickens took place."

The Karako-Kagi site, in what is now Tawaramoto Town, Nara Prefecture, is considered to be a settlement that played the role of a leader of the Kinki region during the Yayoi period. There are multiple archeological digs in the area; one such dig, at the 58th research point, yielded ten phasianid bones, four of which belonged to juvenile birds.

The team used a technique called Zooarchaeology by Mass Spectrometry (ZooMS) to analyze the collagen in two of the juvenile phasianid bones. Previous work by Eda had shown that domestic chicken and Japanese wild pheasants had different ZooMS fingerprints; ZooMS revealed that both the two bones belonged to chickens. The collagen from one of the bones was also carbon dated to 381-204 BCE, corresponding to the middle Yayoi period.

"Ten of the eleven previously-discovered bones of adult chickens from this period have all belonged to males; hence, it was thought chicken breeding could not have occurred on the Japanese archipelago," Eda elaborated. "By identifying bones from juvenile chickens, we provide clear evidence that breeding did occur in that time period -- which is also the earliest time chickens could have been introduced to Japan. In addition, Karako-Kagi is considered an important trade hub of the Yayoi period, so there is a possibility that this status is a factor in chicken breeding during the period."

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New evidence about when, where, and how chickens were domesticated

New research transforms our understanding of the circumstances and timing of the domestication of chickens, their spread across Asia into the west, and reveals the changing way in which they were perceived in societies over the past 3,500 years.

Experts have found that an association with rice farming likely started a process that has led to chickens becoming one of the world's most numerous animals. They have also found evidence that chickens were initially regarded as exotica, and only several centuries later used as a source of 'food'.

Previous efforts have claimed that chickens were domesticated up to 10,000 years ago in China, Southeast Asia, or India, and that chickens were present in Europe over 7,000 years ago.

The new studies show this is wrong, and that the driving force behind chicken domestication was the arrival of dry rice farming into southeast Asia where their wild ancestor, the red jungle fowl, lived. Dry rice farming acted as a magnet drawing wild jungle fowl down from the trees, and kickstarting a closer relationship between people and the jungle fowl that resulted in chickens.

This domestication process was underway by around 1,500 BC in the Southeast Asia peninsula. The research suggests that chickens were then transported first across Asia and then throughout the Mediterranean along routes used by early Greek, Etruscan and Phoenician maritime traders.

During the Iron Age in Europe, chickens were venerated and generally not regarded as food. The studies have shown that several of the earliest chickens are buried alone and un-butchered, and many are also found buried with people. Males were often buried with cockerels and females with hens. The Roman Empire then helped to popularise chickens and eggs as food. For example, in Britain, chickens were not regularly consumed until the third century AD, mostly in urban and military sites.

The international team of experts re-evaluated chicken remains found in more than 600 sites in 89 countries. They examined the skeletons, burial location and historical records regarding the societies and cultures where the bones were found. The oldest bones of a definite domestic chicken were found at Neolithic Ban Non Wat in central Thailand, and date to between 1,650 and 1,250 BC.

The team also used radiocarbon dating to establish the age of 23 of the proposed earliest chickens found in western Eurasia and north-west Africa. Most of the bones were far more recent than previously thought. The results dispel claims of chickens in Europe before the first millennium BC and indicate that they did not arrive until around 800 BC. Then, after arriving in the Mediterranean region, it took almost 1,000 years longer for chickens to become established in the colder climates of Scotland, Ireland, Scandinavia and Iceland.

The two studies, published in the journals Antiquity and Proceedings of the National Academy of Sciences, were carried out by academics at the universities of Exeter, Munich, Cardiff, Oxford, Bournemouth, Toulouse, and universities and institutes in Germany, France and Argentina.

Professor Naomi Sykes, from the University of Exeter, said: "Eating chickens is so common that people think we have never not eaten them. Our evidence shows that our past relationship with chickens was far more complex, and that for centuries chickens were celebrated and venerated."

Professor Greger Larson, from the University of Oxford, said: "This comprehensive re-evaluation of chickens firstly demonstrates how wrong our understanding of the time and place of chicken domestication was. And even more excitingly, we show how the arrival of dry rice agriculture acted as a catalyst for both the chicken domestication process and its global dispersal."

Dr Julia Best, from Cardiff University said: "This is the first time that radiocarbon dating has been used on this scale to determine the significance of chickens in early societies. Our results demonstrate the need to directly date proposed early specimens, as this allows us the clearest picture yet of our early interactions with chickens."

Professor Joris Peters, from LMU Munich and the Bavarian State Collection of Palaeoanatomy, said: "With their overall highly adaptable but essentially cereal-based diet, sea routes played a particularly important role in the spread of chickens to Asia, Oceania, Africa and Europe."

Dr Ophélie Lebrasseur, from the CNRS/Université Toulouse Paul Sabatier and the Instituto Nacional de Antropología y Pensamiento Latinoamericano, said: "The fact that chickens are so ubiquitous and popular today, and yet were domesticated relatively recently is startling. Our research highlights the importance of robust osteological comparisons, secure stratigraphic dating and placing early finds within their broader cultural and environmental context."

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