Never-ending story of Brachyury: From short-tailed mice to tailless primates

From Mendel's peas to genetic regulation of embryogenesis

As scientists, we all stand on the shoulders of giants. This seemingly trivial metaphor reminds us to acknowledge those who laid the groundwork for our careers, to pass on this historical knowledge to future generations, and to cultivate a sense of modesty. In this regard, two prominent biologists from the mid-19th century, Gregor Mendel and Charles Darwin, stand out. Mendel's groundbreaking work laid the foundation for our understanding of genetics, while Darwin's theory illuminated the ever-changing world of plants and animals through the process of evolution. The principles they highlighted proved fundamental for our understanding of embryogenesis. The developmental process from a fertilized egg to an adult organism is paramount for the healthy creation of progeny, whether plants or animals. As with any biological process, embryogenesis is controlled by genes and is greatly impacted by changes in the environment. Pioneers in studying the genetic contribution to normal embryonic development included, among many others, Salome Guecksohn-Waelsch, Ed Lewis, Walter Gehring, Christiane Nüsslein-Volhard, and Eric Wieschaus. They made significant contributions to our understanding of this process, and their work continues to offer valuable insights, which is the subject of this article.

A conserved sequence that sparked the field of evo-devo

The discovery that homeotic genes in Drosophila are conserved and utilized for embryonic development throughout the animal kingdom, including humans, revolutionized the fields of developmental biology and evolutionary developmental biology (evo-devo). In a pair of back-to-back papers published in Cell in 1984, researchers at the Biozentrum in Basel, Switzerland, showed that the homeobox - previously identified as a sequence shared by homeotic genes in Drosophila - was also present in the genome of diverse animals. The first paper (McGinnis et al., 1984a) showed that genomes of both invertebrates and vertebrates contain sequences that cross-hybridized with Drosophila homeobox probes. The second paper (Carrasco et al., 1984) identified a cross-hybridizing sequence in the model system Xenopus laevis. They then isolated the first vertebrate homeobox-containing gene by cloning and sequencing of the corresponding genomic region. Finally, they showed that this gene (AC1, later renamed HoxC6) was expressed during embryonic development, the first evidence that developmentally expressed Drosophila genes could be used to isolate regulators of vertebrate embryonic development. These findings led to a flurry of activity in the evo-devo field, initially focused on isolating Hox genes across diverse species, and then expanding to isolation of other gene families based on Drosophila orthologs, an approach that continues today. This led to the notion of a conserved genetic toolkit for embryonic development, currently accepted, but unexpected at the time of its discovery. We attempt to provide some context for the sea-change in thinking that these discoveries brought about by referring to Jean Piaget's theories about the sequential acquisition of scientific knowledge.

Celebrating the centennial of the most famous experiment in embryology: Hilde Mangold, Hans Spemann and the organizer

This Issue of Cells & Development celebrates the centennial of the Spemann-Mangold organizer experiment. This was the most famous experiment in embryology and its reverberations have greatly influenced developmental biology. This historical issue describes the impact of the discovery and is a prelude to the second volume of this Festschrift, which will consist of the proceedings of the international meeting to be held in Freiburg University, at the place where the organizer was discovered.