Of gradients and genes: Developmental concepts of Theodor Boveri and his students

Gradients, for decades a “dirty word” in developmental biology (Lawrence 1992), have become respectable of late and are likely to stay with us after their role in pattern formation has been demonstrated at the level of gene regulation (Driever and Nusslein-Volhard 1988). The present essay is to illuminate the often misunderstood “relativistic” gradient concepts of Theodor Boveri (who actually introduced the term Gefalle = gradient), and their influences on his students. Boveri also paved the way, by his studies on chromosomes and development (see Essays 15 and 16), for a chromosomal location of the Mendelian hereditary factors (since 1909 called genes). His most influential students reacted quite differently to his insights on both gradients and genes. Hans Spemann (1869–1941) was remarkably reluctant in linking either con-. cept with his organizing center (see Sander 1986; Wolpert 1986). Waldemar Schleip (1879–1948), however, extended Boveri’s idea of interplay between cytoplasmic factors and genes in development, yet maintained a specific “Boverian” definition of gradient. Leopold von Ubisch (1885–1965) finally achieved a synthesis linking the emerging modern concept of morphogenetic gradients with differential gene activity in order to generate spatial patterns.