J. Castelligair, IMPLICATIONS OF THE SPATIAL AND TEMPORAL REGULATION OF HOX GENES ON DEVELOPMENT AND EVOLUTION, The International journal of developmental biology, 42(3), 1998, pp. 437-444
Nearly 20 years have passed since Ed Lewis revealed the importance of
Hox genes in the specification of different segments in the anterior-p
osterior axis of the fly. Pioneering studies by several authors, among
others Garcia-Bellido and his student Gines Morata, helped to elabora
te a theory of segmental specification that was strengthened with the
arrival of molecular techniques to the field of Developmental Biology.
The conservation of Hox genes in metazoans at the level of sequence,
function and complex organization has resulted in the export of this D
rosophila theory as a paradigm to interpret the development of axial s
pecification in organisms less amenable to experimental study. There a
re two main ways to interpret how Hox genes work in Drosophila. One co
nsidering Hox genes as ''segment identity'' factors giving global prop
erties to the segments in which they are active. Another considering H
ox genes as encoding spatially restricted transcription factors requir
ed for a number decisions taken at the cellular level. Here I use publ
ished and unpublished experimental data to illustrate that early activ
ation of the Hox genes does not establish a gene code that leads to ''
segment identity''. I will stress the point that Hox expression patter
ns develop with the embryo, that there are many genes involved in this
modulation, and that the changing pattern of expression is important
to achieve the final shape of the animal. I will show that, by interpr
eting Hox gene function in this way, some apparently paradoxical resul
ts in the Hox field can be answered. Finally, I discuss the implicatio
ns of dynamic Hox gene expression on the evolution of segment morpholo
gy.