Bio Essays Vol. 4, No. 4 147

Homeo Box Fever, Extrapolation and Developmental Biology

For molecular biologists interested in development, the last two years have unquestionably been dominated by a particular event, the discovery of the ‘homeo box’. Although the history of the homeo box, as a scientific investigation, has virtually only just begun, the story already provides an interesting example of the use of language and of the roles of correlation and wishful thinking in science. As such, it reveals much about the current state of developmental biology. At the same time, the story is also a pleasing illustration of the way that scientific investigation provides a built-in check for extrapolative excess.

The term ‘homeo box’ is a generic description. It refers to a family of closely related protein-encoding sequences, of 180 bp length, found in a comparatively small number of copies in many (but not all) animal genomes. The special interest of the homeo box lies in two facts: (1) it was originally isolated as part of the sequence of a few genes whose mutant phenotypes involve either a switch of segment identity or segmental fusion in the fruit fly, and (2) the sequence shows some similarity to that of the DNA binding regions of several known transcriptional regulatory genes in prokaryotes and yeast. The implications were obvious and exciting: the homeo box was found to be an integral part of several developmentally significant genes - genesalready posited as key regulator genes in defined anatomical regions’ - and its structure was consistent with a direct role by these genes in the regulation of transcription of other genes. In effect, homeo boxcontaining genes might be the genetic keys to understanding the control of those major aspects of phenotype designated as ‘pattern formation’2. Excitement grew as similar and perhaps homologous sequences were found in a wide variety of animal genomes, whose embryology or adult pattern involved a metameric (segmented) pattern but not in two animal systems lacking ~egmentation.~ It appeared that a veritable geneticist’s ‘Rosetta tone'^ to understanding the control of animal development might have been found.

Although these expectations may prove true, there are already strong indications that the story will not turn out to be that simple. The initial homeo box sequence isolations were from genes that had something to do with segmentation. However, the expression patterns of homeo box-containing sequences in vertebrates, partic- ularly in amphibians5 and mouse embryonal carcinoma cells and mouse embryos,6* show little obvious corre- lation to the events of segment formation or pattern formation in general. The transcript patterns are indeed complex, as in Drosophih, but seem to reflect the processes of tissue-specific differentiation, a rather diff- erent matter altogether. The vertebrate homeo box- containing genes may yet show themselves to have ‘controlling’ roles in development but the connection to the processes of segmentation and pattern formation seems to be falling by the wayside (at least in its simplest version). At present, it appears just as likely that the homeo box polypeptide has a general biochemical func- tion in genes of widely different developmental

Indeed, the correlation between homeo-box-contain- ing genes and genes involved in segmentation was probably overstated in the first flush of homeo-box fever. Of the first three homeo-box sequences isolated from the Drosophih genome, two were from genetic loci affecting segment identity (homeotic genes of the Anten- napedia and bithorax complexes) and one was from a gene required for the process of segmentation itself (theftz gene of the Antennapedia gene complex.) Gene- tic evidence indicates that these processes - forming segments and setting individual segment phenotypes - are indeed separate events, involving distinctive genetic control^.^ Given the estimated number of homeo box sequences in the fruit-fly genome (from hybridization data, a limit of about 20 is suggested) and the total number of genes known to be required for normal segmental patterning, it seems probable that there are not enough homeo boxes to go around to fill this set of genes (though highly diverged copies may have escaped detection). If so, the homeo box is one key, but not the

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148 Bio Essays Vol. 4, No. 4

EDITORIAL

key to understanding mechanisms of pattern formation in the fruit-fly.

Beyond this qualification of the significance of the homeo box, there is a larger question about the homeo box that will require more thought on everyone’s part. Isn’t it rather a large leap from transcription-regulation, the assumed forte of homeo box-containing genes, to explaining the large-scale interactions and movements of cells that comprise morphogenesis and pattern for- mation? It has been argued that morphogenesis and differentiation are essentially equivalent, with ‘morpho- genesis and differentiation simply being a consequence of differential expression of genes that control develop- mental processes ’.lo The shortcoming of this view is that it makes morphogenesis a trivial consequence of differential gene expression. Furthermore, it may not even be true. Highly plausible models of morphogenesis in animal embryos can be constructed solely on the basis of mechanochemical princip1es.l’ However, even if it should be demonstrated that some differential gene expression is required for morphogenesis in embryos, that knowledge alone will not exphin the complex cell group movements that comprise morphogenesis, any more than simply knowing the components of an auto- mobile can explain how the automobile works. In the end, if we want to understand the generation of pattern and shape in embryos, we will need a set of observations that no description of gene transcription mechanisms by itself can ever provide.

None of the above should be taken as dismissal of the homeo box. The subject is an interesting one and the genes that contain the homeo box are worth all the attention they are getting; the results are bound to have some, perhaps major, developmental significance. However, the rush to pin so much on the homeo box, almost from the first Southern blots showing their existence, reveals the eagerness, even the desperation, of molecular-developmental biologists to find universal or, at least, general principles of genetic control in develop- ment. ‘Master control’ genes may indeed operate in similar fashions in both invertebrates and vertebrates, but it seems just as likely that small genome animals, whose developmental programs contain a large built-in maternal component (e.g. fruit-flies) will employ rather different strategies from those systems (e.g. chickens, mice and men) that have large genomes and little preexisting maternal input for their embryos. In the

pursuit of meaningful general principles, it remains important for molecular biologists to pay attention to the basic distinctions between different developmental processes that are used by developmental biologists. In the long term, clear answers are bound to emerge, but in the short term the blurring of meanings simply muddies the waters for everyone; it may even make it harder to focus priorities in the choice of experiments.

REFERENCES 1 GARCIA-BELLIDO, A. (1975). Genetic control of wing disc develop- ment in Drosophila. In Cell Patterning, CIBA Symposium no. 29 (ed. R. Porter and K. Elliott). Elsevier/North Holland, Amsterdam. 2 STRUHL, G. (1985). A universal genetic key to body plan? Nature 310, 10-11. 3 MCGINNISS, W., GARBER, R. L., WIRZ, J., KUROIWA, A. & GEHRING, W. J. (1984). A homologous proteincoding sequence in Drosophila homeotic genes and its conservation in other metazoans. Cell 37,403-408. 4 SLACK, J. M. W. (1984). A Rosetta stone for pattern formation in animals? Nature 310, 364-365. 5 CARRASCO, A. E., MCGINNIS, W., GEHRING, W. J. & DE ROBERTIS, E. M. (1984). Cloning of an X. laeuis gene expressed diring early embryogenesis coding for a peptide region homologous to Drosophila homeotic genes. Cell 37, 409414. 6 HART, C. P., AWGULEWITSCH, A., FAINSOD, A., MCGINNIS, W. & RLJDDLE, F. H. (1985). Homeo box gene complex on mouse chromo- some. 11. Molecular cloning, expression in embryogenesis, and homology to a human homeo box locus. Cell 43,9-18. 7 HAUSER, C. A., JOYNER, A. L., KLEIN, R. D., LEARNED, T. K., MARTIN, G.R. & TJIAN, R. (1985). Expression of homologous homeo-boxcontaining genes in differentiated human teratocar- cinoma cells and mouse embryos. Cell 43, 19-28. 8 RAFF, E. C. & RAFF, A. (1985). Possible function of the homeo box. Nature 313, 185. 9 NUSSLEIN-VOLHARD, C. & WIFSCHAUS, E. (1980). Mutations affecting segment number and polarity in Drosophila. Nature 287,

10 ROBERTSON, M. (1985). Mice, mating types and molecular mechanisms of morphogenesis. Nature 318, 12-13. 11 ODELL, G. M., OSTER, G., ALBERCH, P. & BURNSIDE, B. (1981). The mechanical basis of morphogenesis. I. Epithelial folding and invagination. Dev. Biol. 85, 446462.

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