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Embryonic Transcription and the Control of Developmental Pathways

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Embryonic Transcription and the Control of Developmental Pathways Copyright 0 1996 by the Genetics Society of America Embryonic Transcription and the Control of Developmental Pathways Eric Wieschaus Department of Molecular Biology, Princeton University, Princeton, New Jersey 08544 It appears that the initial steps up to the [sea urchin] selected to play unique, controlling roles in develop- blastula stage are independent of the quality of the nu- ment. In this admittedly extreme reformulation of BO- clear substance, even though it is essential that the nu- VERI’S observation, I will restrict consideration to those clear substance be of a kind capable of existing in the egg. The necessity for particular chromosomes becomes genes whose products must be supplied zygotically and apparent first with the formation of the primary mesen- to organisms that have minimized their transcriptional chyme and from then on shows up in all processes asfar requirements during embryogenesis. I will first present as development can be observed. With respect to reasons why such a division of labor makes teleological those characters in which we are able to recognize indi- sense and then examine the data from Drosophila to vidual variations, the nuclear substance and not the cyte plasmic cell substance imposes its specific character on determine the extent to which gene activityactually the developing trait. obeys these expectations. Then I will discuss the excep- . Earlier stages, for which according to our results, tions, i.e., certain wellcharacterized cases where partic- specific chromosomes are not necessary, demonstrate a ular Drosophila genes are supplied zygotically but do purely maternal character. I would like to ascribe to not seem to play controlling roles. Lastly, the Drosoph- the cytoplasm of the sea urchin egg only the initial and simplest properties responsible for differentiation. it ila data will be compared to that from other organisms. provides the most general basic form, the framework The simple model: In organisms where embryonic within which all specific details are filled in by the nu- development is rapid and occurs with no increase in size cleus. THEODORBOVERI 1902 before hatching from the egg, it will be advantageous to maximize maternal contributions,because the duration VEN before the molecular nature of the gene was of oogenesis is often much longer than embryogenesis E understood, embryos were known to get the prod- and the ovary provides a more sophisticated and effi- ucts they need from two different sources. In modern cient synthetic machinery. Evolutionary selection for terms, the unfertilized egg contains large stores of ma- efficiency will maximize such maternal contributions. ternal RNA and protein that arederived from transcrip- One major limitation to maternal supplies is the dura- tion during oogenesis. Initially, these proteins provide tion of embryonic development. Maternal supplies the basic machinery for all cellular events. At some must be sufficient to last until a larva hatches and can point these maternally supplied products are supple- obtain additional nutrients from the environment. In mented by transcription from the embryo’sown ge- principle, maternal contributions might consist only of nome (the “nuclear substance” in BOVERI’Sformula- nutrient material (“yolk”) that could be converted by tion). This occurs at different stages in different the embryo to a wide variety of gene products. It may organisms, but significant transcription is usually de- be more efficient, however, thatthe “nutrients” in- tected by the blastula or blastoderm stage. Maternal clude RNA directly encodingthe individual compo- geneproducts persist after this point, however, and nents of most biological processes, including the cellu- most processes during embryogenesis involve both ma- lar machinery required for transcription, translation, ternal and zygotic components. energy utilization and basic cell structure. As the quote from BOVERI’S classicpaper indicates, On the other hand, all embryos have been shown to embryologists at the turn of the century were aware of require certain RNAs supplied by transcription of their both maternal and zygotic contributions to embryonic own genome. Early indications ofthis requirement development. They even considered the possibility that came from the seminal work of BOVERI, as well as spe- maternal components might play different roles in de- cies hybridization experiments and actinomycin and a- velopment than zygotic components. In theeighty years amantin studies (reviewed in DAVIDSON1986). If supply- that followed BOVERI’S work,it became clear that cer- ing gene products maternally is so advantageous, what tain zygotically activegenes play major controlling roles. can zygotic transcription do that just storing maternal Landmark studies were POULSON’S(1940) analysis of transcribed gene products can’t?One possibility is that chromosomal deletions in the Notch region and LEWIS’ zygotic transcription allows much more precise expres- (1978) characterization of the Bithorax complex. sion, putting particular gene products in one cell but In the following pages, I will address the generality not in its immediate neighbors. Such precision is proba- of a hypothesis only partly implied by the BOVERIquote, bly not necessary for most gene products. If a gene namely that in organisms where zygotically required product is needed in some cells but not others,may it be gene activities are rare, all such activitieshave been sufficient (although profligate) to supply it uniformly to Genetics 142 5-10 (January, 1996) 6 E. Wieschaus the whole egg. A patterned expression only becomes from all these studies is that only a small number of important when the absence of a particular gene product loci need betranscribed in the embryo itself to establish is as significant as its presence. This might be the case normal morphology. Even if the Heidelberg screens if the gene product is used as a developmental switch, detected only half the zygotically important loci, the such that its expression in one cell causes that cell to number of such genes would increase only to 300, fewer assume a fate different from its neighbors. While it is than 2% of the estimated 20,000 molecularly defined possible to localize maternal gene products to particu- transcription units and only a small fraction of the tran- lar regions of the egg cytoplasm, localtranscription may scripts and proteins found in the embryo. The remain- allow more precision, and may therefore be more useful der of these proteins and RNAs, and thus the majority as spatial patterns become complex. of the components involved in any particular embry- This view has certain practical consequences. Over onic process, must be supplied maternally. the course of evolution, the efficiencies of maternal If early acting, zygotically transcribed genes are rare, contribution will reduce zygotic transcription to a mini- it should be possible to delete large portions of the mum. That minimum will define a set of genes, each genome with little effect on early stages ofdevelopment. of which is limiting for a specific process, such that its We tested this possibilityby examining the development presence or absence determines when and where in the of embryos homozygous for various cytologically de- embryo that process takes place. Not all controlling fined deletions, and eventually used translocations and elements will be zygotic; a process could be initiated by chromosomal rearrangements to generate embryos de- a localized maternal RNA or ligands (STJOHNSTON and leted for overlapping regions spanning the entire Dro- NUSSLEIN-VOLHARD1992). However, if there is a zygotic sophila genome (MERRILLet al. 1988; WIESCHAUSand component in a process, its expression is likely to play SWEETON1988). In general, such deficiency embryos a controlling role in all events downstream to it. In this show phenotypes that could be explained by point mu- view, selection during evolution has solved the research tations previously located to the deleted regions. There agenda of a large fraction of developmental biologists: are exceptions, notably a group of seven early acting in a complex process involving many gene products, it genes required for cellularization at the onset of cycle has sorted out which one actually plays the controlling 14. Ongoing work in my lab suggests the existence of role. Therefore, to understand how a particular process certain regions with previously undescribed effects on is controlled in the embryo, good candidate genes gastrulation and early morphogenetic movements (E. would be those whose products need to be supplied WIESCHAUS,unpublished results). In general, however, zygotically. For organisms where genetic analyses are such newly discovered loci have been rare, confirming possible, identification of candidates relativelyis the preponderanceof maternal geneproducts and scar- straightfornard; if the genes are removed by mutation, city of gene products that must be supplied by zygotic homozygous mutant embryos will develop abnormally. transcription. Consistentwith behavior as controllingelements, Transcripts that must be supplied to the embryo zy- genes that must be supplied zygotically represent only gotically appear to play special roles in development: a small fraction of the Drosophila genome: The muta- Mostof the mutations identified in theHeidelberg genesis screens that CHRISTIANENUSSLEIN-VOLHARD, screens produce discrete phenotypes, such that differ- GERDJURGENS and I began more than fifteen years ago entiation of most
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