Dual Role for the zeste-white3/shaggy-Encoded Kinase in Mesoderm and Heart Development of Drosophila MAIYON PARK, TYAMAGONDLU V. VENKATESH, AND ROLF BODMER* Department of Biology, University of Michigan, Ann Arbor, Michigan ABSTRACT A Drosophila homolog of the serine/ threonine kinase G SK-3 , encoded by the zest-w hite3 / shaggy gene ( zw3 ), has been implicated as a mater- nally provided antagonist of zygotic signaling by the secreted segmentation gene wingless ( wg ). The wg signal apparently causes a spatially localized inhibition of the ubiquitous repressor function of zw3. This double negative mechanism of signal transduction has been shown to mediate the patterning function of W g in a number of developmental processes. Although wg is absolutely required for specifying the heart progenitors within the mesoderm of Drosophila, the role of zw3 in this process has been unclear. Here, we present evi- dence that zw3 has a dual role in mesoderm develop- ment: (1 ) zw3 acts as an antagonist in cardiogenic wg signal transduction, and (2) zw3 also seems to be required to promote positively the formation of a larger mesodermal region, the tinman- and dpp-dependent ‘‘dorsal mesoderm,’’ which is a prerequisite not only for cardiogenesis, but also for visceral mesoderm formation. W e also demonstrate that a recently identified proximal component of the wg cascade, which is a transcription factor encoded by pangolin/ dTCF ( dTCF ), also seems to mediate wg -dependent cardiogenesis. Further, we present evidence that N otch ( N ), which opposes wg signaling in other situations, is unlikely to be directly involved in the cardiogenic wg pathway, but seems to have multiple other myogenic functions, one of which is to inhibit mesoderm differentiation altogether, when overexpressed as a constitutively active form. Dev. Genet. 22:201–211, 1998.  1998 W iley-Liss, Inc. Key words: cardiogenesis; wingless; G SK-3 -kinase; N otch; pangolin/ TCF/ LEF-1; pattern formation INTRODUCTION The mechanisms involved in mesoderm patterning and subsequent heart formation are beginning to be understood in Drosophila [for review see Bodmer et al., 1997]. As a result of maternal mechanisms of dorsal- ventral axis formation [for review see Govind and Steward, 1991; St. Johnston and Nusslein-Volhard, 1992], one of the first zygotic genes, twist (twi ), a bHLH-type transcription factor, initiates its expression in the mesoderm anlagen in the ventral third portion of the blastoderm embryo [Thisse et al., 1987, 1988]. The absolute requirement of twist for initial mesoderm formation comes from the observation that no meso- derm is formed in twist mutant embryos [Simpson, 1983]. Shortly after the mesodermal anlagen have invaginated during gastrulation along the ventral mid- line, they migrate dorsally in close apposition to the ectoderm. A gene, which is activated in a twi-dependent fashion before gastrulation and the beginning of dorsal migration, is heartless (htl ), which encodes a fibroblast growth factor receptor (FGFR/DFR1) [Shishido et al., 1993]. htl mutants have severe defects in dorsal meso- dermal derivatives, in particular the heart and visceral muscles, but also lack many somatic muscles. This seems mainly to be due to a failure of dorsal mesoder- mal cell migration [Beiman et al., 1996; Gisselbrecht et al., 1996; Shishido et al., 1997; see also Michelson et al., 1998]. As with htl, the homeobox-containing gene tin- man (tin) is expressed in the mesoderm anlagen at blastoderm in a twi-dependent fashion [Bodmer et al., 1990]. tin function is absolutely required, not only for the specification of the heart primordium, but also for the formation of most visceral and dorsal somatic muscles [Azpiazu and Frasch, 1993; Bodmer 1993; Yin and Frasch, 1998]. In contrast to htl, tin is apparently not required for dorsal mesodermal migration, but Contract grant sponsor: NIH (to R.B.); Contract grant sponsor: American Heart Association (to R.B.). *Correspondence to: Dr. Rolf Bodmer, who is now at IBDM, (until July 1998), Campus de Luminy, case 907, F-13288 Marseille, France. E-mail: bodmer@ibdm.univ-mrs.fr After July 1998: Dr. Rolf Bodmer, Dept. of Biology, University of Michigan, 830 N. University, Ann Arbor, MI 48109–1048. E-mail:rolf@umich.edu Dr. T.V. Venkatesh is currently at Synaptic Pharmaceutical Corp., Paramus, NJ. E-mail: tvenkatesh@synapticcorp.com Received for publication 9 January 1998; accepted 19 February 1998. DEVELOPMENTAL GENETICS 22:201–211 (1998)  1998 WILEY-LISS, INC.