cDNAs from a clonal line of the planarian Schmidtea mediterranea. The obtained cDNA sequences, homology comparisons and high-throughput whole-mount in situ

The phylum Platyhelminthes (flatworms) consists of approximately 50,000 different species that populate a remarkable variety of niches (Littlewood and Bray, 2001). In addition to free-living forms, it encompasses parasitic organisms responsible for inflicting debilitating diseases upon hundreds of millions of people throughout the world (see World Health Organization fact sheet 115 at http://www.who.int/inf-fs/en/fact115.html). Platyhelminthes are considered by many to occupy an important position in the evolution of the Metazoa (Adoutte et al., 1999; Henry et al., 2000; Tyler, 2001; Willmer, 1994), and the panoply of developmental properties displayed by these organisms has attracted the attention of generations of biologists (Newmark and Sánchez Alvarado, 2002). For example, the ability of freshwater planarians to regenerate completely from small body fragments has been known for over two centuries (Morgan, 1898; Randolph, 1897), and the life cycles of some digenetic trematodes involve as many as three different hosts as well as both sexual and asexual strategies for their reproduction (Brusca and Brusca, 1990; Hyman, 1951). Yet, as important, abundant and diverse as platyhelminthes are, little is known about the molecular events that guide their sophisticated and often plastic biological properties. Moreover, many members of this phylum possess large populations of undifferentiated mesenchymal stem cells, the study of which could contribute significantly to fundamental biomedical research in the areas of tissue regeneration, stem cell maintenance and degenerative disorders. In most freeliving species these stem cells, which are often referred to as neoblasts, are used for the regeneration of missing body parts and/or the replacement of cells that are lost during the course of physiological turnover (Gschwentner et al., 2001; Ladurner et al., 2000; Newmark and Sánchez Alvarado, 2000). Similarly, free mesenchymal cells in parasitic flukes are known to produce complete larval forms (Brusca and Brusca, 1990; Hyman, 1951), and in the cestode Taenia crassiceps complete cysts can be reconstituted from individual cells (Toledo et al., 1997). Thus, platyhelminthes also provide a unique opportunity for studying the mechanisms that underlie the control of cellular pluripotentiality. To address many of these unsolved problems, we and others (Agata and Watanabe, 1999) have chosen to reintroduce the freshwater planarian as an experimental model. We report the establishment of a clonal line of a diploid, asexual form of the planarian Schmidtea mediterranea (Turbellaria, Tricladida), along with the isolation and sequence characterization of ~3000 non-redundant, expressed sequence tags (ESTs) from this organism. Furthermore, we show the suitability of using planarians for high-throughput mapping of gene expression patterns in the whole animal, and introduce the S. 5659 Development 129, 5659-5665 © 2002 The Company of Biologists Ltd doi:10.1242/dev.00167