Investigating divergent mechanisms of mesoderm development in arthropods: the expression of Ph-twist and Ph-mef2 in Parhyale hawaiensis.
INTRODUCTION The organization of tissues along the dorsoventral (DV) axis is well conserved in arthropods from the germ band stage through to adulthood: the central nervous system (CNS) occupies the ventralmost region, appendages develop from ventrolateral ectoderm, and the heart is positioned dorsally (Brusca and Brusca, 2003). Prior to the germ band stage, however, basic morphogenetic processes such as gastrulation and germ rudiment assembly vary considerably among arthropods. Therein lies a fundamental question of evolutionary biology: how are various animals able to construct a similar end product when they begin with such vastly different starting materials? In the fruit fly Drosophila melanogaster, DV patterning relies on a broad activity gradient of the maternal transcription factor Dorsal that regulates the expression of various tissue-specific target genes in a threshold-dependent fashion (Roth et al., 1989; Jiang et al., 1992; Stathopoulos et al., 2002). Ventrally, high levels of Dorsal activate snail and twist in the presumptive mesoderm, whereas intermediate levels activate the Bone morphogenetic protein (BMP) antagonist short-gastrulation (sog) in the presumptive neurogenic ectoderm (Jiang et al., 1992). Subsequently, secondary DV cell fates, such as the presumptive ventral midline, are established in the embryo. The ventral midline is demarcated by cells expressing the basic helix-loop-helix–Per-ARNT-Sim (bHLH-PAS) transcription factor gene single-minded (sim), which is directly regulated by Dorsal, Snail and Twist, as well as by Notch signaling at the mesoderm-mesectoderm boundary (Thomas et al., 1988; Nambu et al., 1991; Kosman et al., 1991; Kasai et al., 1992; Leptin, 1991; Morel and Schweisguth, 2000). Although sim expression is first visible at the blastoderm stage in two columns of cells flanking the presumptive mesoderm, these cell columns converge during gastrulation to form a single, ventrally located column of cells (Crews et al., 1988). The ventral midline goes on to play a relatively restricted role in subsequent refinement of DV patterning by secreting the EGF ligand Spitz, which helps to ensure proper fate specification within the adjacent neurogenic ectoderm (Golembo et al., 1996; Mayer and Nüsslein-Volhard, 1988; Chang et al., 2000). By contrast, distinctions between germ layer lineages in the amphipod crustacean Parhyale hawaiensis are made by the eightcell stage (Price et al., 2010). In this system, gastrulation occurs as visceral and head mesoderm and germline precursors form an aggregation called the rosette, which is internalized under the developing germ disc. Later, somatic mesodermal precursors ingress along the posterior edge of the germ disc (Price and Patel, 2008). After gastrulation, the germ band for body segments posterior to the mandible consists of an ectodermal grid that is assembled progressively from anterior to posterior (Browne et al., 2005). Furthermore, whereas specification of Drosophila midline cells requires input from Dorsal and mesodermally expressed transcription factors, the ventral midline in amphipods appears to be the first structure to become morphologically and molecularly distinct in the developing germ band (Gerberding and Scholtz, 1999; Browne et al., 2006). Although the ventral midline of Parhyale is assembled in the same manner as the rest of the ectodermal grid, it is assembled from a distinct population of precursor cells, termed midline precursor cells, that can be distinguished from the surrounding ectodermal grid precursor cells by their unique morphology (they are arranged in a wedge shape Development 137, 3469-3476 (2010) doi:10.1242/dev.055160 © 2010. Published by The Company of Biologists Ltd