EVOLUTION & DEVELOPMENT 13:5, 436–447 (2011) DOI: 10.1111/j.1525-142X.2011.00497.x Early patterning and blastodermal fate map of the head in the milkweed bug Oncopeltus fasciatus Michael Birkan, a Nina D. Schaeper, b and Ariel D. Chipman a,* a Department of Ecology, Evolution and Behavior, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, Jerusalem, 91904, Israel b Johann-Friedrich-Blumenbach-Institute of Zoology and Anthropology, Georg-August-Universit¨ at G ¨ ottingen, GZMB, Ernst-Caspari-Haus, Justus-von-Liebig-Weg 11, G¨ ottingen, 37077, Germany ∗ Author for correspondence (email: ariel.chipman@huji.ac.il) SUMMARY The process of head development in insects utilizes a set of widely conserved genes, but this process and its evolution are not well understood. Recent data from Tribolium castaneum have provided a baseline for an un- derstanding of insect head development. However, work on a wider range of insect species, including members of the hemimetabolous orders, is needed in order to draw general conclusions about the evolution of head differentiation and regionalization. We have cloned and studied the expression and function of a number of candidate genes for head de- velopment in the hemipteran Oncopeltus fasciatus. These include orthodenticle, empty spiracles, collier, cap ‘n’ col- lar, and crocodile. The expression patterns of these genes show a broad conservation relative to Tribolium, as well as differences from Drosophila indicating that Tribolium + On- copeltus represent a more ancestral pattern. In addition, our data provide a blastodermal fate map for different head re- gions in later developmental stages and supply us with a “roadmap” for future studies on head development in this species. INTRODUCTION The insect head poses numerous questions in evolution and development. These range from large-scale questions about the homology of individual segments and structures between insects and other arthropods (e.g., Haas et al. 2001; Scholtz and Edgecombe 2006; Bitsch and Bitsch 2010) to questions about the contribution of specific embryonic domains to adult structures (e.g., Posnien and Bucher 2010). The iden- tity of individual segments in the head is one of the defining features of the different arthropod classes, and the specific modifications of these segments are among the defining fea- tures of insect orders, so a detailed understanding of how the head develops and how it has evolved are central to our knowledge of arthropods in general, and of the highly diverse insects in particular. The insect head is formed of six segments (Posnien et al. 2010). The three posterior head segments are known as the gnathocephalon or “gnathal head.” These segments include the labial, maxillary, and mandibular segments. In early em- bryogenesis, they are indistinguishable from trunk segments, and they are patterned through the same set of segmental pat- terning genes (Choe and Brown 2009). The mature segments all include appendages that function within the feeding ap- paratus. The three anterior segments include the intercalary segment, the antennal segment, and the ocular region and are collectively known as the procephalon or “pre-gnathal head.” They are clearly distinct from the trunk at early stages of development and are patterned differently (Posnien et al. 2010). These three segments are also all highly modified and have either no appendages or unusual appendages. Paleon- tological data suggest that the gnathal segments are derived from ancestral trunk segments (Budd 2002). What causes the gnathal segments to transform from “normal” trunk seg- ments to head segments, and how the borders within the head and between the head and the trunk are defined are mostly unknown. Work on the development of Drosophila melanogaster has identified a number of potential participants in the various processes of head development. The first of these were the three genes orthodenticle (otd), buttonhead (btd), and empty- spiracles (ems) (Cohen and J ¨ urgens 1990). These three genes are expressed in partially overlapping anterior domains in the Drosophila blastoderm and are collectively referred to as “head gap-genes.” However, analysis of their precise con- tribution to the mature head and a better understanding of their roles are hampered by the derived and degener- ate form of the Drosophila larval head, which undergoes “head involution” so that morphological markers are few and difficult to identify (Jurgens et al. 1986). A second group of genes involved in Drosophila head development are three genes that are expressed in specific domains and 436 C  2011 Wiley Periodicals, Inc.