In terms of their structural and morphological diversity, as well as sheer numbers, arthropods represent the most successful animal phylum. Although the true phylogeny of arthropods is a much debated topic, four major extant groups can be recognized: insects, myriapods (millipedes and centipedes), crustaceans (e.g., crabs, lobsters, shrimps), and chelicerates (e.g., spiders, ticks, mites, scorpions). These four major lineages are characterized by distinct body plans, which are further modified to a varying degree in each group (for example, in insects, wings may be present or absent, there may be two pairs or one pair, their morphology may be significantly different (butterfly wings vs. fly wings), and so on). The main part of my research program is focused on understanding the molecular basis of this fascinating morphological diversity.

At the macroevolutionary level, I am interested in studying the mechanisms and patterns that have governed the overall evolution of arthropod body plans. How did a relatively undifferentiated, worm-like proto-arthropod form gave raise to the so spectacularly different organisms such as butterflies and lobsters and spiders? With respect to their morphology all arthropods share a common feature: a subdivision of their bodies into distinct segments. This subdivision, and the corresponding evolution of arthropod body plans, must reflect genetic changes in the mechanisms that control body segmentation. In arthropods, it is the homeotic (Hox) genes that are involved in establishing segment identity along the axis of the body. This connection between homeotic genes and segment identity suggests that structural or regulatory changes in Hox genes may very well be responsible for the differences in organization between major body plans. Most of the present work is directed toward the molecular characterization of Hox genes complexes in several arthropod species, especially among chelicerates. The focus on chelicerates is due to the fact that some of the members of this group are among the oldest extant arthropods. Equally important is the fact that chelicerates do not have a head distinct from the rest of the trunk. Instead, their body is divided into prosoma (cephalothorax) and opisthosoma (abdomen). Thus, chelicerates can provide an important insight into the developmental changes that have resulted in the evolution of a novel body region (the head) in other arthropod classes. Another aspect of this research focuses on examining the evolutionary dynamics of regulatory elements in selected Hox genes. At present, we do not know if and how much evolutionary forces like selection and mutation affect the evolution of genes that regulate developmental processes. I am interested in studying the nature and amount of molecular variation in the regulatory elements of Hox genes as a way of understanding partitioning of genetic variation in developmental loci.

My second mid-term goal centers on the comparative analysis, at the molecular level, of specific morphological features. For this purpose, a number of developmental genes are being used as molecular probes to study structural changes that occurred in these characters during arthropod evolution. A good illustration of this approach is my recent work on the origins of the arthropod mandible. Traditionally, insect and myriapod mandibles are thought to be composed of a whole limb, in contrast to crustacean mandibles which are regarded as being formed from a limb base only. My data show, however, that insects and crustaceans actually have a similar, gnathobasic mandibular structure. This finding has important consequences for resolving phylogenetic relationships between insects, crustaceans, and myriapods.

In summary, I am interested in both the molecular evolution of genes per se as well as the links between molecular and morphological evolution. In my mind, the integration of evolutionary biology with the currently expanding field of developmental genetics provides the most promising and exciting possibilities for a new understanding of morphological evolution in arthropods.

1. Popadicī , A., Rusch, D., Peterson, M., Rogers, B.T., and Kaufman, T.C. 1996. Origin of the arthropod mandible. Nature 380: 395.

2. Popadicī , A., Abzhanov, A., Rusch, D., and Kaufman, T.C. 1998. Understanding the genetic basis of morphological evolution: The role of homeotic genes in the diversification of the arthropod bauplan. Int. J. Dev. Bio. 42: 453-461.

3. Popadicī , A., Kaufman, T.C., Panganiban, G., and Shear, W.S. 1998. Molecular evidence for the gnathobasic derivation of arthropod mandibles and for the appendicular origin of the labrum and other structures. Dev. Gen. Evol. 208: 142-150.

4. Peterson, M., Popadicī , A., and Kaufman, T.C. 1998. The expression of two engrailed-related genes in an apterygote insect and a phylogenetic analysis of insect engrailed-related genes. Dev. Gen. Evol. 208: 547-557.

5. Peterson, M., Rogers, B.T., Popadicī , A., and Kaufman, T.C. 1999. The expression pattern of labial, posterior homeotic complex genes and the teashirt homologue in an apterygote insect. Dev. Gen. Evol. 209: 77-90.

6. Abzhanov, A., Popadicī , A., and Kaufman, T.C. 1999. Chelicerate Hox genes and the homology of arthropod segments. Evol. Dev., in press.