The movement from water to land was a critical evolutionary transformation for both vertebrates and arthropods (insects, spiders, millipedes, crustaceans, etc.). Different groups of arthropods used different methods to enable breathing air, a critical requirement for life on land, a change that occurred separately in at least 7 groups. The goal of this project is to investigate how an air-breathing respiratory system evolved separately in multiple arthropod groups. Did evolution of aerial respiration involve alteration to the same anatomical structures and genes in each group, or does each arthropod group use a different solution to the problem of breathing air? To resolve this question, the embryonic formation of respiratory organs in eight arthropod species will be investigated. Genes and proteins will be tested to determine whether the same genes are used in the respiratory system of all arthropods. Genome sequencing will further test and identify genes that create unique respiratory structures (e.g., the book lung of spiders). The outcomes of this research will be integrated into a new laboratory-based course on invertebrate zoology, to be taught annually through the Department of Zoology, using live specimens. A museum exhibit will be developed on the movement of arthropods onto land, and using evidence from fossils, genes, and developmental biology. One postdoctoral fellow, one graduate student, and ten summer undergraduates will be trained during the work, emphasizing recruitment of women and underrepresented groups in science.
Investigating the parallel evolution of aerial respiration in Arthropoda has previously been limited to studies of functional morphology or expression data from a few taxa. Some developmental data suggest that mandibulate and chelicerate respiratory systems are directly homologous, whereas a separate set of data suggests that respiratory organs of arachnids arose independently from modified walking legs. Respiratory systems of Myriapoda have not been studied in a developmental genetic context at all. To resolve the origins of internalized respiratory systems of arthropods, the proposed work will examine positional, genetic, and serial homology of respiratory organs in eight emerging model arthropod systems. To establish the serial homology of arachnid respiratory organs, Hox misexpression experiments will be conducted to derepress appendages in posterior segments. To establish degree of conservation of the gene regulatory network (GRN) underlying tubulogenesis across Arthropoda, gene expression assays will be performed for candidate genes known to be required for establishment of the Drosophila melanogaster tracheal tubule system. To assess functional correspondence, knockdown experiments will be conducted in two chelicerate and three mandibulate exemplars for five critical genes in the Drosphila GRN. To investigate the patterning of the arachnid book lung, comparative transcriptomic data will be generated from book lung primordia of spiders and scorpions, toward identifying a set of novel candidate genes putatively involved in book lung morphogenesis for further functional screening. This work will directly address the two competing scenarios of arthropod terrestrialization that persist in the literature, using independent tests of positional, serial, and genetic homology.
