A complete understanding of evolution requires an understanding of how phenotypic variation is generated. Since animal morphologies are a consequence of development, we must therefore understand how developmental processes change through evolutionary time
A comparative approach allows us to infer what developmental changes have occurred during evolution. I have therefore focused on how different insect species generate their segmented body plans. Insects come in essentially two flavors; long germ insects form their entire set of body segments nearly simultaneously while short and intermediate germ insects initially only form their anterior body regions with posterior segments arising sequentially in an anterior to posterior progression during a later stage of development. Although these two modes of segmentation are quite different, they are evolutionarily linked---short germ insects are ancestral and from these, long germ insects evolved. Studies in the fruitfly, Drosophila melanogaster, have elucidated the genetic mechanisms underlying long germ segmentation. Unfortunately, much less is known about the short germ mode
I have therefore studied the expression and function of several homologous segmentation genes in an intermediate germ insect, the large milkweed bug, Oncopeltus fasciatus (Hemiptera). I have broken down the larger problem of short vs. long germ segmentation into its two constituent parts. First, I investigate anterior patterning by determining the role of the Oncopeltus gap gene homologs hunchback and Kruppel then I ask how metameric pattern in the posterior is generated by examining the homolog of the pair-rule gene even-skipped
I found that both hunchback and Kruppel are expressed in similar gap-like domains but their roles in development are quite different. I depleted Kruppel function and found that resulting embryos lacked both blastoderm- and germband-derived segments, indicating that both types of segments share common molecular underpinnings despite their different modes of generation. hunchback depletions yielded no gap-like phenotype. Instead, results suggests that hunchback in ancestral insects was important to impart segmental identity but not formation in the anterior. My results with Oncopeltus even-skipped show that it is neither expressed nor functions in a pair-rule fashion implying that its pair-rule role evolved only in the higher insects
