Meeting Abstract
Reconstructing a sequence of evolutionary events requires as a minimum a comparison among three taxa – two sister taxa and an outgroup. For understanding the evolution of segmentation in insects, we have three ideally positioned species, which form such a three-taxon group: The fruitfly Drosophila melanogaster and the red flour beetle Tribolium castaneum form a sister group relationship within Holometabola, while the milkweed bug Oncopeltus fasciatus forms a hemimetabolous outgroup. We have been focusing on Oncopeltus, dissecting its segmentation process in detail. Our results offer a comparative view and allow a reconstruction of the stages in the evolution of the different segmentation modes in insects. We have previously shown that simultaneous segmentation most likely appeared before the origin of Holometabola, and has been lost several times, including in Tribolium. We now add details about the evolution of different modes of sequential segmentation. We present the cascade involved in differentiating new segments from a posterior growth zone, and show that a hierarchy reminiscent of the classical Drosophila hierarchy is found in sequential segmentation as well. The cascade begins with primary pair-rule genes followed by secondary pair-rule genes, which regulate segment polarity genes. The cascade is highly redundant and RNAi phenotypes of most genes are surprisingly minor. This hierarchy was most likely ancestrally of a single-segment periodicity. However, there is evidence for a two-segment periodicity in the differentiation of the segments after their formation in Oncopeltus, perhaps giving a hint to the origin of the pair-rule pattern found in both Tribolium and Drosophila.
