Access Type

Open Access Dissertation

Date of Award

January 2015

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Biological Sciences

First Advisor

Aleksandar Popadic

Abstract

Insects display the greatest amount of structural and functional variation among animal groups, particularly in regard to their appendage morphology. These differences can range from the diverse pigmentation patterns between fore- and hindwings to changes in the size and shape of legs. The greatly enlarged jumping hind leg in crickets and grasshoppers is one of the best known illustrations of such diversity, representing a unique feature for the entire order of these insects (Orthoptera). Previous work from our lab has shown that the homeotic gene Ultrabithorax (Ubx) plays a key role in the enlargement of hind legs not only in orthopterans, but other insect species as well. Another example of a greatly modified hind leg is seen in the honeybee, Apis mellifera, which has been adapted for the collection and carrying of pollen. To determine if these modifications are also regulated by Ubx, we used the RNAi approach to examine its function in the developing Apis legs. Our results show that Ubx is indeed responsible for the formation of the pollen basket and its associated structures, and is also expressed in the same region in another bee species, Bombus impatiens. These results indicate that this gene likely has a conserved role in the formation of the pollen basket, a unique structure common to all social bees. Therefore, Ubx may not only responsible for the enlargement, but also for the specific morphological specialization of the T3 leg in insects.

In addition to legs, wings are another set of appendages that display a wide range of morphological diversity. They also represent a novel adaptation that allowed for the rapid and highly successful radiation of insects into almost every ecological niche on Earth. Hence, to better understand the evolution of insect wings we must elucidate both the origins and divergence of these structures. Here we use an unbiased global transcriptome analysis combined with a candidate gene approach to begin to answer these questions. Our data show that knocking down wing genes affects specific regions in the prothorax (T1 segment), which may be considered to be wing serial homologs. This is the first evidence that such structures were identified in hemimetabolous insects. Furthermore, the depletion of the same wing genes (by RNAi) also plays a role in shaping of the scutellum, the dorsal plate on the second thoracic (T2) segment. This result suggests that the wings and dorsal plates may have co-evolved, which in turn, may have enabled for proper wing folding. Finally, our data reveal that the fore- and hindwings are composed of both dorsal and ventral components confirming the predictions of the recently proposed combinatorial model. However, the ectopic wings on T1 were found to lack any ventral contribution and constitute strictly a dorsal wing program. This may explain why fossil hemipteroids featured underdeveloped T1 wings, leading to their ultimate loss in the lineages giving rise to modern-day insects. Overall, the present work provides several new insights that further our understanding of wing origins and divergence, and help establish a robust and testable framework for future studies of wing evolution.

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