Access Type

Open Access Dissertation

Date of Award

January 2010

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Biological Sciences

First Advisor

Aleksandar Popadic

Abstract

The vast diversity of insect appendages provides an excellent model for examining the cellular and molecular basis of phenotypic changes in nature. One of the hallmarks of evolution of insect appendages is the differential enlargement of hind (T3) legs compared to their fore (T1) and mid (T2) counterparts. While basic information regarding the genetic mechanisms that regulate the size of the T3 legs became available in the past several years, virtually nothing is known about cellular mechanisms involved. To address this issue, we measured the relative mitotic activity (RMA) in Acheta domesticus (house cricket) and Oncopeltus fasciatus (milkweed bug), two hemimetabolous insect species with a vast difference in the magnitude of the T3 leg enlargement. We found that while crickets exhibit an increasing trend in RMA, the situation is reversed in milkweed bugs. Interestingly, the mitotic activity arrests entirely in both species when the size variation between the T2 and T3 legs is most pronounced. This finding suggests that additional cellular processes (cell size, cell shape, and cell rearrangement) may contribute to differential leg growth during this period of time. Additionally, we were able to assign the importance of allometric changes at embryonic and post-embryonic levels with regard to the final adult morphology. We found that most of the differential T3 leg growth occurs during embryogenesis in Acheta, while it is post-embryonic allometry that contributes the most to size differences between mid and hind legs in Oncopeltus. Despite their vast morphological diversity, all insects share the same modular leg organization composed of five segments and a pair of claws. Built upon this common design, each species exhibits its own unique morphology with regard to the size, shape, and function of individual leg segments (podomeres). Previous functional studies in Oncopeltus, combined with our current findings in Acheta, show that the POU homeodomain gene nubbin (nub) play a pivotal role in the growth of particular podomeres. The abolition of nub via RNAi affects the size of the femur in milkweed bugs and the length of the tibia and tarsus in crickets. Additionally, Acheta nub-depleted individuals loose a joint between the tibia and tarsus that leads to their fusion. This finding is significant because it provides the first evidence for the role of nub in leg segmentation of hemimetabolous insects.

Unlike nub, which regulates isometric growth of all thoracic legs in Acheta, the homeotic gene Ultrabithorax (Ubx) is responsible for allometric differences in size between the mid and hind legs. While the embryonic role of Ubx has been well documented, virtually nothing is known about its post-embryonic function in hemimetabolous insects. In the present study, we show that Ubx defines the third thoracic (T3) segment and its associated appendages (hind wings and T3 legs) in Acheta adults. The depletion of Ubx causes the T3 segment to assume the T2 identity. While similar transformations (T3 toward T2) were observed in both Tribolium (beetle) and Oncopeltus, the Acheta Ubx was found to regulate the identity of the posterior region of the T2 segment as well. Taken together, these results indicate that while the role of Ubx in the third thoracic segment and its appendages is conserved, its function in the second thoracic segment is species-specific.

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