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Beetle horns evolved from wing serial homologs

ABSTRACT: Understanding how novel complex traits originate is a foundational challenge in evolutionary biology. Yet how descent with modification in developmental evolution may lead to morphological innovation remains poorly understood. We investigated the origin of thoracic horns in scarabaeine beetles, one of the most dramatic classes of secondary sexual traits in the animal kingdom. We show that thoracic horns derive from bilateral source tissues, that diverse wing genes are functionally required for instructing this process, and that in the absence of Hox-input thoracic horn primordia transform to contribute to ectopic wings. Once induced, however, the transcriptional profile of thoracic horns diverges markedly from that of wings and other wing serial homologs. Our results provide evidence for the serial homology between thoracic horns and insects wings, and suggest that other insect innovations may similarly derive from wing serial homologs and the concomitant recruitment of diverse genes from outside a wing formation context.

ORGANISM(S): Onthophagus taurus

PROVIDER: GSE137455 | GEO | 2020/04/02

REPOSITORIES: GEO

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Project description:Amongst the various different insect groups, there is remarkable diversity in the number and size of wings. However the development of the basic body plan in insects is similar to a large extent. The genes of the hox complex regulate various pathways to bring about the development or modification of different organs. Ubx, a gene of the bithorax hox complex is expressed in the third thoracic segment of insects and is known to specify the fate of wing appendage in that segment.To understand the role of Ubx and how its regulatory mechanism has evolved through the course of evolution we have compared its genome wide targets in different insect orders. The identification of regulatory pathways and the key players Ubx regulates is crucial to understand how it has controlled wing development across insect orders. Our lab has previously identified direct targets of Ubx in Drosophila using ChIP-chip (Agrawal et al, 2011). To further our knowledge on the role of regulation in development and modification of hind wing appendage we have studied the targets in the hind wings of other insects (silk moth; Lepidoptera and honeybee; Hymenoptera) and performed a comparative analysis.To understand the differential development of wing appendages in insects we intend to compare the differential expression of fore and hind wing appendages in Diptera and Lepidoptera. We have employed RNA-seq using by illumina sequencing to identify the genes that are differentially expressed between fore and the hind wing bud of the Bombyx larvae. Only one replicate was performed as the intended study was to validate the differential targets in comparison to ChIP studies and other methods of select candidates. Total RNA was extarcted from wing buds of IV instar Bombyx larvae and sequenced on an illumina sequencer. Wing buds were collected directly in liquid nitrogen and 80 such buds each for fore and hind wing were collected from IV instar larvae. RNA was isolated from them and processed for Library preperation and illumina sequencing. One replicate was done as a supplement to ChIP studies.

Project description:Developmental mechanisms play an important role in determining the costs, limits, and evolutionary consequences of phenotypic plasticity. One issue central to these claims is the metaphor of developmental “decoupling,” where alternate morphs result from evolutionarily independent developmental pathways. We test this assumption through a microarray study that explores differences in gene expression between alternate morphs relative to differences between sexes, a classic example of developmental decoupling. We then examine whether morph-biased genes are less conserved, relative to morph-shared genes, as predicted if developmental decoupling relaxes pleiotropic constraints on divergence. We focus on the developing horns and brains of two species of horned beetles with spectacular sexual- and morph-dimorphism in the expression of horns and fighting behavior. We find that patterns of gene expression were as divergent between morphs as they were between sexes. However, overall patterns of gene expression were also highly correlated across morphs and sexes. Morph-biased genes were more evolutionarily divergent, suggesting a role of relaxed pleiotropic constraints or relaxed selection. Together these results suggest that alternate morphs are somewhat developmentally decoupled, and that this decoupling has significant evolutionary consequences. However, alternative morphs may not be as developmentally decoupled as sometimes assumed and such hypotheses of development should be revisited and refined. We compared gene expression in three focal epidermal tissues (head, prothorax, legs) relative to a "control" tissue (dorsal abdominal epidermis) without any outgrowths. We also surveyed gene expression in the brain, relative to ganglionic neural tissue. We compared such patterns of gene expression between two male morphs (horned, fighter and hornless, sneaker males) and between males and females. We focused our array analyses (N = 48 arrays) on Onthophagus taurus (the species for which the array was designed), but also ran 19 arrays on thoracic tissue of Onthophagus nigriventris, a species which expresses thoracic horns as adults (O. taurus expresses head horns). Finally, we included a small subset of arrays (N = 4) directly hybridizing head epidermis tissue of O. taurus male morphs to validate our overall estimates of morph-biased expression. For more detail, refer to Snell-Rood et al. 2010, Evolution.

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Beetle horns evolved from wing serial homologs

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