Comparative gene expression to study the developmental basis of organ diversification.
by Elisa Buchberger
Date of Examination:2019-09-03
Date of issue:2019-09-30
Advisor:Dr. Nico Posnien
Referee:Dr. Nico Posnien
Referee:Prof. Dr. Daniel John Jackson
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EnglishThe striking diversity in adult morphologies is the result of millions of years of adaptation of species to different environments and habitats. Fixed changes in populations or species are the consequence of mutations in the genome and thus in the developmental programs of body plans, their structures and organs. Years of studies in the field of ‘Evo-Devo’ have revealed that there exists only a limited number of genes, governing basic developmental processes, and that these so-called ‘toolkit genes’ are highly conserved even between distantly related species. It is nowadays accepted, that morphological diversification is often driven by changes in gene expression and subsequently the interplay of gene products. Since the expression of genes is tightly controlled in a spatiotemporal manner on several molecular levels, also the wiring of such gene regulatory networks is highly context dependent. Therefore, single cells, tissues and organs are characterized by a unique set of expressed transcripts and proteins which are specifically intertwined and govern their developmental programs. The advent of high throughput sequencing techniques provides nowadays the opportunity to analyze the transcriptome of developing structures in a highly specific manner and opens the possibility to understand how these toolkit genes are differentially used and rewired in different developmental and evolutionary contexts. In Chapter I of this thesis, I studied gene expression in a developmental context, using the emerging model species Schistocerca gregaria to understand the development and function of pleuropodia - small glandular structures forming on the first abdominal segment of many insect embryos. In Chapter II, I used a comparative transcriptomic dataset of developing eye-antennal discs in two closely related species of the Drosophila melanogaster subgroup to study the molecular basis of evolution of complex traits. The size and shape of the compound eyes and head structures vary extensively between D. melanogaster and D. mauritiana and show a typical trade-off between eye-size and head width. I could show that differential expression of pannier (pnr) underlies natural variation of eye size, ommatidia number and head width between these two species. In Chapter III, I combined an allele specific expression dataset of F1 hybrids between D. melanogaster vs. D. mauritiana and D. simulans vs. D. mauritiana with a newly generated comparative ATAC-seq dataset, to study gene expression divergence and sought to recapitulate the observed patterns in terms of nucleotide turnover and accessibility of regulatory regions. In summary, this works shows that the combination of methods and various datasets allows to gain major insights into development, function, and evolution of morphological traits.
Keywords: evolution; development; Drosophila; eye size; head shape; gene expression; Schistocerca