Sorry, this entry is only available in Icelandic.
We co-authored a manuscript submitted recently to a special issue of Developmental dynamics.
Palsson A, Wesolowska N, Reynisdóttir S, Ludwig MZ, Kreitman M (2014) Naturally Occurring Deletions of Hunchback Binding Sites in the Even-Skipped Stripe 3+7 Enhancer. PLoS ONE 9(5): e91924. doi:10.1371/journal.pone.0091924
Transcription factors (TF) bind to specific DNA sequence to regulate expression of genes. Evolutionary studies of enhancers show that some TF binding sites are well conserved while others are less constrained. The regulatory elements of the even-skipped (eve) gene in Drosophila are a textbook example of regulatory function, as specific activating and repressing TFs bind to specific binding sites and control the spatio-temporal expression of the gene. These enhancers are also a textbook example of TF binding site “turn-over” and compensatory evolution within regulatory elements.
In this study we examine natural sequence polymorphism in characterized enhancers in Drosophila melanogaster, and find the TF binding sites to be preserved by selection. Curiously we find two exceptions, both in the same enhancer of eve (that forms stripes 3 and 7 in the embryo). Both mutations are large deletions (larger than 45 bp) and both remove conserved binding sites for the same transcription factor. Hunchback is an important developmental regulator that affects expression of eve in the embryo. Both deletions are at high frequency in fly populations, and thus do not seem to be harmful for the flies. By analogy one might say that a tree is struck twice by lighting, but doesnt catch fire.
The most puzzling result of this study is that both Hb binding sites are conserved and the two deletions removing them are at up to 35% frequency in the population.
Furthermore, one of the Hb sites was not noticed earlier, because the Drosophila reference genome is homozygous for the deletion allele.
We postulate that coevolution between Hb function and its target sequences best explains the data. In other words, this could reflect compensatory evolution of cis and trans factors, that is developmental system drift in the gene regulatory network controlling stripe formation in fruitfly embryos.
The charr group had one paper accepted in BMC EvoDevo last week. The title is:
Ehsan P Ahi, Kalina H Kapralova, Arnar Palsson, Valerie H Maier, Johannes Gudbrandsson, Sigurdur S Snorrason, Zophonias O Jonsson and Sigridur R Franzdottir Transcriptional dynamics of a conserved gene expression network associated with benthic-limnetic craniofacial divergence in Arctic charr. Abstract to follow.
A review article in Icelandic about the interface of development and evolution, published in the Icelandic Naturalist (Natturufraedingurinn).
Book review about, the translation of Chance and necessity by Jacques Monod, published in the Icelandic Naturalist (Natturufraedingurinn).
This past Friday (August 29th. 2014) I enjoyed the great fortune of visiting the Center for Ecological and Evolutionary Synthesis at the University of Oslo. A colleague from the Chicago years, Lee Hsiang Liow was our host.
The title of our talk was “On rapid and repeated evolution via transcriptional cooption and decay” and the abstract reads like:
The function and evolution of gene regulatory mechanisms and networks has implications for development, diseases and ecology. In this talk I will describe our work on transcriptional evolution, drawing on studies in two systems. I will outline population genetic, morphometric and transcriptomic analyses of parallel evolution of recently evolved dwarfism and associated phenotypes in Arctic charr (Salvelinus alpinus). Secondly I will focus on naturally occuring deletions of transcription factor bindings sites in characterized enhancers of the even-skipped gene in Drosophila melanogaster. Lastly I will outline musings about general principles of evolution by gene recruitment and transcriptional decay, and predictions that follow.
The talk summarized the Arctic charr work that we participate in and two projects on regulatory evolution. I got great questions and had wonderful conversations while in Oslo. Hopefully I can visit again soon.