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Neural crest cells and sensory neurons are two prominent cell populations which are induced at the border between neural and non-neural ectoderm during early vertebrate development. The neural crest cells are multipotent and highly migratory precursors that give rise to face cartilage, peripheral neurons, glia cells, pigment cells and many other cell types unique to vertebrates. Sensory neurons are located dorsally in the neural tube and are essential for sensing and converting environmental stimuli into electrical motor reflexes. In my PhD thesis, I obtained novel insights into the complex processes of cell induction at the neural plate border by investigating the regulation and function of mdkb in zebrafish. First, it was possible to demonstrate that mdkb expression is spatiotemporally correlated with the induction of neural crest cells and primary sensory neurons at the neural plate border. Second, it became evident that the expression of mdkb is activated by known neural crest cell inducing signals, like Wnts, FGFs and RA, but that it is independent of Delta-Notch signals essential for lateral inhibition. Knockdown experiments showed that mdkb function is necessary for induction of neural crest cells and sensory neurons at the neural plate border, probably through determination of a common pool of progenitor cells during gastrulation. The present study also used the advantages of the zebrafish model system to investigate the in vivo function of all midkine gene family members during early brain development. In contrast to the situation in mouse, all three zebrafish genes show distinct expression patterns throughout CNS development. mdka, mdkb and ptn expression is detected in mostly non-overlapping patterns during embryonic brain development in the telencephalon, the mid-hindbrain boundary and the rhombencephalon. The possibility of simultaneously knocking down two or even three mRNAs by injection of morpholino mixtures allowed the investigation of functional redundancy of midkine factors during brain formation. Knockdown of Midkine proteins revealed characteristic defects in brain patterning indicating their association with the establishment of prominent signaling centers such as the mid-hindbrain boundary and rhombomere 4. Interestingly, combined knockdown of mdka, mdkb and ptn or single knockdown of ptn alone prevented correct formation of somites, either by interfering with the shifting of the somite maturation front or interferance with cell adhesion in the PSM. Thus, Ptn was identified as a novel secreted regulator of segmentation in zebrafish.
How genomic and ecological traits shape island biodiversity - insights from individual-based models
(2020)
Life on oceanic islands provides a playground and comparably easy\-/studied basis
for the understanding of biodiversity in general. Island biota feature many
fascinating patterns: endemic species, species radiations and species with
peculiar trait syndromes. However, classic and current island biogeography
theory does not yet consider all the factors necessary to explain many of these
patterns. In response to this, there is currently a shift in island biogeography
research to systematically consider species traits and thus gain a more
functional perspective. Despite this recent development, a set of species
characteristics remains largely ignored in island biogeography, namely genomic
traits. Evidence suggests that genomic factors could explain many of the
speciation and adaptation patterns found in nature and thus may be highly
informative to explain the fascinating and iconic phenomena known for oceanic
islands, including species radiations and susceptibility to biotic invasions.
Unfortunately, the current lack of comprehensive meaningful data makes studying
these factors challenging. Even with paleontological data and space-for-time
rationales, data is bound to be incomplete due to the very environmental
processes taking place on oceanic islands, such as land slides and volcanism,
and lacks causal information due to the focus on correlative approaches. As
promising alternative, integrative mechanistic models can explicitly consider
essential underlying eco\-/evolutionary mechanisms. In fact, these models have
shown to be applicable to a variety of different systems and study questions.
In this thesis, I therefore examined present mechanistic island models to
identify how they might be used to address some of the current open questions in
island biodiversity research. Since none of the models simultaneously considered
speciation and adaptation at a genomic level, I developed a new genome- and
niche-explicit, individual-based model. I used this model to address three
different phenomena of island biodiversity: environmental variation, insular
species radiations and species invasions.
Using only a single model I could show that small-bodied species with flexible
genomes are successful under environmental variation, that a complex combination
of dispersal abilities, reproductive strategies and genomic traits affect the
occurrence of species radiations and that invasions are primarily driven by the
intensity of introductions and the trait characteristics of invasive
species. This highlights how the consideration of functional traits can promote
the understanding of some of the understudied phenomena in island biodiversity.
The results presented in this thesis exemplify the generality of integrative
models which are built on first principles. Thus, by applying such models to
various complex study questions, they are able to unveil multiple biodiversity
dynamics and patterns. The combination of several models such as the one I
developed to an eco\-/evolutionary model ensemble could further help to identify
fundamental eco\-/evolutionary principles. I conclude the thesis with an outlook
on how to use and extend my developed model to investigate geomorphological
dynamics in archipelagos and to allow dynamic genomes, which would further
increase the model's generality.