My research mainly revolves around the developmental genetic basis of divergent traits in cichlid fishes. More precisely, I am trying to find out which genes underly differences in the development of certain traits. The visual system and jaws are the two traits that I have focused on primarily. Both of these traits are extremely important for cichlid fishes as they have been hypothesized to be crucial for the immense species richness of this fish family.
Light cxonditions under water can be extremely challenging for visually oriented organisms. Humans cannot see differences in light environments, except for differences in light intensity comparing day and night. The only time when we can observe the presence of different wavelengths or colors of light is during sunrise and sunset. During that time, mainly long-wavelength red-light passes from the atmosphere into our eyes, making the sky appear yellow to red. Contrary to air, water has a strong and predictable impact on the spectral composition of light due to absorbance and scattering. For example, long-wavelength red-light is filtered in the water column and in just a few meters of depth only short-wavelength blue-light is present. On the other hand, in turbid waters short-wavelength blue-light is filtered by suspended particles so that mainly red-light remains. Fish have to cope with these different light conditions under water when colonizing new environments or due to temporal variation within environments. One way to do so is to tune their color vision to match the present light environment. In the past years I have tried to understand how cichlids can change their color vision as a response to new conditions.
Once you tuned your visual system to the conditions under water, you can search, find and consume food. Cichlid jaws have evolved many different phenotypes to exploit any possible resource available. So can fish with small teeth efficiently process soft plant based material while fish with massive big teeth can crush hard snails without breaking their teeth. Especially the pharyngeal jaw (a second set of jaws that fish have in the back of their throat) has been proposed to have facilitated the diversification of cichlid fishes. Not only have many different phenotypes of jaws evolved in cichlids, but certain phenotypes have evolved multiple times independently, making cichlid pharyngeal jaws a prime example of convergent evolution. But how many different ways are there to make the same phenotype? Unfortunately, it is not a simple answer.
If you are interested in further detail, below you can find short summaries of a some studies that I have been involved in.
Light cxonditions under water can be extremely challenging for visually oriented organisms. Humans cannot see differences in light environments, except for differences in light intensity comparing day and night. The only time when we can observe the presence of different wavelengths or colors of light is during sunrise and sunset. During that time, mainly long-wavelength red-light passes from the atmosphere into our eyes, making the sky appear yellow to red. Contrary to air, water has a strong and predictable impact on the spectral composition of light due to absorbance and scattering. For example, long-wavelength red-light is filtered in the water column and in just a few meters of depth only short-wavelength blue-light is present. On the other hand, in turbid waters short-wavelength blue-light is filtered by suspended particles so that mainly red-light remains. Fish have to cope with these different light conditions under water when colonizing new environments or due to temporal variation within environments. One way to do so is to tune their color vision to match the present light environment. In the past years I have tried to understand how cichlids can change their color vision as a response to new conditions.
Once you tuned your visual system to the conditions under water, you can search, find and consume food. Cichlid jaws have evolved many different phenotypes to exploit any possible resource available. So can fish with small teeth efficiently process soft plant based material while fish with massive big teeth can crush hard snails without breaking their teeth. Especially the pharyngeal jaw (a second set of jaws that fish have in the back of their throat) has been proposed to have facilitated the diversification of cichlid fishes. Not only have many different phenotypes of jaws evolved in cichlids, but certain phenotypes have evolved multiple times independently, making cichlid pharyngeal jaws a prime example of convergent evolution. But how many different ways are there to make the same phenotype? Unfortunately, it is not a simple answer.
If you are interested in further detail, below you can find short summaries of a some studies that I have been involved in.
Development of the visual system depends on photic conditions
Color vision under water can be challenging, because the light under water changes drastically with depth and turbidity. These factors can change throughout the life-time of a fish. To be best suited for under-water vision, cichlid fishes adapt to match their needs at different stages during development. The different stages can be affected strongly by different light environments, but the strongest effect can be observed when fish are reared in complete darkness: color vision progresses through all the developmental stages within only few days and reaches an almost adult phenotype in very young embryos. The explanation for this is potentially the disruption of hormones in darkness that, as a side-effect, accelerates the development of color vision. However, the disruption of hormones comes with great costs, because nearly all dark-reared fish develop strong malformations of their spines. Milder forms of darkness-induced spinal deformities with lower incidence can also be observed in a species of Poecilia, another freshwater fish, which hints towards a developmental constraint for some species to live in complete darkness, e.g. in caves.
Heterochronic opsin expression due to early light deprivation results in drastically shifted visual sensitivity in a cichlid fish: Possible role of thyroid hormone signaling Evolution in caves: selection from darkness causes spinal deformities in teleost fishes |
Phenotypic plasticity in cichlid color vision is extremely fast
The light environment of fishes can change rapidly, for example, due to algal blooms or pollution. Such events can drastically shift the light from blue light in clear waters to more green or red light. Can fish adapt to such rapid changes to maintain visual functions? For cichlid fishes, this is true. Within few days Nicaraguan cichlids can match their color vision to their environment. This process is not only a one-way road where fish rapidly adapt to different wavelengths of light, they can reverse changes when their light environment changes a second time. At least as juveniles.
Reverting ontogeny: rapid phenotypic plasticity of colour vision in cichlid fish |
New genes associated with convergence in tooth size
One of the hypothesized reasons why cichlid fishes are among the most species rich vertebrate groups are their incredibly diverse and well adapted pharyngeal jaws. Depending on the food items different species feed on, they develop different phenotypes. For example, species feeding on soft items like algae develop countless small and pointed teeth to shred their food. Other species that feed on hard items like snails develop few large and massive teeth. These two extreme phenotypes evolved numerous times independently in different cichlid species. We have shown that gene birth gave rise to a larger repertoire of tooth genes in cichlids that are associated with different tooth sizes, where species with larger teeth show higher expression levels. These and potentially many more new genes in cichlids could attribute to their evolvability.
A genomic cluster containing novel and conserved genes is associated with cichlid fish dental developmental convergence |
Dental plasticity could drive adaptation and diversification
Teeth are usually regarded as strong and rigid structures that once they develop are immutable. Even though this is largely true, a lot of phenotypic plasticity has been demonstrated in teeth already. Tooth sizes in cichlid fishes can increase as a response to harder food items or growth rates in rabbit molars increase when wear is strong. Evidence exists that certain phenotypes like tooth size can become fixed in species that are derived from a flexible ancestor. Therefore, phenotypic plasticity in teeth could have impacted the evolution of teeth and contributed to the diversity we can observe today. I aim to understand the relationship between tooth size and the environment in cichlid fishes on a developmental genetic basis.
Phenotypic plasticity in vertebrate dentitions |