Lecture 13 Model Systems
Its additional sensory capability is to generate an electric field. This represents an adaptation to living in murky water with poor visibility. For instance, when a potential prey animal enters the fish's electric field, it will create a disturbance and alert the fish as to its proximity.
Its additional sensory capability is its ability to figure out its relative position to the sun, as it possesses polarized light detectors in its ommatidia.
Its additional sensory capability is echolocation-ultrasonic (high frequency sounds) imaging of its environment.
Its additional sensory capability is detection of magnetism; this capability informs the pigeons' flight direction.
Its additional sensory capability is the ability to detect heat waves, using its pit organs.
Star nosed mole
The star nosed mole is a blind mole with no eyes and an elaborated star derived from whiskers found in the Eastern United States only in swamps. Kenneth Catania is a pioneer that examines thes alternate systems. The star nosed mole picks up food in complete darkness with a directed set of movements (video from Lecture 13 shown by Professor Clandinin). In real time, the movements are remarkably fast and demonstrate evolution of sensory structures to quickly puruse prey in a specialized way. The behavioral strategy of the star nosed mole is to first make contact to the star with a directed movement over the prey item. The saccade, or sensory fovea, then decides whether the prey is worth eating or not. Catania has studied how the underlying brain structure of the star nosed mole has changed, in particular showing that the somatosensory cortex of the star nosed mole is built around a star. The sensory periphery has the characteristic structure of the star where star number 11 is the sensory fovea. There is a densely packed array of nasal mechanoreceptors that serve as a sensitive touch organ. In the cortex, barrels correspond one-to-one to the stars on the nose. Number 11 is overrepresented in the cortex as well. The star nosed mole serves as an exmaple of balancing cost and reward. Profitability is equal to energy/handling time. The standard mole has large prey and a large handling time (low cost, low payoff), whereas the star nosed mole has small food items and a low handling time (high cost, high payoff).
Euglena is a photosynthetic protist with eyespots, demonstrating photosensitivity within unicellular eukaryotes. The eyespot is contained within a chloroplast where there is a stack of photoreceptor pigments. Rhodopsin encodes a 7-transmembrane protein that is fused directly to an ion channel.
Otherwise known as the sea wasp, the cubozoan jellyfish has very advanced optics, and has eye 'master regulatory genes.' Its eyes are set up to be eyes on stalks. It has a small lens eye, a large lens eye, and four small eye spots, all joined to a simple nervous system.
The studies of yeast give us some clues about the prevalence of G protein cascades in the majority of organisms. Yeast have two sexes, a and alpha, which are each related to their own unique pheromones. Haploid yeast cells consist of a-cells that secrete a-factor pheromone and have alpha-factor receptors as well as alpha-cells that secrete alpha-factor pheromone and have a-factor receptors. The mating of a and alpha is regulated by a G protein cascade that triggers the joining of two yeast partners of opposite pheromones.
Lecture 13 Techniques
Analyzing the fossil record is one way to study evolution that is useful for learning about the evolution of skeletal structure, in particular. The fossil record, however, proves to be less useful when studying sensory systems whose components are poorly preserved.
Comparing DNA and protein sequences
Examinination of sequence homology comparisons across species and proteins can be used to determine how far apart genes and proteins are related evolutionarily. For example, the photoreceptors in modern rod and cone cells can be linked to a common ancestral receptor, c-opsin. Sequence homology comparisons can also help to distinguish homologs from analogs derived from convergent evolution (for example, bacterio-rhodopsin versus vertebrate rhodopsin).
Molecular resurrection is the use of PCR to amplify fragments of DNA extracted from well-preserved extinct organisms. Although DNA usually decays rapidly, more complete DNA comes from better preserved specimens kept in cold temperatures for thousands of years and from highly concentrated mitochondrial DNA. Once the DNA is amplified, it can be analyzed, although contamination and incompleteness can pose challenges.
Long-term field studies
Long-term field studies of emerging species can provide greater understanding of variation in species. For example, more than 30 years of studying Darwin's finches provides systematic analysis of how changes in the Galapagos islands relates to changes in beak structure.