CH. 21 For BIO

Antenna The antenna is not a body segment of a fruit fly embryo.

Offspring will show a mutant phenotype if the mother has a mutant genotype. This statement is true; the gene is transcribed in the mother, and the mRNA is delivered to the zygote.

True Pattern formation is the process by which the location of an embryo's body segments is determined.

Fertilized egg bicoid mRNA is translated at the anterior pole of the fertilized egg.

Abdomen The regions with low concentrations of Bicoid protein develop into posterior structures such as the abdomen.

The Bicoid protein is a regulatory transcription factor. The Bicoid protein regulates expression of the embryo's early developmental genes.

The larva would be normal with one head at the anterior pole. The injected mRNA would rescue the mutant phenotype of the egg.

two-headed fly The head develops where there is a high concentration of bicoid protein.

the establishment of the anterior-posterior axis The concentration gradient of bicoid protein determines the anterior-posterior axis of a developing Drosophila.

head

fertilization of the egg

egg-polarity Egg-polarity genes are responsible for establishing the polarity of the egg.

abdomen

diffusion Bicoid protein is produced at the anterior end and diffuses toward the posterior, resulting in a gradient.

fertilized egg

nurse cells

Differential gene expression affects the developmental process in animals. Homeotic genes code for transcription factors that control the development of segment-specific body parts. Cells receive molecular signals that communicate their position in relation to other cells. Positional information controls pattern formation.

Some stickleback populations became trapped in lakes that formed at the end of the last ice age. Freshwater stickleback populations were established when marine populations became trapped in the freshwater lakes where they migrate to spawn annually. At the end of the last ice age, these lakes, formerly connected to the ocean, were cut off by retreating ice fields.

In lakes where there are no large predatory fish, there is no advantage to having pelvic spines. In lakes with dragonfly larvae, pelvic spines can be disadvantageous, allowing the predatory larvae to grab the fish. Different environments can provide different selective pressures on an organism's morphology. In the threespine stickleback, pelvic spines provide a selective advantage in environments with large predatory fish but are a liability in environments with dragonfly larvae.

To find the location of the gene(s) causing the difference between stickleback populations with and without spines.

It occurred in a similar DNA region in freshwater stickleback populations all over the world. It is found in a regulatory region (a "switch") upstream of the coding region of the Pitx1 gene. Many mutations of evolutionary importance are found in regulatory regions. The gene remains intact, but the location of its expression changes—conveying a new phenotype without losing existing capabilities.

The population of fish with pelvic spines that arrived in the lake at time B evolved a reduced pelvis over time (beginning at time C). Having a reduced pelvis must have been advantageous for this ancient stickleback population, just as it is for many current freshwater stickleback populations.

. The fossil data show a pattern of evolution over long stretches of time. 2. If the same morphological changes occur in the fossil record as in living populations, we might deduce that the genetic mechanism discovered in the living populations might be responsible for the changes observed in fossils. 3. Genetic evidence reveals the precise molecular mechanism responsible for the change in pelvic structures in stickleback populations. 4. Data obtained by analyzing living fish in lakes show the selective pressures present in different environments. Evolution repeats itself. The film uses three complimentary lines of evidence from field studies, molecular genetics, and fossil populations to show evolution of the same trait over and over again, across hundreds of thousands of years.