GENETICS 3020: CH 18 Develop. Genetics

Gradients of maternal effect gene products along anterior-posterior axis of embryo regulate the expression of what genes?

- Segmentation genes ( gap, pair-rule, and segment polarity genes---- classes of phenotypes) - Homeotic selector (Hox) genes----specifies the fate of each segment

Developmental geneticists study:

- What genes are expressed and when? - In what part of developing embryo (or plants) gene are they expressed? - How is the expression of these genes regulated? - What happens when these genes are defective?

Developmental Mechanisms

- how the adult body plan of animals is laid down in the embryo - the program of gene expression that turns undifferentiated cells into differentiated cells - the role of cell-cell communication in development

Homeotic Genes (Floral Meristem)

3 class of homeotic genes control the development of the 4 types of organs: - Class A along: Sepal development - Class A & B: Petal development - Class B & C: Stamen development - Class C: Carpel development

Synpolydactyly (SPD)

A form of mutation caused to Hox genes. It is caused by mutations on HOXD13 and is characterized by extra toes and fingers & bone abnormalities in hands and feet.

Model organism used for the study of development & underlying assumptions for using these organisms

Drosophilia melonogaster (fruit flies), Caenorhabditis elegans (nematode), Arabidopsis thaliana (flowering plant) common- not species-specific- set of genes and developmental mechanisms control development

Stages of Drosophila Development (10 day cycle)

Embryo, 3 larval stages, Pupal stage, Adult stage

Homeotic Genes and Mutants

Homeotic genes/selector genes are activated as targets of zygotic genes. They determine which adult structures will be formed by each body segment such as antennae, mouth parts, legs, wings, thorax, and abdomen. Homeotic mutants occur when one segment is transformed so that it forms the same structure as another segment.

What are the underlying assumptions of Variable Gene Activity Hypothesis?

It assumes that cells contains entire genome and different transcription of selected genes controls development and differentiation of each cell.

Floral Meristem

It is a group of undifferentiated cells that give rise to flowers. Each flower consists of 4 organs that develop from concentric rings of cells within meristem: sepals, petals, stamens, and carpel. It is in Arabidopsis thaliana.

2 gene sets that control embryonic development in Drosophila

Maternal-effect genes and Zygotic genes

What are the impact of segmentation genes in mice?

Mice with one copy of the mutant runt homolog have skeletal abnormalities as seen in humans. However, mice with 2 copies of it have complete absence of bone formation, with skeletons containing only cartilage. This shows that the runt gene is important in controlling the initiation of bone formation.

Mutations in the organ development in plants

Mutations in homeotic genes in plants cause the organs to develop in abnormal locations. For example, APETALA2 mutants cause the order to change to carpel, stamen, stamen, and carpel instead of sepal, petal, stamen, and carpel.

Bithorax (BX-C) complex

One of the two clusters of homeotic selector genes (Hox genes) on chromosome 3 that encode for transcription factors. It contains the 3 genes that specify structures in the 2nd thoracic segment, entire 3rd thoracic segment, and abdominal segment.

Antennapedia (ANT-C) complex

One of the two clusters of homeotic selector genes (Hox genes) on chromosomes 3 that encode for transcription factors. It contains 5 genes that specify the structures in head and first 2 segments of the thorax.

Comparison of the developmental patterns btw plants and animals.

Plants and animals diverged from a common unicellular ancestor 1.6 billion years ago. Thus, developmental patterns evolved independently in plants and animals. However, plant pattern formation can be studied by using the model organism Arabidopsis thaliana.

What are the impact of segmentation genes in humans?

Runt, one of the pair-rule genes in Drosophila, is also in other mammals such as in humans and mice. For example, the human disorder called cleidocranial dysplasia (CCD) is caused by a mutation in the RUNX2/CBFA gene, a human homolog of runt. It is characterized by numerous skeletal defects.

What occurs in the early stages of Drosophila development?

The egg is fertilized. The zygote nucleus undergoes nuclear division w/o cytokinesis (syncytium). After the 10th division, nuclei moves to the periphery of the egg. Germ cells are formed. Nuclei becomes enclosed in membrane (blastoderm). Segmentation pattern is also established.

Variable Gene Activity Hypothesis

The idea that differentiation is accomplished by activating and inactivating genes at different times in different cells.

Maternal-Effect Genes

The products (mRNA and protein) are deposited in the egg cytoplasm by "mother" fly. The products are distributed in gradient/concentrated specific regions of the cell. These genes encode for transcription factors and proteins that regulate gene expression; they also activate or repress expression of zygotic genome.

Segment Polarity Genes

They are controlled by transcription factors that are encoded by pair-rule genes. Within each of the segments created by the pair-rule genes, these genes become active in a single band of cells that extends around the embryo's circumference to divide the embryo into 14 segments.

Pair-Rule Genes

They are expressed as series of 7 narrow bands/stripes that extend around the circumference of the embryo. The expression of this gene does 2 things: establishes boundaries of segments and then programs the developmental fate of the cells within each segment by controlling expression of the segment polarity genes. At least 8 genes act to divide the embryo into a series of stripes.

What are the fundamental roles of Hox genes?

They are in the genomes of all animals where they play a fundamental role in shaping body and appendages. In vertebrates, they also control development along the anterior-posterior axis and the formation of appendages (as in Drosophila). The difference btw vertebrates and Drosophila is that vertebrates have 4 clusters of hox genes (HOXA, HOXB, HOXC, and HOXD) while in drosophila there is only 1 cluster.

Maternal Gene products

They are placed in eggs during oogenesis and are activated immediately after fertilization. They also establish the anterior-posterior axis of embryo. They encode transcription factors such as activate transcription of gap genes; this is important because expression of gap genes divides the embryo into a series of regions corresponding with head, thorax, and abdominal regions of adults.

MADS-Box Proteins

They are the transcription factors in Arabidopsis homeotic genes (like what hox genes do in Drosophila). They are characterized by sequence of 58 amino acids not similar to amino acid sequences in Hox genes. The divergence in Arabidopsis and Drosophila is shown in that the homeotic genes of Arabidopsis do not share sequence homology with Drosophila Hox genes.

Zygotic Genes

They are transcribed in nuclei of developing embryo. They are transcribed in specific regions in response to distribution of maternal-effect proteins. Deleterious recessive mutations in these genes can lead to embryonic lethality in homozygotes.

Gap Genes

Transcription of gap genes divides the embryos into head, thorax, and abdomen. However, mutations in these genes can cause large gaps in embryo's segmentation pattern.

Development

attainment of differentiated state by all cells of an organism