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12
The Genetic Control of
Development
Gene Regulation in Development
• Key process in development is pattern
formation = emergence of spatially organized
and specialized cells in the embryo
• Spatial organization of gene products
determines cell fate
• Genomic developmental program results in
differential gene expression and pattern
formation
Genetic Control of Cell Lineages
• Lineage = ancestor-descendant
relationships among a group of cells
• Lineage diagrams show the cell
relationships within a lineage
• Differentiation in cell lineages is under
genetic control
• Development in the
nematode follows a
fixed program of
lineage diversification
Gene Regulation in Development
• Cell fate is progressively restricted in animal
development
• Fertilization initiates
cleavage divisions in
embryo to generate
multicellular blastula
• Infolding of blastula forms
gastrula
• Cell fate is determined by
autonomous development
and/or intercellular
signaling
Gene Regulation in Development
• Cell fate refers to developmental outcome
of cells within a lineage
• Autonomous developmental restriction is
controlled by genetically programmed
changes
• Positional information refers to
developmental restrictions imposed by the
location of cells in embryo
• Morphogen = controls development
Lineage Mutations
• Division pattern and cell fate are
determined by many genes
• Mutations in any of these genes can affect
the pattern of development
• Developmental mutations can be
classified by their effect on lineages
• Transformation mutations = cells undergo
developmental fate characteristic of other
types of cells
Lineage Mutations
• Segregation mutations = sister cells
or parent-offspring cells fail to
differentiate
• These mutations occur because
differentiation factors are not
distributed properly among cell
lineages during cell division
Lineage Mutations
• Loss of function mutations are recessive
mutations in development control genes
which result in a failure to produce mRNA
or protein
• Gain of function mutations are dominant
mutations which result in overexpression
or inappropriate
Programmed Cell Death
• Programmed cell death (PCD)
occurs in developmental pathways
• PCD or apoptosis is a form of cell
suicide that removes specific cells as
part of pattern formation
• Mutations in cell death genes may
cause tissue malformations or
abnormal cell growth patterns
Genes in Pattern Formation
• Pattern formation in Drosophila is a
model for gene regulation
• Segmental organization is
controlled by segmentation
genes which specify the
pattern of repeat units
• Parasegments contain the
posterior region of one
segment and the
anterior region of the next
Genes in Pattern Formation
Regional differentiation of Drosophila
embryo is regulated by coordinate
genes that establish polarity due to
concentration gradients:
• Anterior genes affect head and thorax
• Posterior genes affect
abdomen
• Terminal genes affect
both ends
Genes in Pattern Formation
Segmentation Genes:
• Gap genes are expressed in a set of
contiguous segments
• Pair-rule genes are expressed in alternating
segments
• Segment-polarity genes
are expressed in each
segment and affect
anterior to posterior
polarity
Developmental Progression
Early development in Drosophila is
controlled by a hierarchy of regulatory
interactions in which each gene is
controlled by a combination of other
genes:
• Coordinate genes regulate polarity
• Gap genes regulate regionalization
• Pair-rule genes and segment polarity genes
regulate repeat patterns
Homeotic Genes
• Homeotic genes
determine segment
specification and differentiation
• Imaginal disks in larvae give rise to adult
tissues and organs
• Mutations in homeotic genes result in the
transformation of one body segment into
another
• Homeotic genes are transcriptional regulators
Homeotic Genes
• Homeotic (HOX) genes contain a
consensus sequence = homeobox
• Homeobox is highly conserved in
evolution = found in many species
• Homeobox sequences encode DNA
binding domains important in
transcriptional activation
• HOX genes function in regulatory
hierarchy
Plant Development
• Development in higher plants uses some of
the same mechanisms as in animals
• In higher plants developmental fate of
proliferating cells called meristems into
vegetative or floral
tissue is determined
largely by position