LE 21-11a Sperm Molecules of a cytoplasmic determinant Fertilization Nucleus Molecules of another cytoplasmic determinant Unfertilized egg cell Zygote (fertilized egg) Mitotic cell division Two-celled embryo Cytoplasmic determinants in the egg

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The other important source of developmental information is the environment around the cell, especially signals from nearby embryonic cells In the process called induction, signal molecules from embryonic cells cause transcriptional changes in nearby target cells

LE 21-11b Early embryo (32 cells) Signal transduction pathway Signal receptor Signal molecule (inducer) NUCLEUS Induction by nearby cells

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 21.3: Pattern formation in animals and plants results from similar genetic and cellular mechanisms Pattern formation is the development of a spatial organization of tissues and organs It occurs continually in plants, but it is mostly limited to embryos and juveniles in animals Positional information, the molecular cues that control pattern formation, tells a cell its location relative to the body axes and to neighboring cells

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Drosophila Development: A Cascade of Gene Activations Pattern formation has been extensively studied in the fruit fly Drosophila melanogaster Combining anatomical, genetic, and biochemical approaches, researchers have discovered developmental principles common to many other species, including humans

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Life Cycle of Drosophila After fertilization, positional information specifies the body segments in Drosophila Positional information triggers the formation of each segment’s characteristic structures Sequential gene expression produces regional differences in the formation of the segments

LE Egg cell developing within ovarian follicle Follicle cell Nucleus Egg cell Fertilization Nurse cell Fertilized egg Embryo Nucleus Laying of egg Egg shell Multinucleate single cell Early blastoderm Plasma membrane formation Late blastoderm Yolk Body segments Cells of embryo Segmented embryo 0.1 mm Hatching Larval stages (3) Pupa Metamorphosis Adult fly Head Thorax Abdomen 0.5 mm BODY AXES Dorsal Ventral PosteriorAnterior

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetic Analysis of Early Development: Scientific Inquiry Study of developmental mutants laid the groundwork for understanding the mechanisms of development Mutations that affect segmentation are likely to be embryonic lethals, leading to death at the embryonic or larval stage

LE Eye Antenna Leg Wild typeMutant

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Axis Establishment Maternal effect genes encode for cytoplasmic determinants that initially establish the axes of the body of Drosophila These maternal effect genes are also called egg- polarity genes because they control orientation of the egg and consequently the fly

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings One maternal effect gene, the bicoid gene, affects the front half of the body An embryo whose mother has a mutant bicoid gene lacks the front half of its body and has duplicate posterior structures at both ends This phenotype suggests that the product of the mother’s bicoid gene is concentrated at the future anterior end This hypothesis is an example of the gradient hypothesis, in which gradients of substances called morphogens establish an embryo’s axes and other features

LE 21-14a Head Tail Wild-type larva Mutant larva (bicoid) Drosophila larvae with wild-type and bicoid mutant phenotypes

LE 21-14b Developing egg cell Bicoid mRNA in mature unfertilized egg Nurse cells Egg cell bicoid mRNA Bicoid protein in early embryo Fertilization Translation of bicoid mRNA 100  Anterior end Gradients of bicoid mRNA and Bicoid protein in normal egg and early embryo m

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Animation: Development of Head-Tail Axis in Fruit Flies Animation: Development of Head-Tail Axis in Fruit Flies

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The bicoid research is important for three reasons: 1. It identified a specific protein required for some early steps in pattern formation 2. It increased understanding of the mother’s role in embryo development 3. It demonstrated a key developmental principle that a gradient of molecules can determine polarity and position in the embryo

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Segmentation Pattern Segmentation genes produce proteins that direct formation of segments after the embryo’s major body axes are formed Positional information is provided by sequential activation of three sets of segmentation genes: gap genes, pair-rule genes, and segment-polarity genes

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Identity of Body Parts The anatomical identity of Drosophila segments is set by master regulatory genes called homeotic genes Mutations to homeotic genes produce flies with strange traits, such as legs growing from the head in place of antennae

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

C. elegans: The Role of Cell Signaling The nematode C. elegans is a very useful model organism for investigating the roles of cell signaling, induction, and programmed cell death in development Researchers know the entire ancestry of every cell of an adult C. elegans—the organism’s complete cell lineage Video: C. elegans Embryo Development (time lapse) Video: C. elegans Embryo Development (time lapse)

LE Nervous system, outer skin, mus- culature Time after fertilization (hours) Musculature, gonads 0 10 Zygote First cell division Hatching Outer skin, nervous system Intestine Musculature Germ line (future gametes) Intestine EggsVulva ANTERIOR 1.2 mm POSTERIOR

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Induction As early as the four-cell stage in C. elegans, cell signaling helps direct daughter cells down the appropriate pathways, a process called induction

LE 21-16a Anterior EMBRYO Posterior Receptor Signal protein Signal Posterior daughter cell of 3 Anterior daughter cell of 3 Will go on to form muscle and gonads Will go on to form adult intestine Induction of the intestinal precursor cell at the four-cell stage

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Induction is also critical later in nematode development, as the embryo passes through three larval stages prior to becoming an adult

LE 21-16b Epidermis LARVA ADULT Gonad Anchor cell Signal protein Vulval precursor cells Inner vulvaOuter vulva Induction of vulval cell types during larval development

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A number of important concepts apply to C. elegans and many other animals: – In the developing embryo, sequential inductions drive organ formation – The effect of an inducer can depend on its concentration – Inducers produce their effects via signal transduction pathways, as in adult cells – The induced cell often responds by activating genes that establish a pattern of gene activity characteristic of a particular kind of cell

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Programmed Cell Death (Apoptosis) Cell signaling is involved in apoptosis, programmed cell death During apoptosis, a cell shrinks and becomes lobed (called “blebbing”); the nucleus condenses; and the DNA is fragmented

LE  m

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In C. elegans, a protein in the outer mitochondrial membrane is a master regulator of apoptosis Research on mammals has revealed a prominent role for mitochondria in apoptosis A built-in cell suicide mechanism is essential to development in all animals

LE Ced-9 protein (active) inhibits Ced-4 activity Mitochondrion Ced-4Ced-3 Inactive proteins Death signal receptor No death signal Death signal Ced-9 (inactive) Cell forms blebs Active Ced-4 Active Ced-3 Other proteases Nucleases Activation cascade Death signal

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The timely activation of apoptosis proteins in some cells functions during normal development and growth in both embryos and adult In vertebrates, apoptosis is part of normal development of the nervous system, operation of the immune system, and morphogenesis of hands and feet in humans and paws in other mammals

LE Interdigital tissue 1 mm

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plant Development: Cell Signaling and Transcriptional Regulation Genetic analysis of plant development, using model organisms such as Arabidopsis, has lagged behind that of animal models because fewer researchers work on plants Plant research is now progressing rapidly, thanks to DNA technology and clues from animal research

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mechanisms of Plant Development In general, cell lineage is much less important for pattern formation in plants than in animals The embryonic development of most plants occurs inside the seed, and thus is relatively inaccessible to study However, other important aspects of plant development are observable in plant meristems, particularly apical meristems at the tips of shoots In meristems, cell division, morphogenesis, and differentiation give rise to new organs

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pattern Formation in Flowers A floral meristem has three layers of cells (L1-L3), all of which participate in forming a flower with four types of organs: carpels (containing egg cells) stamens (containing sperm-bearing pollen) petals sepals (leaflike structures outside the petals)

LE Cell layers L1 L2 L3 Floral meristemAnatomy of a flower Stamen Carpel Petal Sepal Tomato flower

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings To examine induction of the floral meristem, researchers grafted stems from a mutant tomato plant onto a wild-type plant and then grew new plants from the shoots at the graft sites The new plants were chimeras, organisms with a mixture of genetically different cells The number of organs per flower depended on genes of the L3 (innermost) cell layer

LE Sepal Petal Carpel Stamen Wild-type, normal Fasciated (ff), extra organs Graft Chimeras Key Wild-type (FF) Fasciated (ff) Floral meristem L1 L2 L3 PlantFlowerPhenotypeFloral Meristem Wild-type parent Fasciated (ff) parent Wild-type Fasciated Wild-type Chimera 1 Chimera 2 Chimera 3

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In contrast to genes controlling organ number in flowers, genes controlling organ identity (organ identity genes) determine the types of structures that will grow from a meristem Organ identity genes are analogous to homeotic genes in animals Mutations cause plant structures to grow in unusual places, such as carpels in place of sepals

LE Wild typeMutant

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 21.4: Comparative studies help explain how the evolution of development leads to morphological diversity Evolutionary developmental biology (“evo-devo”) compares developmental processes of different multicellular organisms

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Widespread Conservation of Developmental Genes Among Animals Molecular analysis of the homeotic genes in Drosophila has shown that they all include a sequence called a homeobox An identical or very similar nucleotide sequence has been discovered in the homeotic genes of both vertebrates and invertebrates

LE Adult fruit fly Fruit fly embryo (10 hours) Fly chromosome Mouse chromosomes Mouse embryo (12 days) Adult mouse

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Related genetic sequences have been found in regulatory genes of yeasts, plants, and even prokaryotes In addition to developmental genes, many other genes are highly conserved from species to species Sometimes small changes in regulatory sequences of certain genes lead to major changes in body form, as in crustaceans and insects

LE ThoraxAbdomen Genital segments ThoraxAbdomen

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In other cases, genes with conserved sequences play different roles in different species In plants, homeobox-containing genes do not function in pattern formation as they do in animals

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Comparison of Animal and Plant Development In both plants and animals, development relies on a cascade of transcriptional regulators turning genes on or off in a finely tuned series But genes that direct analogous developmental processes differ considerably in sequence in plants and animals, as a result of ancestry