Lecture 3 BIO3147 (2024) PDF

Figure 1.12 Evolution of pharyngeal arch structures in the vertebrate head Thus, the cells that form gill supports in fish form the middle ear bones in mammals. Figure 1.18 Larval stages reveal the common ancestry Larval stages reveal the common ancestry of two crustacean arthropods, barnacles (A) a...

Figure 1.12 Evolution of pharyngeal arch structures in the vertebrate head Thus, the cells that form gill supports in fish form the middle ear bones in mammals. Figure 1.18 Larval stages reveal the common ancestry Larval stages reveal the common ancestry of two crustacean arthropods, barnacles (A) and shrimp (B). Barnacles and shrimp both exhibit a distinctive larval stage (the nauplius) that underscores their common ancestry as crustacean arthropods, even though adult barnacles—once classified as mollusks—are sedentary, differing in body form and lifestyle from the free-swimming adult shrimp. A larva is shown on the left in each pair of images, an adult on the right. What do we have in common with sea squirts? Tunicate (urochordate; wikipedia ) Amphioxus (cephalochordate; wikipedia) Larval stage of the ascidian tunicate C. intestinalis reveal its common ancestry with other chordates. Station Biologique Roscoff Ciona intestinalis (Adult form) Linnaeus grouped sea squirts (tunicates) as mollusks (1767) Dorsal nervous system and a notochord (Kowalevsky 1866) What do we have in common with sea squirts? Corbo et al., 2001 Phylogenetics first confirmed that sea squirts were chordates (left) and then demonstrated that they are the closest invertebrate relative to vertebrates! (right) This means that tunicates lost somites, which cephalochordates and vertebrates have Delsuc et al., 2006 Figure 1.20 Transitional states over the course of animal evolution (Part 2) (B) A late Jurassic (~150 mya) fossil of Archaeopteryx showing its distinctive features of both a reptilian skeleton and avian feathered wings. Figure 1.20 Transitional states over the course of animal evolution (Part 3) (C) Tiktaalik roseae emerged 375 mya from the water to be the first animal hypothesized to walk on land. This fossil (upper) and reconstruction (lower) revealed characteristics of both fish fins and amphibian forelimbs, among other characteristics. [found in Nunavut, Canada by Daeschler, Shubin, and Jenkins] Figure 1.23 The tree of life—an illustration of the major branches of life Figure 1.21 Homologies of structure among a human arm, a seal forelimb, a bird wing, and a bat wing; homologous supporting structures are shown in the same color Homology of structure among a human arm, a seal forelimb, a bird wing, and a bat wing; homologous supporting structures are shown in the same color. All four limbs were derived from a common tetrapod ancestor and thus are homologous as forelimbs. The adaptations of bird and bat forelimbs to flight, however, evolved independently of each other, long after the two lineages diverged from their common ancestor. Therefore, as wings they are not homologous, but analogous. Figure 1.22 Development of bat and mouse forelimbs (Part 1) Pentadactyl groundplan Figure 1.22 Development of bat and mouse forelimbs (Part 2) Figure 1.22 Development of bat and mouse forelimbs (Part 3) (C) Comparison of mouse and bat forelimb morphogenesis Both limbs start as webbed appendages, but the webbing between the mouse’s digits dies (apoptosis) at embryonic day 14 (arrow). The webbing in the bat forelimb does not die and is sustained as the fingers grow. Figure 1.24 The developmental evolution of life This illustration depicts key developmental adaptations that occurred over the course of evolutionary history in animals. The last eukaryotic common ancestor (LECA) gave rise to both plants and animals 2000 million years ago (mya). (1)Colonization of choanoflagellate cells. (2) Development of a 2-layered organism with a proliferative inner layer and an epithelial filter-feeding outer layer. (3) Digestive architectures emerge with the evolution of tighter junctions and extracellular matrix (neon blue). (4) A primitive gut with aboral and oral openings appears, as in the sponge. (5) Ctenophores, such as this comb jelly, exhibit the first interconnected system of nerve-like cells. (6) Cnidarians such as the sea anemone show the first signs of gastrulation. (7) Bilateral symmetry evolves (aceols) and (8) segmentation emerges, generating (9,10) a diversity of arthropod lineages. (11) Adaptation of mesoderm produces the first axial derivative—the notochord (red)—giving rise to chordates. (12–14) From jawless fish (12, lamprey) to jawed fish (13, teleost) and from paired fins to articulating forelimbs (14, Tiktaalik), metazoans walk out of the water. (15,16) Among the terrestrial tetrapods, reptiles (15) further adapt their forelimbs into wings, giving rise to avian species (16). Remarkable complexity of development (Amphibian) Jan van Ijken Figure 2.1 Cell fate determination (A) Two differently positioned blastula cells are specified to become distinct muscle and neuronal cells when placed in isolation. (B,C) The two different blastula cells cultured together. (B) Red cell specified—but not determined—to form muscle, adopts neuronal fate due neighbour interactions. (C) Red cell specified and determined to become muscle at the time of culturing, will continue to differentiate into a muscle cell despite neighbour interactions. Three Major Modes of Cell Specification Figure 2.2 Autonomous specification (A–C) Differentiation of trochoblast (ciliated) cells of the snail Patella starting at 16-cell stage. Figure 2.3 Autonomous specification (D–G) Differentiation of a Patella trochoblast cell isolated from the 16-cell stage and cultured in vitro. Even in isolated culture, cells divide and become ciliated correctly Figure 2.3 Autonomous specification of the tunicate (A) Originally, described by Conklin, the yellow crescent is seen in the tunicate from the egg to the larva (dense yellow-orangered coloration). (B) Schematic of a Styela partita zygote (left), shown shortly before the first cell division, with the fate of the cytoplasmic regions indicated. The 8-cell embryo on the right shows these regions after three cell divisions. (C) Confocal section through a larva of the tunicate Ciona savignyi. Different tissue types were pseudocolored. Figure 2.3 Autonomous specification of the tunicate (Part 4) (D) A linear version of the S. partita fate map, showing the fates of each cell of the embryo. Can trace the lineage of cells. Figure 2.4 Autonomous specification in the early tunicate embryo When the four blastomere pairs of the 8-cell embryo are dissociated, each forms the structures it would have formed had it remained in the embryo. (However, the tunicate nervous system, seen at bottom of a4.2, is conditionally specified) Figure 2.5 The macho mRNA in the oocyte regulates muscle development in tunicates (Part 1) macho transcript is localized to the vegetal-most end of the egg and differentially expressed only in the B4.1 blastomere. Figure 2.5 The macho mRNA in the oocyte regulates muscle development in tunicates (Part 2) (B) Knockdown of macho function by injection of targeting antisense oligonucleotides causes reductions in muscle differentiation, whereas ectopic misexpression of macho in other blastomeres results in expanded muscle differentiation. Figure 2.6 Conditional specification (A) What a cell becomes depends on its position in the embryo. Its fate is determined by interactions with neighboring cells. (B) If cells are removed from the embryo, the remaining cells can compensate for the missing part. Figure 2.7 Driesch’s demonstration of conditional specification (A) An intact 4-cell sea urchin embryo generates a normal pluteus larva. (B) When one removes the 4-cell embryo from its fertilization envelope and isolates each of the four cells, each cell can form a smaller, but normal, pluteus larva. (C) Note that the four larvae derived in this way are not identical, despite their ability to generate all the necessary cell types. (All larvae are drawn to the same scale.) Figure 2.7 Driesch’s demonstration of conditional specification Historical tangent: Driesch's goal was to explain development in terms of the laws of physics and mathematics. This strikingly modern concept of nuclear-cytoplasmic interaction and nuclear equivalence eventually caused Driesch to abandon science. Because the embryo could be subdivided into parts that were each capable of reforming the entire organism, he could no longer envision it as a physical machine. In other words, Driesch had come to believe that development could not be explained by physical forces. Harking back to Aristotle, he invoked a vital force, entelechy (“internal goal-directed force”), to explain how development proceeds. Essentially, he believed that the embryo was imbued with an internal psyche and wisdom to accomplish its goals despite the obstacles embryologists placed in its path. Unable to explain his results in terms of the physics of his day, Driesch renounced the study of developmental physiology and became a philosophy professor Figure 2.9 The syncytial blastoderm in Drosophila melanogaster (A) Schematic of the progression of blastoderm cellularization in Drosophila (nuclei are red). (B) Still frames from a timelapse movie of a developing Drosophila embryo with nuclei that are premitotic (blue) and actively dividing in mitosis (purple). Cellularization only happens at cycle 14! Nuclei are dynamically ordered within the syncytium of the early embryo, holding their positions using cytoskeletal elements. Figure 2.10 Positioning of nuclei during the interphase stage of nuclear cycle 13 of the Drosophila melanogaster syncytium Syncytium: The nuclei undergo S-phase (DNA replication) and sister chromatids get pulled apart and re-assembled into nuclei containing full sets of homologous chromosomes, but cytokinesis does not occur. Thus, the nuclei multiply in a common cytoplasmic space. Nuclei are dynamically ordered within the syncytium of the early embryo, holding their positions using the cytoskeletal elements associated with them (‘asters’). Fly Embryogenesis LA Royer Figure 2.11 Morphogen gradients during syncytial specification in Drosophila melanogaster The amounts of each morphogen differentially activate transcription of the various nuclear genes that specify the segment identities of the larval and the adult fly. Anterior-posterior specification originates from morphogen gradients in the egg cytoplasm, specifically of the transcription factors Bicoid and Caudal. Morphogen gradients during syncytial specification in Drosophila melanogaster Figure 2.12 The developmental landscape of cell fate maturation (Part 1) Figure 2.12 The developmental landscape of cell fate maturation (Part 2) (B) T-distributed stochastic neighbor embedding (tSNE) plots for each developmental time point, with cells colored according to their expressed genes of known germ layer identity. Figure 2.13 Developmental ‘trees’ (2018 Science Breakthrough of the year) (C) Visualization of a full gene expression landscape with representative cell states over the course of the first 24 hours of zebrafish embryonic development. The earliest time points are at the image’s center, with more differentiated cells emanating outward. Figure 2.14 The developmental landscape of cell fate maturation (Part 5)