Chordate Phylogeny and Its Origins

Chordate Phylogeny

• Chordates are a large phylum of animals that includes vertebrates, sea squirts, and lancelets.
• Chordates have a notochord, a dorsally situated central nervous system, and gill slits at some stage in development.
• Chordates emerged from a deuterostome ancestor during the Cambrian period.

Echinoderm Origin

• Johannes Muller (1860) proposed that echinoderm larvae are the ancestors of chordates.
• Echinoderms are deuterostomes and have mesodermal skeletal elements made of calcium carbonate.

Hemichordate Origin

• Romer (1959) suggested ancestral deuterostomes were sedentary tentacle feeders with mucous-laden ciliated tentacles that trapped plankton.
• Hemichordates resemble modern hemichordates with their tentacled feeding mechanism.

Urochordate Origin

• W. Garstang (1928) and N.J. Berrill (1955) proposed that the tadpole larvae of urochordates carried typical chordate characters.
• Garstang suggested that chordates evolved from sessile filter-feeding urochordates, with the larval stage becoming the adult stage through neoteny.

Cephalochordate Origin

• Chamberlain (1900) proposed that cephalochordates are both primitive and advanced, possessing all chordate characters in a typical state.
• The fossil Pikaia gracilens shows evidence of cephalochordate ancestry.

Combined Theory

• E.J.W. Barrington (1965) proposed a combined theory—the common ancestor of echinoderms and chordates was a sessile ciliary arm feeder.
• This ancestor evolved into a free-swimming form during a time of food scarcity.
• Fossils like Yuknessia and Pikaia gracilens support this hypothesis.

Chordate Characteristics

• Dorsal hollow nerve cord
• Notochord
• Pharyngeal pouches
• Tail

Embryology

• The study of prenatal development, including gametes, fertilization, and embryo/fetus development.

Ernst Haeckel's Theory

• Ontogeny recapitulates phylogeny: Embryonic development stages retrace the evolutionary history of a species.

Gavin de Beer's Modification

• Embryonic structures may be retained, lost, or modified.
• Developmental stages may be present in earlier stages but not in later stages.

Gametes

• Reproductive cells (sex cells) containing haploid sets of chromosomes.

Gametogenesis

• The process of gamete formation.

Spermatogenesis

• The process of sperm cell formation, occurring within the seminiferous tubules of the testes.
• Leydig cells produce testosterone in the presence of luteinizing hormone (LH).
• Sertoli cells are "nurse calls" that support spermatogenesis.

Spermiogenesis

• The final stage of spermatogenesis, in which spermatids mature into spermatozoa.

Oogenesis

• The process of egg cell formation, occurring within the ovaries.
• Corpus luteum releases hormones that thicken the endometrium of the uterus.

Fertilization

• Fusion of haploid egg and sperm, creating a diploid zygote.

Cleavage

• Repeated mitotic cell divisions of the zygote.

Morula

• A solid ball of 16 totipotent cells.

Blastula

• A hollow ball of cells surrounding a fluid-filled cavity.

Animal Pole

• Region of small, rapidly dividing cells.

Vegetal Pole

• Region of large, slowly dividing cells and yolk.

Influence of Yolk on Cleavage

• Holoblastic: Cleavage furrow bisects the entire egg.
• Meroblastic: Cleavage furrow restricted to the active cytoplasm, typically in the animal pole of macrolecithal eggs.

Classification of Eggs by Yolk Amount

• Alecithal: No yolk (eutherian mammals).
• Microlethical: Small or negligible yolk (amphioxus, tunicates).
• Mesolecithal: Moderate yolk (dipnoans, petromyzon).
• Macrolecithal/Polylecithal: High yolk (reptiles, birds).

Distribution of Yolk

• Isolecithal/Homolecithal: Uniformly distributed yolk (echinoderms, amphioxus, mammals).
• Telolecithal: Unequal yolk distribution, concentrated in the vegetal pole (diploans, reptiles, birds).

Cleavage in Amphibians

• Mesolecithal and holoblastic, with unequal-sized blastomeres.
• Vegetal pole: Larger blastomeres, slower development.
• Animal pole: Smaller blastomeres, rapid development.
• Blastocoel: Toward the animal pole.

Cleavage in Birds

• Macrolecithal and meroblastic, with unequal-sized blastomeres.
• Vegetal pole: Large yolk mass, too large for cleavage furrows.
• Animal pole: Smaller blastoderm.

Cleavage in Mammals

• Microlecithal and holoblastic, with unequal-sized blastomeres.

Gastrulation

• Reorganization of the single-layered blastula into a multilayered gastrula.
• Cell movements result in the formation of three germ layers.

Outcomes of Gastrulation

• Formation of three germ layers (ectoderm, mesoderm, endoderm).
• Formation of the embryonic gut (archenteron).
• Appearance of the major body axes.

Three Germ Layers

• Ectoderm: Outermost layer, forms skin, brain, nervous system, and external tissues.
• Mesoderm: Middle layer, forms muscle, skeletal system, circulatory system.
• Endoderm: Innermost layer, forms gut lining and internal organs.

Mesenchymal Stem Cells

• Multipotent stromal cells that can become a variety of cell types.
• Unspecialized tissue cells that contribute to specialized tissues.

Diploblasts and Triploblasts

• Diploblasts: Organisms with only two germ layers (ectoderm and endoderm, e.g., sponges and coelenterates).
• Triploblasts: Organisms with three germ layers (ectoderm, mesoderm, and endoderm, e.g., most multicellular animals).

Protostomes and Deuterostomes

• Protostomes: Blastopore becomes the mouth (invertebrates).
• Deuterostomes: Blastopore becomes the anus (echinoderms and chordates).

Gastrulation in Amphibians

• Invagination of the blastula forms the archenteron (primitive gut).
• The blastopore becomes the anus.

Gastrulation in Chicken

• The epiblast layer invaginates, forming the primitive streak and then the primitive groove.
• The primitive groove forms the archenteron.

Neurulation

• Folding process in vertebrate embryos where the neural plate transforms into the neural tube.
• The embryo at this stage is called the neurula.

Organogenesis

• Development of internal organs from the three germ layers.
• Mesenchyme migration contributes to organ development.

Organogenesis: Endoderm

• Forms the lining of the digestive tract, respiratory system, liver, pancreas, and other internal organs.

Organogenesis: Mesoderm

• Forms the muscles, skeletal system, circulatory system, excretory system, and reproductive organs.

Organogenesis: Ectoderm

• Forms the skin, nervous system, sensory organs, and linings of the mouth and anus.

Extraembryonic Membranes

• Membranes assisting in embryo development.
• Originate from the embryo but are not considered part of it.
• Functions include nutrition, gas exchange, and waste removal.

Extraembryonic Membranes in Mammals

• Amnion: Protective membrane filled with amniotic fluid.
• Yolk sac: Contains yolk, a food source for the developing embryo.
• Chorion: Lines the inner surface of the shell, participates in gas exchange.
• Allantois: Stores metabolic wastes and participates in gas exchange.

Extraembryonic Membranes in Fish

• Only yolk sac is formed.
• Gas exchange and waste removal occur directly with the environment.

Extraembryonic Membranes in Reptiles and Birds

• Amnion protects the embryo in a fluid-filled sac.
• Yolk sac provides sustenance.
• Chorion lines the inner surface of the shell, permitting gas exchange.
• Allantois stores excretory waste and assists in gas exchange.

Extraembryonic Membranes in Monotremes

• Produce shelled eggs like their reptilian ancestors.
• Possess a yolk sac for nourishment.

Extraembryonic Membranes in Metatherians

• Pouched mammals, including marsupials.
• Embryo has minimal yolk, and the yolk sac provides a rudimentary connection to the mother's blood.
• Young are born in an immature state and rely on the pouch until they mature.

Extraembryonic Membranes in Eutherians

• Placental mammals, where extraembryonic membranes form a placenta and umbilical cord.
• Placenta facilitates gas exchange, metabolic transfer, hormone secretion, and fetal protection.

Explore the fascinating evolution of chordates, including their distinct traits and ancestral links to echinoderms and hemichordates. This quiz will cover key theories and contributions from notable scientists such as Johannes Muller and W. Garstang. Test your understanding of the origins and characteristics of this vast animal phylum.