Lab 1&2 Answers

© BIOL204 UBCVancouver 2023Jan Lab 1: Sample Solutions Vertebrate Development Table 1.1. Term Translation of Greek and Latin roots Definition Ectoderm ect-: outside -derm: skin Definition: Outer skin The outermost embryonic germ layer Notochord not-: back -chord: cord Definition: Back cord A long axial rod, located dorsal to the digestive tract. It is composed of a wall of fibrous connective tissue surrounding cells and/or a fluid-filled space. Gastrulation gastrula-: stomach, belly - tion: formation of Definition: Gut formation Early stage of embryonic development in which the 3 germ layers differentiate, cells migrate from the outer surface of the embryo to inside the embryo, and (in many taxa) the early gut is formed. Mesenchyme mes-: middle -chym: juice Definition: Inner juice Loosely associated cells of mesodermal origin. Blastopore blast-: budding -pore: passage or opening Definition: budding opening Earliest opening in the developing embryo, this opening forms in the blastula at the beginning of gastrulation. Cells from the outer surface of the embryo migrate through the blastopore during gastrulation. (Note: in the chick the cells migrate through the primitive streak instead.) Macrolecithal macro-: large -lecith: yolk Definition: Large yolk Pertaining to eggs with large quantities of stored yolk. Archenteron arch-: beginning, first -enter: intestine, gut Definition: beginning gut The early embryonic gut – this space is lined with endoderm Sclerotome scler-: bone, hard material -tome: part, section Definition: bone section The region of the somites (epimere mesoderm) that gives rise to the vertebrae and ribs. 1

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© BIOL204 UBCVancouver 2023Jan Other useful diagrams of frog embryos from lecture slides (cross-section and lateral view) showing epimere mesoderm (dermatome, sclerotome and myotome), mesomere mesoderm, and hypomere mesoderm 4

© BIOL204 UBCVancouver 2023Jan Figure 1.8: Four serial cross-sections (A, B, C, and D and one para-sagittal section (E) through the 24-hour chick embryo. Refer to Figure 1.7 for the location of these sections on a whole-mount. Development Discussion Questions SAMPLE SOLUTIONS 1. Consider the models of development in frogs ( Xenopus ) and amphioxus ( Branchiostoma ). a) Describe the major visible difference during cleavage for these two organisms. During cleavage in amphioxus, at any given time (for example, when the embryo has 8 cells, or 16 cells) all the cells are approximately the same size. However, in frogs, the cells at the animal pole are smaller in size than those at the vegetal pole. b) Frog eggs are mesolecithal while amphioxus eggs are microlecithal – how does this difference in amount of yolk affect cleavage in these two organisms? Relate this difference in amount of yolk to your observations from part a). Frog eggs contain more yolk than amphioxus eggs. In frogs, the yolk is noticeably more concentrated at the vegetal pole than the animal pole. Larger amounts of yolk decrease the rate of cell division, so the embryonic cells of the vegetal pole divide (cleave) more slowly than those in the animal pole. As a result, at any given time during cleavage, the cells at the vegetal pole tend to be 5

© BIOL204 UBCVancouver 2023Jan a) splanchnic hypomere mesoderm b) It is important that blood vessels develop early in the embryo because blood vessels transport oxygen and nutrients (e.g., glucose) to developing tissues and remove wastes (e.g., carbon dioxide) from the developing tissues. During the earliest stages of development, the embryo is small enough that substances are transported using only diffusion, but as the embryo increases in volume, diffusion alone is too slow to meet the metabolic requirements of the growing tissue, so a more rapid method of transport is required or else the cells of the embryo will die from lack of nutrients or will be poisoned by their own wastes. c) The vitelline vessels transport nutrients from the yolk to the developing tissues of the embryo. 5. View a microscope slide showing a transverse-section through the trunk of the 48-hour chick embryo. Draw a quick schematic of this transverse section. Label the notochord, neural tube, epidermal ectoderm, endoderm, coelom, and all the different regions of mesoderm, including the approximate locations of each of the three parts of the epimere mesoderm (somite). Which adult structures will arise from each of these three parts of the epimere mesoderm? Three parts of the epimere mesoderm: ● Myotome – gives rise to most skeletal muscles ● Dermatome – gives rise to the dermis of the skin ● Sclerotome – gives rise to the vertebrae and ribs Splanchnopleure = endoderm and splanchnic hypomere mesoderm (need to add to drawing) Somatopleure = ectoderm and somatic hypomere mesoderm. (need to add to drawing above) 6. How is the neural crest ectoderm different from other ectoderm? Most ectoderm (neural plate ectoderm and epidermal ectoderm) is organized into sheets of closely connected cells that all move together. In comparison, neural crest cells are only loosely associated with each other, and so can move relatively independently and quickly throughout the embryo. Neural crest ectoderm, neural plate ectoderm, and epidermal ectoderm each give rise to different structures in the adult. 7. What is mesenchyme? How is it different from other mesoderm? Most mesoderm is organized into sheets of closely connected cells that move through the embryo together. In comparison, mesenchyme cells are only loosely associated with each other, and so can move relatively independently and quickly 7

© BIOL204 UBCVancouver 2023Jan throughout the embryo. Mesenchyme also gives rise to different structures in the adult than each of the other types of mesoderm. 8. Consider development in the frog. a) What is the coelom? Provide a detailed definition. b) How is the coelom different from the blastocoel? Explain. c) How is the coelom different from the archenteron? Explain. a) The coelom is a fluid-filled body cavity – it is completely enclosed within the body, and is surrounded by splanchnic hypomere mesoderm and somatic hypomere mesoderm. The coelom gives rise to the main body cavities of the adult body, which will contain internal organs such as the liver and intestines. b) The blastocoel is the hollow space within the blastula, formed during the stage of blastula formation, but it is displaced and shrinks during the process of gastrulation. The coelom is not formed until after gastrulation (during neurulation), and is not lost – it gives rise to the main body cavities. The blastocoel is surrounded by undifferentiated cells while the coelom is only surrounded by mesoderm. c) The archenteron forms during gastrulation while the coelom forms after gastrulation (during neurulation). The archenteron will give rise to the lumen (the space inside) of the digestive tract, while the coelom will give rise to the major body cavities. The archenteron is surrounded by endoderm while the coelom is surrounded by mesoderm. The archenteron is (usually) open to the environment through the blastopore, while the coelom is completely enclosed within the body. Lab 2: Sample Solutions Introduction to Phylogeny Table 2.1 Character Matrix (Data Collected from Preserved Specimens in Lab) Characters Echinodermat a Cucumaria Vertebrata Petromyzon Cephalochordata Branchiostoma Hemichordata Balanoglossus Urochordata Clavelina Dorsal hollow nerve cord - + + - Nerve cord is not hollow + Notochord - + + - + Pharyngeal slits - Secondary loss + + + + Post-anal tail - + + - + Cranium - + - - - Tunic - - - - + Endostyle - + In ammocoetes, is the thyroid in adult + - + Deuterostom e Development + + + + + 8

© BIOL204 UBCVancouver 2023Jan · In this lab we investigated only one representative species for each taxon. We could examine multiple representative species for each taxon to improve our dataset, because having any taxon represented by only one species could potentially be misleading, especially when considering large taxa containing a great diversity of species (for example, using a lamprey to represent all vertebrates). · In this lab, we examined only one or two individuals for each species. Examining more replicates for each species would improve our data collection, because with more replicates we would reduce the effects on our data of individual variation within a species. 11. a) This is a phylogeny that includes the taxa that you observed today. On this tree, indicate where each of the nine characters from Table 2.1. have evolved. b) The phylogeny (the branching pattern and the relationships among the taxa) above is based on a phylogeny of the deuterostomes from the 8 th Edition of our textbook that incorporates molecular data and new anatomical evidence (Kardong, p.55, 2019) . Compare this tree to the deuterostome phylogenetic tree below; it is from the 4 th Edition (page 53, published in 2006). 10

© BIOL204 UBCVancouver 2023Jan What differences do you notice in the hypothesized relationships among vertebrata, cephalochordata, and urochordata in the two phylogenies? Explain a possible reason for these differences. In the 2006 tree, the Cephalochordata and the Vertebrata are sister groups (are the most closely related), whereas in the 2019 tree, Urochordata and Vertebrata are sister groups (are the most closely related). A phylogenetic tree is a hypothesis that explains the available data, so when more data becomes available, we sometimes have to adjust the hypothesis to reflect this new data (or in other words, the new data supports a different tree). In recent years, extensive genetic analysis of deuterostomes has generated a great deal of new data. The more recent phylogenetic tree (8 th Edition) is based on this new genetic and morphological evidence in addition to the previous available data, whereas the older phylogenetic tree (4 th Edition) is based primarily on morphological characters only. Superficially, the overall appearance of Cephalochordata seems more similar to Vertebrata than to an adult Urochordata such as Clavelina , but the new molecular data draws attention to the importance of looking at the larval stages of Urochordata in which we can see that many morphological features closely resemble those of Vertebrata. 12. Tunicates such as Clavelina have a dramatic metamorphosis during the transition from larvae to adults. Consider the general body structure of the adult compared to the larva. Based on the differences in body structure between these two stages of the life cycle, predict a possible function of the larval stage in this animal. The adults do not have any structures specialized for swimming. They are sessile – they attach to a hard structure and do not move from place to place. The larval stage has a muscular post-anal tail supported by a notochord – they can swim and therefore can move from place to place relatively easily, so the larval stage is useful for dispersal. (The adults are benthic while the larvae are pelagic, so you could also suggest that living in these different environments may minimize the competition for resources between adults and larvae. However, in most tunicate species the larval stage lasts less than a day and does not feed, so there would not be competition between adults and larvae for food anyway.) 13. Both Branchiostoma and Clavelina are filter feeders as adults. Describe filter feeding in these two organisms. Both Branchiostoma and Clavelina use cilia to pump water. The water is pumped in through the mouth (or incurrent siphon) and into the pharynx, through the pharyngeal slits into the atrium, then exits the body through the atriopore (or excurrent siphon). Food particles in the water are captured on the bars of the pharyngeal basket by the sticky mucous produced by the endostyle and are transported by cilia into the digestive regions of the digestive tract. 14. The endostyle is one of the five chordate characteristics. 11