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Amniotes: Lecture 7. Diversity and Reproduction

Question Answer
What defines an Anapsid?Continuous bony covering posterior to orbit
What defines a Diapsid/Synapsid?Two/one fenestra posterior to orbit
What groups are within the Diapsids?Probably turtles, lepidosaurs and archosaurs (crocodiles and birds)
Lepidosaurs (reptiles with overlapping scales) are a superorder containing which two orders?Sphenodontia (two species endemic to New Zealand, genus Sphenodon) and Squamata (lizards and snakes; lizards form a paraphyletic group)
Mammals are a class of animals forming which three groups?Monotremes (echidna and platypus; only mammals to possess a true cloaca), Marsupials and Placental mammals.
Which groups make up the therians?Marsupial and placental mammals
Name 7 Amniote characteristics1) Lack of metamorphosis and larval stage 2) Egg with shell and extraembryonic membranes. 3) Reduced permeability of skin 4) Water absorbing cloaca (posterior opening for intestinal, reproductive and urinary tract) 5) Internal fertilisation 6) Two or more vertebrae in sacrum (at base of spine between lumbar and cocyx) 7) Single astragalus (tarsal bone) in ankle.
Compare bird eggs to salamandersBird eggs tend to be relatively larger, fewer per clutch, a large yolk, lined by several membranous extensions of the embryo, dessicant resistant yet permeable to oxygen.
How may extraembryonic membranes do Amniotes possess?4
What does the yolk sac provide?Food supply. Formed from mesoderm and endoderm
What is the function of the Amnion?It is a fluid filled protective membrane formed from ectoderm and mesoderm.
What is the Chorion?An external continuation of the amnion, also formed from ectoderm and mesoderm.
What is the Allantois?A site for water storage and waste collection formed from mesoderm and endoderm.
Besides exchanging nutrients and waste and allowing sufficiently independent offspring that can proceed rapidly to adult hood, what is another important function of the Amniote egg?It resists the maternal immune system
Define oviparityEggs develop outside the mother's reproductive tract, eg turtles, crocodiles, birds, monotremes, some squamates
Define ovoviviparityEggs develop inside the reproductive tract with minimal nutrients beyond yolk, some squamates
Define vivparityEggs develop inside female reproductive tract, dependent on nutrients beyond yolk, therians (placentals and marsupials)
What do newborn marsupials have?Accelerated development of forelimb, shoulder girdle and facial skeleton, the capacity to find the teat and suckle to allow them to travel from maternal genital tract to pouch.
What sort of placenta do placental mammals have?A chorioallantoic placenta. This has a small yolk sac, intimate maternal-foetal exchange via uterine wall and a trophoblast which stops the immune repsonse of the mother and allows a longer intrauterine time.
What sort of placenta do marsupial mammals have?A choriovitelline placenta. This has a large yolk sac, less intimate maternal-foetal exchange and a "trophoblast" with little immunological protection.
Compare placental and marupial mammals at birthPlacental mammals are much more precocious than marsupials, for example a human baby as 6 layers of cerebral cortex, its eyes open and thermoregualtion prior to birth. Marsupial mammals are dependent on lactation and postnatal development to reach adult body size and shape.
Why does vivparity evolve so frequently among squamates?Egg retention allows a more thermally stable environment for development, it is associated with the thinning of the eggshell in utero.
Give an example of evolution towards vivparity in squamatesPackard et al 1977 showed that Green snakes in North Michigan where winters were very cold retained their eggs for longer than Green snakes in South Michigan, 300 miles South. N. Michigan eggs hatched 4-14 days after deposition, S. Michigan eggs, around 30 days. They suggest that selection for thermal stability in unstable environments leads to repeated evolution of ovoviviparity.
Why doesn't viviparity evolve in birds, crocodiles or turtles?Too dependent on the uptake of calcium and magnesium from the egg shell for the developing embryo. Squamates have a greater dependence on yolk for mineral uptake.
Which Carboniferous early amniote showed which Amniote features?Paleothyris had a pterygoid transverse process (part of vertebrae where muscles and ligaments attach); single circular occipital condyle (part of skull which articulates with vertebrae) and a single astragalus (lower part of ankle joint)
Describe a typical amphibian lifestyleNumerous externally fertilised eggs are laid in an aquatic environment, aquatic larval stage, terrestrial adults.
Give two examples of diversity in amphibian reproductioncave dwelling plethodontids (lungless salamanders) like Eurycea do not metamorphose but remain in larval form for life: paedomorphosis. Pipa pipa (Silurian toad) has direct ovoviviparous development, her offspring develop underneath the skin on her back.
Some sub famililes of Plethodontidae show what sort of developments?Direct. The eggs are laid on land and hatch as adults, there is no metamorphosis, the yolk sac is relatively large, the adults do not exceed 10cm in length.
What reproductive strategy might early amniotes have used?Laying eggs terrestrially which then develop directly (no metamorphosis). Terrestrially deposited eggs may have relieved predator pressure.
From Elinson et al. Development in frogs with large eggs and the origin of amniotes. How large is the egg of the Puerto Rican tree frog?3.5 mm, has an alteration of its fate map and a secondary coverage of its yolky cells,
What localisation within the egg are likely to change with increased egg size?mRNAs at the vegetal pole specifying embryonic germ layers and the dorsal axis. Elinson et al

Amniotes: Lecture 8, Constraint and Circulation

Question Answer
What is Neo-Darwinism?George Romanes coined the term neo-Darwinism to refer to the version of evolution advocated by Alfred Russel Wallace and August Weismann with its heavy dependence on natural selection. It is the "Modern Synthesis" of genetics, systematics and paleontology.
How did E. Mayr in 1963 define neo-Darwinism?"The proponents of the synthetic theroy maintain that all evolution is due to the accumulation of small genetic changes, guided by natural selection"
What did R. A Fisher write in 1930?"The Genetical Theory of Natural Selection", it rejected the idea of "blending inheritance"
What did Theodosius Dobzhansky write in 1937? "Genetics and the Origin of the Species"
What is the purpose of the "spandrel" analogy put forward by Gould and Lewontin in 1979?To highlight the importance of architectural constraints in evolutionary biology and to move away from Panglossian adaptionism "Our noses were made to carry spectacles, so we have spectacles"
Give four examples of potential "spandrels" in biology, a morphology or adaptation that is limited by structural constraint.1) Lack of vivparity in crocodiles, birds and turtles. 2) Lack of large-size in plethodontid (lungless, directly developing) salamanders. 3) Relative lack of forelimb diversity in marsupials. 4) Basic plan of Amniote circulation, eg single arch in mammals and birds.
What is structuralism and who advocated it?Geroges Cuvier. Emphasis on adaption to explain all aspects of biological form "form follows function."
Who advocated formalism and what is it?Etienne Geoffroy St. Hilare. Emphasis on constraint, "function follows form"
What is Haeckel's Biogenetic law?Ontogeny recapitulates phylogeny, novel phenotypes are added as terminal stages onto a conserved developmental pattern.
Describe circulation in lungfishIt can be directed to either the lungs or the gills to maximise oxygenation of the blood.
Describe circulation in a salamanderIt can be directed to the gills or lungs, corresponding to water breathing as larva and air-breathing as adult. Neotony has been observed in all salamander families, gills may be retained during sexual maturity; usually however metamorphosis continues with the loss of gills and the ability to live terrestrially.
Describe the "reptile" heart2 divided atria, 1 ventricle. However crocodiles have a 4 chambered heart.
In birds what develops into the dorsal aorta?aortic arch (4) on the right side
In mammals, what develops into the dorsal aorta?aortic arch (4) on the left side
Define systemic archIn the embryo of a tetrapod, the fourth aortic arch which, in the adult, becomes the main source of the blood supply to all parts of the body other than the head. In amphibians and 'reptiles' both right and left arches are present in adults. In birds only the right arch is present in adults and in mammals only the left arch is present in adults.
What is counter-intuitive about ventricles and systemic arches in crocodiles?The left systemic arch comes off the right ventricle during the right to left shunt, and vice versa.
Describe what happens normally in a crocodile heartBoth systemics are functionally supplied from the left ventricle. The valve (in the left systemic arch) is open.
Describe what occurs during the right to left shunt in a crocodile heartThe pulmonary artery is constricted, this results in a build up of pressure in the right ventricle, causing the valve to open. The right ventricle predominantly supplies the left systemic arch instead of the pulmonary artery.
What does the right to left shunt result in?A decrease in metabolic rate, and greater uptake of oxygen from the blood as it is continually pumped round the body.
What are the three arterial arches in Amniotes and which aortic arches do they form from?Carotid (the third), systemic (the fourth), and pulmonary (the sixth) arches.
What is the ductus arteriosus?In human foetus it connects the beginning of the left pulmonary artery to the aortic arch so that blood bypasses the lungs in embryonic circulation to facilitate gas exchange at the placenta. It is the remains of the 6th branchial arch.
What does the ductus arteriosus degenerate to in adults (within 3 weeks of birth in humans)?Ligamentum arteriosum
What provides a similar function to the incomplete septum in ectotherms (or the foramen of Panizza in crocodiles) in allowing blood to enter the left atrium from the right atrium and along with the ductus arteriosus allows blood to bypass the pulmonary system in foetal mammals?Foramen ovale. Develops during the fourth week of gestation in humans.
What does the complete division of ventricles ensure?The complete seperation of oxygenated and deoxygenated blood, and a continuous supply to the lungs.
What could result in increased pressure on lungs and what are its consequences?Being underwater. Increased resistance to blood entering pulmonary artery in non-mammalian Amniotes, blood is directed to the systemic aorta
What may cardiac defects in humans resemble?Cardiac conditions in other amniotes, eg persistant truncus arteriosus (does not properly divide into aorta and pulmonary artery), persistant non-vestigial ligamentum arteriosum (blood from pulmonary artery diverted to aortic arch) and persistant foramen ovale (atria never properly seperated)
What does the single systemic arch in both mammals and birds reflect?The need for relatively high blood pressure for oxygen intensive systemic flow.
If form follows function then should aquatic mammals have similar circulation as other aquatic diving vertebrates?Yes, they should have incomplete septum seperation. This is not the case: despite frequent and deep diving sperm whales retain complete dual circuit vasculature and a 4 chambered heart.
Name five adaptations in the circulation of diving mammals1) Brachycardia ("heart slowness"), 2) flat heart 3) high red blood cell concentration 4) High concentration of myoglobin 5) Increased reliance on anaerobic respiration coupled with a greater tolerance of lactic acid
What does the form of the whale heart reflect?The constraints inherited from terrestrial ancestors.
How do catfish (a teleost) breathe?Through their alimentary canal (farting!) can oxygenate their blood via their intestinal tract.