UNIT 5:ANIMALREPRODUCTION ANDDEVELOPMENTCampbell & Reece, 2010:Chapters 46, 47
INTRODUCTION Animal reproduction takes manyforms Animal form and function can beviewed broadly as adaptationscontributing to reproductivesuccess. Both asexual and sexualreproduction occur in the animalkingdom.
SEXUAL REPRODUCTIONIs the creation of an offspringby fusionof a male gamete (sperm)and female gamete (egg)to form a zygote
ASEXUAL REPRODUCTIONIs creation of offspringwithout the fusion of egg andsperm.
MECHANISMS OF ASEXUAL REPRODUCTIONMany invertebrates reproduceasexually.One of these is by the processof fission.FISSION: Is the separation ofa parent into two or moreindividuals of about thesame size.FISSION OF ANAMOEBA
Fission of a sea anemone to producertwo daughter organisms
Second method: BUDDINGIn budding, new individuals arisefrom outgrowths of existing onesBUDDING OF AHYDRA
Third method: FRAGMENTATIONIs breaking of the body into pieces,some or all of which develop intoadults.Fragmentation must beaccompanied by regeneration -regrowth of lost body partsFRAGMENTATION ANDREGENERATION OF SEA
Fourth method: ParthenogenesisIs the development of a newindividual from an unfertilized egg
INTRODUCTION TO SEXUAL REPRODUCTIONIN ANIMALSAlmost all eukaryotic speciesreproduce sexually.Sexual reproduction results ingenetic recombination –variation within species.
GENETIC RECOMBINATION PROVIDES POTENTIALADVANTAGES:1. An increase in variation inoffspring,2. An increase in the reproductivesuccess of parents in changingenvironments.3. An increase in the rate ofadaptation.4. Elimination of harmful genes from
REPRODUCTION CYCLES AND PATTERNSOvulation is the release of matureeggs at the midpoint of a femalecycle.Most animals exhibit reproductivecycles related to changing seasons.Reproductive cycles are controlledby hormones and environmentalcues
Animals may reproduce asexually orsexually, or they may alternate thesemethods.Lizards reproduce only by a complexform of parthenogenesis that involvesthe doubling of chromosomes aftermeiosisAsexual whiptail lizards are descendedfrom a sexual species, and females stillexhibit mating behaviours.
Sexual reproduction is a specialproblem for organisms thatseldom encounter a mate.One solution is hermaphroditism,in which each individual hasmale and female reproductivesystemsSome hermaphrodites can self-fertilize.C. elegans makes both sperm andeggs and can reproduce by self-fertilization.
Individuals of some speciesundergo sex reversalsSome species exhibit male tofemale reversal (for example,certain oysters),while others exhibit female to malereversal (for example, a coral reeffish)
HOW DOORGANISMSENSURE THESURVIVAL OFTHEIROFFSPRING?
ENSURING THE SURVIVAL OF OFFSPRINGAll species produce more offspringthan the environment can handle.The proportion that survives issmall.Species with external fertilizationproduce more gametes thanspecies with internal fertilization.
Species with internal fertilization providegreater protection of the embryos andmore parental care.The embryos of some terrestrial animalsdevelop in amniote eggs with protectivelayers.Other animals retain the embryo, whichdevelops inside the female.In many animals, parental care helpsensure survival of offspring.
Complex systems contain manysets of: accessory tubes and glands thatcarrynourish,and protect gametes anddeveloping embryos.
Some organisms have a cloaca: is acommon opening between theexternal environment and thedigestive, excretory, andreproductive systemsCommon in non-mammalianvertebrates;Mammals usually have a separateopening to the digestive tract.
THREE METHODS OF REPRODUCTION IN WHICH EGGSARE LAID AND EMBRYOS DEVELOPOviparyOvoviviparyVivipary
OVIPARYOUS ORGANISMSReproduction in which eggs are laid andembryos develop outside the mothersbody.Each egg eventually hatching into ayoung animal.Little or no development occurs within themothers body.Most invertebrates and many vertebratesreproduce in this way.
OVOVIVIPAROUS:Animals produce eggs, but instead oflaying the eggs, the eggs developwithin the mothers body.The young eat unfertilized eggs in thewomb for nourishmentThe young are born alivewithout placental attachment (umbilicalcord),as certain reptiles, fishes, sharks,insects.
VIVIPAROUSWhere the embryo developswithin the uterus.and is nourished through a yolksac placenta from the mother’sblood.Young are born alive.most mammals and somereptiles and fishes.
INTRODUCTION TO ANIMAL DEVELOPMENTThe question of how a zygote becomes ananimal?The first step is cell division (mitosis)Second step: Cell differentiation (Is thespecialization of cells in structure andfunction)Third step: Morphogenesis (Is the processby which an animal takes shape.)
FERTILIZATIONFertilization depends onmechanisms that bring togethersperm and eggs of the samespecies.All fertilization requires: critical timing,mediated by:environmental cues,pheromones, and/orcourtship behaviour.
FERTILIZATIONTwo types of fertilizations: External fertilization Internal fertilization
EXTERNAL FERTILIZATIONIn external fertilization,eggs shed by the femaleare fertilized by sperm of malein the external environment.
INTERNAL FERTILIZATIONIn internal fertilization,sperm are deposited in or near thefemale reproductive tract,and fertilization occurs within thetract
PROCESS OF FERTILIZATIONFertilization brings the haploid nuclei ofsperm and egg together, forming adiploid zygote.The sperm’s contact with the egg’ssurface initiates metabolic reactions inthe egg that trigger the onset ofembryonic development.Metabolic reactions:Acrosomal reactionCortical reaction
ACROSOMAL REACTIONTriggered when the sperm meets theegg.The acrosome, at the tip of the spermreleases hydrolytic enzymes thatdigest material surrounding the egg.Gamete contact depolarizes the eggcell membrane and sets up a fastblock to polyspermy.
THE CORTICAL REACTIONInitiated by the fusion of egg andsperm.It causes a rise in Ca2+.This stimulates cortical granules torelease their contents outside theegg.Cause formation of a fertilizationenvelope (functions as a slow blockto polyspermy.)
RISE OF CALCIUM IN EGG AND FORMATION OFFERTILIZATION ENVELOPE
ACTIVATION OF THE EGGThe sharp rise in Ca2+ in the egg’scytosol increases the rates ofcellular respiration and proteinsynthesis by the egg cell.Now the egg is said to be activated.The sperm nucleus merges with theegg nucleus to form a diploid zygoteand cell division begins.
EMBRYONIC DEVELOPMENT AFTER FERTILIZATION3 PROCESSES OCCURE IN AN EMBRYOAFTER FERTILIZATION:1.Cleavage2.Gastrulation3.Organogenesis
1. CLEAVAGEA period of rapid cell divisionwithout growth.This creates a hollow ball of cellscalled a blastula.The blastula consists of manysmaller cells called blastomeresThe hollow part of the blastula isfilled with fluid and called ablastocoel.
CELL DIVISION OF A FERTILIZED EGG FORMING ABLASTULA
TWO TYPES OF CLEAVAGEMeroblastic cleavage, incompletedivision of the egg, occurs in specieswith yolk-rich eggs, such as reptilesand birds.Holoblastic cleavage, complete divisionof the egg, occurs in species whoseeggs have little or moderate amountsof yolk, such as sea urchins andfrogs
2. GASTRULATIONGastrulation –rearranges the cells of ablastula into a three-layeredembryo, called a gastrula,which has a primitive gut(archenteron)and opens in ablastopore.
THREE EMBRYONIC GERM LAYERS OF THEGASTRULAThe ectoderm forms the outerlayerThe endoderm lines the digestivetract.The mesoderm partly fills thespace between the endodermand ectoderm.
3. ORGANOGENESISDuring organogenesis, variousregions of the germ layersdevelop into organs.Early in vertebrateorganogenesis, the notochordforms from mesoderm, and theneural plate forms fromectoderm.
The neural plate soon curvesinward, forming the neural tubeThe neural tube will become thecentral nervous system (brainand spinal cord).
THE FATE OF THE 3 GERM LAYERSFig. 47-14ECTODERM MESODERM ENDODERMEpidermis of skin and itsderivatives (including sweatglands, hair follicles)Epithelial lining of mouthand anusCornea and lens of eyeNervous systemSensory receptors inepidermisAdrenal medullaTooth enamelEpithelium of pineal andpituitary glandsNotochordSkeletal systemMuscular systemMuscular layer ofstomach and intestineExcretory systemCirculatory and lymphaticsystemsReproductive system(except germ cells)Dermis of skinLining of body cavityAdrenal cortexEpithelial lining ofdigestive tractEpithelial lining ofrespiratory systemLining of urethra, urinarybladder, and reproductivesystemLiverPancreasThymusThyroid and parathyroidglands
DEVELOPMENTAL ADAPTATIONS OF AMNIOTESEmbryos of birds, other reptiles, andmammals develop in a fluid-filledsac in a shell or the uterusOrganisms with these adaptationsare called amniotes.
During amniote development, fourextraembryonic membranes form aroundthe embryo:The chorion functions in gas exchangeThe amnion encloses the amniotic fluidThe yolk sac encloses the yolkThe allantois disposes of waste productsand contributes to gas exchange.
PRECOCIAL ANDALTRICIALDEVELOPMENTPRECOCIAL YOUNGALTRICIAL YOUNG
At birth the young may be well-developedand able to move about at once, this iscalled precocial development.If at birth the young are maybe blind,hairless, and essentially helpless, it iscalled altricial development.In general, precocial young are born after arelatively long gestation period and in asmall litter.Hares and many large grazing mammals bearprecocial offspring.Rabbits, carnivores, and most rodents bearaltricial young.