UNIT 5:ANIMALREPRODUCTION ANDDEVELOPMENTCampbell & Reece, 2010:Chapters 46, 47
INTRODUCTION Animal reproduction takesmany forms Animal form and function canbe viewed broadly asadaptations contributing toreproductive success. Both asexual and sexualreproduction occur in theanimal kingdom.
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 AsexualReproductionMany invertebratesreproduce asexually.One of these is by theprocess of 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 be accompaniedby regeneration - regrowth of lostbody partsFRAGMENTATION ANDREGENERATION OF SEA STARS
Fourth method: Parthenogenesis Is the development of a newindividual from an unfertilized egg
Introduction to sexualReproduction in animalsAlmost all eukaryotic speciesreproduce sexually.Sexual reproduction results ingenetic recombination –variation within species.
Genetic recombination providespotential advantages:1. An increase in variation in offspring,2. An increase in the reproductivesuccess of parents in changingenvironments.3. An increase in the rate ofadaptation.4. Elimination of harmful genes froma population.
REPRODUCTION CYCLES ANDPATTERNSOvulation is the release of matureeggs at the midpoint of a female cycle.Most animals exhibit reproductivecycles related to changing seasons.Reproductive cycles are controlled byhormones and environmental cues
Animals may reproduce asexually orsexually, or they may alternate thesemethods.Lizards reproduce only by a complex formof parthenogenesis that involves thedoubling 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 has maleand female reproductive systemsSome hermaphrodites can self-fertilize.C. elegans makes both spermand eggs and can reproduceby self-fertilization.
Individuals of some species undergosex 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 moreoffspring than the environmentcan handle.The proportion that survives issmall.Species with external fertilizationproduce more gametes thanspecies with internal fertilization.
Species with internal fertilizationprovide greater protection of theembryos and more parental care.The embryos of some terrestrial animalsdevelop in amniote eggs withprotective layers.Other animals retain the embryo,which develops 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 a common opening betweenthe external environment and thedigestive, excretory, andreproductive systemsCommon in non-mammalianvertebrates;Mammals usually have aseparate opening to the digestivetract.
Three Methods of reproduction inwhich eggs are laid and embryosdevelopOviparyOvoviviparyVivipary
OVIPARYOUS ORGANISMSReproduction in which eggs are laidand embryos develop outside themothers body.Each egg eventually hatching into ayoung animal.Little or no development occurs withinthe mothers body.Most invertebrates and manyvertebrates reproduce in this way.
Ovoviviparous:Animals produce eggs, but insteadof laying the eggs, the eggs developwithin the mothers body.The young eat unfertilized eggs inthe womb for nourishmentThe young are born alivewithout placental attachment(umbilical cord),as certain reptiles, fishes, sharks,insects.
VIVIPAROUSWhere the embryo developswithin the uterus.and is nourished through ayolk sac placenta from themother’s blood.Young are born alive. most mammals and somereptiles and fishes.
Introduction to animal developmentThe question of how a zygote becomesan animal?The first step is cell division (mitosis)Second step: Cell differentiation (Is thespecialization of cells in structure andfunction)Third step: Morphogenesis (Is theprocess by which an animal takesshape.)
PROCESS OF FERTILIZATIONFertilization brings the haploid nucleiof sperm and egg together, forming adiploid zygote.The sperm’s contact with the egg’ssurface initiates metabolic reactionsin the egg that trigger the onset ofembryonic development.Metabolic reactions:Acrosomal reactionCortical reaction
ACROSOMAL REACTIONTriggered when the sperm meetsthe egg.The acrosome, at the tip of thesperm releases hydrolytic enzymesthat digest material surrounding theegg.Gamete contact depolarizes theegg cell membrane and sets up afast block to polyspermy.
Sperm releases haploid nucleus in egg cytoplasm– fast block prevent 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 slowblock to polyspermy.)
Rise of calcium in egg andformation of fertilization 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 diploidzygote and cell division begins.
EMBRYONIC DEVELOPMENT AFTERFERTILIZATION3 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 FERTILIZEDEGG FORMING A BLASTULA
Two types of cleavageMeroblastic cleavage, incompletedivision of the egg, occurs in specieswith yolk-rich eggs, such as reptilesand birds.Holoblastic cleavage, completedivision of the egg, occurs in specieswhose eggs have little or moderateamounts of yolk, such as sea urchinsand frogs
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 layersof the gastrulaThe ectoderm forms the outerlayerThe endoderm lines thedigestive tract.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 becomethe central nervous system(brain and spinal cord).
THE FATE OF THE 3 GERMLAYERSFig. 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 ofAmniotesEmbryos of birds, other reptiles,and mammals develop in a fluid-filled sac 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.
PRECOCIALAND ALTRICIALDEVELOPMENTPRECOCIAL 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 aftera relatively long gestation period and in asmall litter.Hares and many large grazing mammalsbear precocial offspring.Rabbits, carnivores, and most rodentsbear altricial young.