reproduction and development two modes of reproduction asexual: one parent; offspring are clonal...
TRANSCRIPT
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Reproduction and development
Two modes of reproductionasexual: one parent; offspring are clonal
sexual: two parents produce gametes,which fuse to form a zygote
“Advantages” and “disadvantages” to each
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How does reproduction take place?
Asexual- no gametesfission: binary and multiple
binary: bacteria and protozoacan be lengthwise or transversemultiple: nucleus divides rapidlybefore cytoplasm dividesseen in some parasitic protozoa
budding: new individual arises as out-growth of the oldseveral animal phyla, esp. cnidarians,
tunicates
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Asexual, continued
Gemmulation- formation of cell aggregationsurrounded by a capsule (gemmules)
Fragmentation- lots of animals can reproducethis waysponges, cnidarians, annelids, tunicates
regeneration is part of the process, butsome animals can regenerate body partswithout actually reproducing
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Advantages to asexual reproduction
rapidisolated animals can reproduceis successful if animals are well adaptedto their environment
Some animals can reproduce both ways, depending on circumstances
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Modes of sexual reproduction
Bisexual- involving two individuals
HermaphroditismParthenogenesis- one individual
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Hermaphroditic (monoecious)- male and femalereproductive systems in same individual
Usually one individual fertilizes another
Some fishes are sequentially hermaphroditic
Wrasses start out as females and change to malesprotogynous (female first)protandrous (male first)
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Parthenogenesis (“virgin birth”)
Embryo develops from unfertilized egg, ormale and female nuclei do not unite
Ameiotic parthenogenesis (“asexual”)egg forms by mitotic divisionoccurs in some flatworms, rotifers,crustaceans, insects and others
offspring are genetically identical toparent
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Meiotic parthenogenesis
Ovum is formed by meiosis (i.e., is haploid)sperm may or may not activate egg
Various forms of these are described
Some fishes: sperm activates egg but isrejected before it fuses with egg nucleus
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Haploid egg can begin developing simultaneouslychromosomes replaced by duplication
Flatworms, rotifers, annelids, mites, insects
Bees, wasps, ants
Fertilized eggs become diploid females(queens, females)
Unfertilized eggs become drones
Whiptail lizards become clones of (female) parents
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Why parthenogenesis?
If males and females cannot be brought together
Not viable in mammals (fetuses can developin mice)
Parthenogenesis was achieved recently in mice(with a little genetic manipulation)
Kono et al., reported in Nature, 4/21/04
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Not the usual type of parthenogenesis
Chromosomes form two different female micewere used
One was modified so that a certain gene wasdeleted, allowing a growth factor (IgF2)to be expressed
Genetic imprinting; in embryos this gene isexpressed on the paternal chromosome
Only 2 out of 500 attempts were successfulOne parthenogenetic mouse reproduced
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Sexual vs asexual reproduction
Asexual more “energy-efficient”May be advantageous if environment is stable
Otherwise diversity provided by sexual reproduction is advantageous
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Origin of germ cells (as opposed to somaticcells)
VertebratesPrimordial germ cells formed from endodermMigrate to gonadsDevelop exclusively into eggs and sperm
InvertebratesDistinct germ cells may form, or they may
derive later from somatic cells
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Sex determination
Chromosomal in many animals
Sometimes dependent on temperature or otherstimuli
Alligators: eggs incubated at low temperaturesbecome female; high temperatures, male(no sex chromosomes)
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(in humans)
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TDF is the product of the SRY gene on the Ychromosome
Testes develop much more rapidly than ovaries(7 weeks vs. 15 weeks)
TDF initiates a sequence of events that leads tothe formation of testes and male externalgenitalia
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Disorders of embryonic sexual development
Hermaphroditism- both ovarian and testiculartissue
Pseudohermaphroditismcongenital adrenal hyperplasia
testicular feminization
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Sperm vary greatly in size among species
Sperm production greatly outnumbers eggproduction
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Oogenesis
Oogonia- earliest forms; diploid; divide bymitosis
Primary oocytes do not divide equally(polar body)
Secondary oocytes are haploid
One functional ovum is ultimately formedfrom a germ cell
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In many animals meiosis is not completebefore fertilization
Birds, most mammals- at ovulation
Many invertebrates, fishes, reptiles, amphibians-after fertilization
Humans- arrested in prophase I in fetal stageresumes at ovulationis completed only on fertilization
Yolk is distinctive: greatly enlarges egg cell
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Reproductive patterns- internal, externalfertilization
Oviparous- egg-laying (invertebrates and somevertebrates)fertilization internal or external
Ovoviviparous- eggs retained in body, nourishedby yolk (some annelids, arthropods,gastropods, some fishes and reptiles
Viviparous- develop in oviduct or uterus,nourished by mother(mostly mammals, some fishes, scorpions)
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Reproductive systems
Primary organs (gonads)
Secondary organs- assist with formation and delivery of gametesmay support embryo
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Invertebratesmay expel gametes, som system is very simple
insects can be quite complex
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Reproductive cycles
estrus- most mammalsbrief receptivity to male during cycle
menstrual cyclesexual activity can occur throughout
cycleuterine lining is shed
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(human)
Estrogen surge causesrelease of GnRH. Thiscauses release of FSHand LH
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Pregnancy and birth
Fertilization usually occurs in uterine tubeBlastocyst is formed by the time it reaches the
uterus
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“Extraembryonic membranes”
Start forming after implantation
Yolk sac- transport of nutrients, red bloodcell formation. Role reduced> 6 weeks
Amnion- encloses amniotic cavity. Fluid cushionsdeveloping embryo/fetus
Allantois- forms urinary bladder; umbilical cordChorion- blood vessels help nourish embryo;
develops into placenta. Secretes hCG, which stimulates corpus luteum to secreteestrogen and progesterone
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Placenta
Umbilical arteries and veins provide fetalcirculation
Maternal circulation does not actually mixwith fetal blood
Gas and nutrient exchange takes place here
Secretes estrogen and progesterone to maintain endometrium (corpus luteumdoes that up to 3rd month)
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Four weeks
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Labor and childbirth
Laboroxytocin (hypothalamus)prostaglandins
Fetal adrenal gland produces cortisol and anestrogen presursor; makes uterus moresensitive to oxytocin and prostaglandins
CRH secretion by placenta triggers fetaladrenal gland activity
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Single or multiple births?
Multiparous- several eggs develop at onceArmadillos always give birth to four offspring,
all the same sex
Humans tend to be uniparoustwinning is monozygotic or dizygotic(“identical” or “fraternal”)
Monozygotic twinning is uniformDizygotic twinning seems to vary with ethnicity
and/or geography
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Monozygotic twinning
One fertilized zygote splits and forms two embryos
Depending on timing of split, twins may developseparate placentas (2-cell stage); oneplacenta and two amnions (complete splitof cell mass) or share placenta and amnion(later in development; conjoined twinningis a risk here)