Reproduction and Development-S2

8/13/2019 Reproduction and Development-S2

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Reproduction and development

Two modes of reproduction

asexual: 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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Asexual, continued

Gemmulation- formation of cell aggregation

surrounded by a capsule (gemmules)

Fragmentation- lots of animals can reproduce

this waysponges, cnidarians, annelids, tunicates

regeneration is part of the process, but

some animals can regenerate body partswithout actually reproducing


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Advantages to asexual reproduction

rapidisolated animals can reproduce

is successful if animals are well adapted

to their environment

Some animals can reproduce both ways,

depending on circumstances


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Modes of sexual reproduction

Bisexual- involving two individuals

Hermaphroditism

Parthenogenesis- one individual


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Hermaphroditic (monoecious)- male and female

reproductive systems in same individual

Usually one individual fertilizes another

Some fishes are sequentially hermaphroditic

Wrasses start out as females and change to

males

protogynous (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 is

rejected before it fuses with egg nucleus


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Haploid egg can begin developing simultaneously

chromosomes 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 develop

in 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 was

deleted, 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 successful

One parthenogenetic mouse reproduced


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Sexual vs asexual reproduction

Asexual more energy-efficientMay 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 somatic

cells)

Vertebrates

Primordial germ cells formed from endoderm

Migrate to gonadsDevelop exclusively into eggs and sperm

Invertebrates

Distinct germ cells may form, or they mayderive later from somatic cells


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Sex determination

Chromosomal in many animals

Sometimes dependent on temperature or other

stimuli

Alligators: eggs incubated at low temperatures

become 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 Y

chromosome

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 external

genitalia


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Disorders of embryonic sexual development

Hermaphroditism- both ovarian and testiculartissue

Pseudohermaphroditism

congenital 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 formed

from a germ cell


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In many animals meiosis is not complete

before fertilization

Birds, most mammals- at ovulation

Many invertebrates, fishes, reptiles, amphibians-

after fertilization

Humans- arrested in prophase I in fetal stage

resumes at ovulation

is completed only on fertilization

Yolk is distinctive: greatly enlarges egg cell


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Reproductive patterns- internal, external

fertilization

Oviparous- egg-laying (invertebrates and some

vertebrates)

fertilization internal or external

Ovoviviparous- eggs retained in body, nourished

by 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 gametes

may support embryo


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Reproductive cycles

estrus- most mammalsbrief receptivity to male during cycle

menstrual cycle

sexual activity can occur throughoutcycle

uterine lining is shed


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(human)

Estrogen surge causes

release of GnRH. This

causes release of FSH

and 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 blood

cell formation. Role reduced> 6 weeks

Amnion- encloses amniotic cavity. Fluid cushionsdeveloping embryo/fetus

Allantois- forms urinary bladder; umbilical cord

Chorion- blood vessels help nourish embryo;

develops into placenta. Secretes hCG,

which stimulates corpus luteum to secrete

estrogen and progesterone


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Placenta

Umbilical arteries and veins provide fetalcirculation

Maternal circulation does not actually mix

with fetal blood

Gas and nutrient exchange takes place here

Secretes estrogen and progesterone to

maintain endometrium (corpus luteum

does that up to 3rdmonth)


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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 more

sensitive to oxytocin and prostaglandins

CRH secretion by placenta triggers fetal

adrenal 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 uniparous

twinning is monozygotic or dizygotic

(identical or fraternal)

Monozygotic twinning is uniform

Dizygotic twinning seems to vary with ethnicity

and/or geography


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Monozygotic twinning

One fertilized zygote splits and forms twoembryos

Depending on timing of split, twins may develop

separate placentas (2-cell stage); oneplacenta and two amnions (complete split

of cell mass) or share placenta and amnion

(later in development; conjoined twinning

is a risk here)

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Reproduction and Development-S2