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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. How does reproduction take place? Asexual- no gametes fission: binary and multiple

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slide1

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

slide2

How does reproduction take place?

Asexual- no gametes

fission: binary and multiple

binary: bacteria and protozoa

can be lengthwise or transverse

multiple: nucleus divides rapidly

before cytoplasm divides

seen in some parasitic protozoa

budding: new individual arises as out-

growth of the old

several animal phyla, esp. cnidarians,

tunicates

slide4

Asexual, continued

Gemmulation- formation of cell aggregation

surrounded by a capsule (gemmules)

Fragmentation- lots of animals can reproduce

this way

sponges, cnidarians, annelids, tunicates

regeneration is part of the process, but

some animals can regenerate body parts

without actually reproducing

slide5

Advantages to asexual reproduction

rapid

isolated animals can reproduce

is successful if animals are well adapted

to their environment

Some animals can reproduce both ways,

depending on circumstances

slide6

Modes of sexual reproduction

Bisexual- involving two individuals

Hermaphroditism

Parthenogenesis- one individual

slide7

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)

slide8

Parthenogenesis (“virgin birth”)

Embryo develops from unfertilized egg, or

male and female nuclei do not unite

Ameiotic parthenogenesis (“asexual”)

egg forms by mitotic division

occurs in some flatworms, rotifers,

crustaceans, insects and others

offspring are genetically identical to

parent

slide9

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

slide10

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

slide12

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

slide13

Not the usual type of parthenogenesis

Chromosomes form two different female mice

were 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 is

expressed on the paternal chromosome

Only 2 out of 500 attempts were successful

One parthenogenetic mouse reproduced

slide14

Sexual vs asexual reproduction

Asexual more “energy-efficient”

May be advantageous if environment is stable

Otherwise diversity provided by sexual

reproduction is advantageous

slide16

Origin of germ cells (as opposed to somatic

cells)

Vertebrates

Primordial germ cells formed from endoderm

Migrate to gonads

Develop exclusively into eggs and sperm

Invertebrates

Distinct germ cells may form, or they may

derive later from somatic cells

slide17

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)

slide19

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 to

the formation of testes and male external

genitalia

slide21

Disorders of embryonic sexual development

Hermaphroditism- both ovarian and testicular

tissue

Pseudohermaphroditism

congenital adrenal hyperplasia

testicular feminization

slide23

Sperm vary greatly in size among species

Sperm production greatly outnumbers egg

production

slide26

Oogenesis

Oogonia- earliest forms; diploid; divide by

mitosis

Primary oocytes do not divide equally

(polar body)

Secondary oocytes are haploid

One functional ovum is ultimately formed

from a germ cell

slide28

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

slide29

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)

slide30

Reproductive systems

Primary organs (gonads)

Secondary organs- assist with formation and

delivery of gametes

may support embryo

slide31

Invertebrates

may expel gametes, som system is very

simple

insects can be quite complex

slide35

Reproductive cycles

estrus- most mammals

brief receptivity to male during cycle

menstrual cycle

sexual activity can occur throughout

cycle

uterine lining is shed

slide36

(human)

Estrogen surge causes

release of GnRH. This

causes release of FSH

and LH

slide37

Pregnancy and birth

Fertilization usually occurs in uterine tube

Blastocyst is formed by the time it reaches the

uterus

slide41

“Extraembryonic membranes”

Start forming after implantation

Yolk sac- transport of nutrients, red blood

cell formation. Role reduced> 6 weeks

Amnion- encloses amniotic cavity. Fluid cushions

developing 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

slide42

Placenta

Umbilical arteries and veins provide fetal

circulation

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 3rd month)

slide44

Labor and childbirth

Labor

oxytocin (hypothalamus)

prostaglandins

Fetal adrenal gland produces cortisol and an

estrogen presursor; makes uterus more

sensitive to oxytocin and prostaglandins

CRH secretion by placenta triggers fetal

adrenal gland activity

slide47

Single or multiple births?

Multiparous- several eggs develop at once

Armadillos 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

slide48

Monozygotic twinning

One fertilized zygote splits and forms two

embryos

Depending on timing of split, twins may develop

separate placentas (2-cell stage); one

placenta and two amnions (complete split

of cell mass) or share placenta and amnion

(later in development; conjoined twinning

is a risk here)