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

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

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

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

Modes of sexual reproduction

Bisexual- involving two individuals

HermaphroditismParthenogenesis- one individual

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)

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

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

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

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

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

Sexual vs asexual reproduction

Asexual more “energy-efficient”May be advantageous if environment is stable

Otherwise diversity provided by sexual reproduction is advantageous

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

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)

(in humans)

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

Disorders of embryonic sexual development

Hermaphroditism- both ovarian and testiculartissue

Pseudohermaphroditismcongenital adrenal hyperplasia

testicular feminization

Sperm vary greatly in size among species

Sperm production greatly outnumbers eggproduction

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

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

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)

Reproductive systems

Primary organs (gonads)

Secondary organs- assist with formation and delivery of gametesmay support embryo

Invertebratesmay expel gametes, som system is very simple

insects can be quite complex

Reproductive cycles

estrus- most mammalsbrief receptivity to male during cycle

menstrual cyclesexual activity can occur throughout

cycleuterine lining is shed

(human)

Estrogen surge causesrelease of GnRH. Thiscauses release of FSHand LH

Pregnancy and birth

Fertilization usually occurs in uterine tubeBlastocyst is formed by the time it reaches the

uterus

“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

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)

Four weeks

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

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

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)