1. 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

2. 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

4. 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

5. 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

6. Modes of sexual reproduction
Bisexual- involving two individuals
Hermaphroditism
Parthenogenesis- one individual

7. 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)

8. 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

9. 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

10. 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

12. 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

13. 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

14. Sexual vs asexual reproduction
Asexual more “energy-efficient”
May be advantageous if environment is stable
Otherwise diversity provided by sexual
reproduction is advantageous

16. Origin of germ cells (as opposed to somatic
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

17. 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)

18. (in humans)

19. 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

21. Disorders of embryonic sexual development
Hermaphroditism- both ovarian and testicular
tissue
Pseudohermaphroditism
congenital adrenal hyperplasia
testicular feminization

23. Sperm vary greatly in size among species
Sperm production greatly outnumbers egg
production

26. 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

28. 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

29. 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)

30. Reproductive systems
Primary organs (gonads)
Secondary organs- assist with formation and
delivery of gametes
may support embryo

31. Invertebrates
may expel gametes, som system is very
simple
insects can be quite complex

35. Reproductive cycles
estrus- most mammals
brief receptivity to male during cycle
menstrual cycle
sexual activity can occur throughout
cycle
uterine lining is shed

36. (human) Estrogen surge causes release of GnRH. This
causes release of FSH
and LH

37. Pregnancy and birth
Fertilization usually occurs in uterine tube
Blastocyst is formed by the time it reaches the
uterus

41. “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

42. 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)

43. Four weeks

44. 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

47. 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

48. 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)