CHAPTER 7
The Reproductive
Process
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Nature of the Reproductive Process
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Reproduction is one of the most
important properties of life
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Two modes of reproduction
 Asexual
 Sexual
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Examples of Sexual and Asexual Reproduction
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Nature of the Reproductive Process

Asexual Reproduction
Involves only one parent
 No special reproductive organs or cells
 Genetically identical offspring
 Production of offspring is simple, direct,
and rapid - increase population fast
 Widespread in bacteria, unicellular
eukaryotes and many invertebrate phyla
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Nature of the Reproductive Process
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Asexual Reproductive Methods
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Binary Fission
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Multiple Fission
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Common among bacteria and protozoa
The parent divides by mitosis into two parts
Each grows into an individual similar to the parent
Nucleus divides repeatedly
Cytoplasmic division produces many daughter cells
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Nature of the Reproductive Process
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Budding
Unequal division of an organism
 Bud is an outgrowth of the parent
 Develops organs and then detaches
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Fragmentation
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Multicellular animal breaking into many fragments that
become a new animal
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Nature of the Reproductive Process

Sexual Reproduction
Generally involves two parents
 Special germ cells (gametes) unite to form a
zygote
 Sexual reproduction recombines parental
characters
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A richer, more diversified population results
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Nature of the Reproductive Process

Sexual Reproductive Methods
 Bisexual
Reproduction
 Most
common form
 Produces offspring from union of gametes
from two genetically different parents
 Generally, individuals are male or female
 Organisms are dioecious

Sexes are separate
 Gonads
(Found in most vertebrates and
invertebrates)
 Organs that produce gametes (testes,
ovaries)
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Nature of the Reproductive Process
 The
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female produces the ovum
Large with stored yolk and nonmotile
 Spermatozoa
(sperm) are produced by
the male
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Small, motile and much more numerous
 Meiosis

- used to make gametes
Produces four haploid cells
 Fertilization
Two haploid cells combine
 Restores the diploid chromosome number in
the zygote
 Zygote divides by mitosis
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Sexual Life Cycle
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Nature of the Reproductive Process
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Hermaphroditism
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Both male and female organs in the same individual
(monoeicious, hermaphrodites)
Many sessile, burrowing and/or endoparasitic
invertebrates and some fish
 Most avoid self-fertilization
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Each individual produces eggs, increases #
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Hermaphroditic species could potentially produce twice as
many offspring as dioecious species
Sequential Hermaphroditism
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Exchange gametes with member of same species
A genetically programmed sex change occurs with an
individual organism
 Ex: Clownfish - born male, change to female if
dominant female is removed
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Hermaphroditic Earthworms Mating
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Nature of the Reproductive Process
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Parthenogenesis
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Development of an embryo from an unfertilized egg
Male and female nuclei fail to unite after fertilization
Egg begins development without sperm
Narrows the diversity available for adaptation to
new conditions - not clones of female (haploid cells
replicate)
Examples: fleas, bees, aphids, some fish and
lizards
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Nature of the Reproductive Process
Why do so many animals reproduce
sexually rather than asexually?

The costs of sexual reproduction are greater
than asexual methods - negatives of sexual
reproduction:
Requires more time
 Uses more energy
 The cost of meiosis to the female involves passing
only half of her genes to offspring
 Production of males reduces resources for
females that could produce eggs - more females =
more offspring
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Nature of the Reproductive Process
However:
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Sexual organisms
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Produce more diverse genotypes to survive in times of
environmental change
diversity prevents extinction
On a geological time scale
 Sexual lineages with less variation are prone to
extinction
Many invertebrates with both sexual and asexual
modes enjoy the advantages of both
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Example: Starfish, Lizard
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Unisex - ALL female lizards
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Bisexual - Both Male and Female Present
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Sex Determination
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Sex Determination
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At first, gonads are sexually identical
In human males
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SRY (sex determining region Y) on the Y chromosome
organizes the gonad into a testis
Once formed, the testis
 Secretes testosterone which, masculinizes the
fetus
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Development of a penis, scrotum and male ducts,
and glands
Females have no “Y”, so gonads never
change into testes, therefore Testosterone
is never secreted
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Sex Determination
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Absence of testosterone in a genetic female
embryo
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Genetics of sex determination vary:
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Promotes development of female sexual organs
 Vagina, clitoris and uterus
XX-XY
Haplodiploid (males are formed from unfertilized eggs)
XX-XO
Temperature
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Incubation Temperature determine sex of offspring
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Germ Cells
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Gametogenesis
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Gametes formation
 Spermatogenesis (Testes)
 Oogenesis
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(Ovaries)
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Seminiferous Tubule containing sperm
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Germ Cells
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Spermatogenesis
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Formation of sperm
Parts of Sperm
 Haploid nucleus condenses into a head
 A midpiece forms containing mitochondria
 The whiplike flagellar tail provides locomotion
Sperm head contains an acrosome
 Often contains enzymes to aid in penetration of egg
layers
 Enzymes are specific to a species. Why??
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Acrosome produces
specific enzymes, so they
only digest their species’
egg’s membrane.
This helps aquatic
animals who might spawn
at the same time.
Ex. Coral sperm can’t fertilize
Sea Star eggs.
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Germ Cells
 Oogenesis
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Formation of ovum (egg) - 3 polar bodies and 1 egg
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Reproductive Patterns
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TYPES OF BIRTH
Oviparous (“egg-birth”) Animals
Lay eggs outside the body
 Fertilization may be internal (before eggs
are laid) or external (after laid)
 Some animals abandon eggs; others
provide extensive care
 Examples: reptiles, birds, amphibians, fish
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Reproductive Patterns
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Ovoviviparous (“egg-live-birth”) Animals
Retain eggs in their body
 Essentially all nourishment is derived from
the yolk not the mother.
 Fertilization is internal
 Common in some invertebrate groups and
aquatic animals, certain fishes (sharks) and
reptiles
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Reproductive Patterns
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Viviparous (“live-birth”) Animals
Give birth to young in a more advanced
stage of development
 Eggs develop in oviduct or uterus
 Embryos continuously derive nourishment
from the mother
 Fertilization is internal
 Occurs in mammals and some fishes
 Provides more protection to offspring
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Plan of Reproductive Systems
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Invertebrate Reproductive Systems
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Invertebrates that transfer sperm for
internal fertilization require complex
organs
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Insects (Crickets) - have an ovipositor - helps
deposit eggs
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Crickets’ Sex Organs
Sperm is stored in a sac (spermatophore) and deposited into the genital bursa of the female.
The female then controls the release of a few sperm to fertilize her eggs at the moment they are laid, using the ovipositor.
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Plan of Reproductive Systems
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Vertebrate Reproductive Systems
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Reproductive and excretory systems are called
the urogenital system
 Close anatomical connection
In male fishes and amphibians
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In all vertebrates except most mammals
 Ducts open into a cloaca
 In females with cloacas, the oviduct also opens
into cloaca
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Most female mammals have separate excretory
and reproductive systems
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Male Reproductive Parts:
Scrotum, Testis, Epididymis,
Vas deferens, Prostate,
Seminal Vesicles,
Urethra, Penis
Female Reproductive Parts:
Vagina, Ovary, Cervix,
Fallopian Tubes
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Endocrine Events that Orchestrate Reproduction
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Hormonal Control of Timing of Reproductive
Cycles
 Vertebrate reproduction
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Seasonal or cyclic
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Sexual cycles
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Offspring arrive when food is available and other
environmental conditions are optimal for survival
Controlled by hormones that respond to food
intake, photoperiod, rainfall, temperature or
social cues
Hypothalamus controls release of
hormones
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Childbirth
 Multiple
 Many
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animals are multiparous
Give birth to many offspring at one time
 Some
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Births
are uniparous (elephants)
Give birth only to one at a time
 Exceptions
occur
Armadillos gives birth to four young, all
male or all female
 Derived from one zygote - IDENTICAL
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Endocrine Events that Orchestrate Reproduction
 Identical
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Twins - separate
One-third have separate placentas and
amniotic sacs
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Indicates fertilized egg separated at an early
stage
 Two-thirds
share a placenta with separate
amniotic sac
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Splitting occurred after implantation
few share 1 amniotic sac and 1 placenta
Indicates that separation of the zygote
occurred after day 9 of pregnancy, when the
amnion has formed
 These twins risk becoming conjoined
(Siamese twinning)
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1/3
2/3
Rare - possible
Conjoined twins
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