Animal Reproduction

Animal Reproduction

• Reproduction is asexual or sexual

– asexual reproduction

• offspring genetically identical to parent and siblings

• saves costs of sexual reproduction

• resulting population lacks genetic variability

Animal Reproduction

• Reproduction is asexual or sexual

– asexual reproduction

• various methods

–budding

»outgrowth from body of parent

–regeneration

»regrowth of lost body parts or whole organisms

budding of hydraregeneration of a sea star

Figure 43.1

parthenogenesis &

viviparyin

aphid

Animal Reproduction

• Reproduction is asexual or sexual

– asexual reproduction

• various methods

–parthenogenesis

»production of offspring from unfertilized eggs

»may require mating behavior to stimulate development

parthenogenesis in whiptail lizardsFigure 43.2

Animal Reproduction

• Reproduction is asexual or sexual

– sexual reproduction

• offspring differ genetically from parents

• involves significant cost

–gametogenesis

–mating (getting gametes together)

–fertilization (syngamy - gamete fusion)

Animal Reproduction

• Reproduction is asexual or sexual

– sexual reproduction

• offspring differ genetically from parents

• involves significant cost

• resulting population exhibits genetic variability

Animal Reproduction

• Reproduction is asexual or sexual

– sexual reproduction - genetic variability

• combines genetic contributions from male and female parents

• gamete genomes contain recombined chromosomes

–crossing over (prophase I)

–independent assortment (metaphase I)

Animal Reproduction

• Reproduction is asexual or sexual

– sexual reproduction

• alternates the halving and doubling of genomes

–meiosis reduces 2n genome to 1n

–fertilization combines two 1n gametes

Animal Reproduction

• meiosis is part of gametogenesis

– gametogenesis occurs in the primary sex organs - the gonads

• motile sperm are produced in testes

• non-motile eggs (ova) are produced in ovaries

Early stages

of spermatogenesis

and oogenesis

Figure 43.3

Animal Reproduction

• gametogenesis

– germ cells enter the gonads during embryonic development

– germ cells divide mitotically to produce spermatogonia or oogonia

– spermatogonia and oogonia divide mitotically to produce primary spermatocytes and primary oocytes

– meiosis produces spermatids and ootids

Animal Reproduction• spermatogenesis

– meiosis I divides a primary spermatocyte• produces two secondary spermatocytes

–haploid –connected due to incomplete

cytokinesis– meiosis II divides secondary spermatocytes

• produces four linked spermatids• spermatids develop into sperm

complete spermatogenesisFigure 43.3

Animal Reproduction• oogenesis

– primary oocyte enters prophase I• arrests development at prophase I• remains in prophase I for some time• grows & accumulates nutrients, energy, RNA, ribosomes, organelles

human gametesdiffer

in size

Animal Reproduction• oogenesis

– primary oocyte resumes meiosis I• asymmetric division produces two cells

–secondary oocyte–first polar body

• secondary oocyte divides asymmetrically in meiosis II–ootid–second polar body

complete oogenesis, minus pausesFigure 43.3

Animal Reproduction• oogenesis

– timing of meiosis and egg development varies• secondary oocyte pauses in development prior to meiosis II

• meiosis II may occur after expulsion from the ovary

• human ova completes meiosis II after fertilization

Fertilization

• sperm/egg recognition

– recognition molecules ensure that sperm interact with eggs of the same species

• jelly layer activates acrosome

• acrosomal enzymes digest jelly layer

• acrosomal process bears bindin proteins

• plasma membrane bears bindin receptors

sea urchin sperm/egg recognitionFigure 43.4

Fertilization

• mammalian sperm/egg recognition

– sperm capacitation occurs in female reproductive tract

– superficial sperm enzymes digest cumulus

– zona pellucida glycoproteins bind sperm head recognition proteins

– acrosomal enzymes digest zona pellucida

– plasma membrane proteins bind sperm adhesion proteins

(follicle cells)

(glycoprotein)

mammalian barriers to sperm entryFigure 43.5

fast block to polyspermy in sea urchin: prevents entry by multiple sperm

plasma membrane electrical potential rapid Na+ influx

sperm entry

Ca2+ release

exocytosis

fertilization envelope

slow block

to polyspermyFigure 43.6

Fertilization• egg activation

– mammals• calcium release from ER• cortical granule fusion• removal of sperm-binding proteins• metabolic activation

–increased cytoplasmic pH–increased O2 consumption–increased protein synthesis

• nuclear fusion ~12 h after sperm entry

simultaneous hermaphroditic fertilization

Animal Reproduction• sexual identity in animals

– many animals are dioecious– other animals are monoecious

(hermaphroditic)• simultaneous hermaphrodites• sequential hermaphrodites

fertilization• fusion of sperm and egg

– produces zygote– requires sperm & egg to be in close

proximity

release of

sperm and eggs

mating behavior• external fertilization

– release of gametes into aquatic environment– chance meeting of egg & sperm

• synchronized release by sessile animals• simultaneous release by motile animals

simultaneous release of sperm & eggs

mating behavior• internal fertilization

– protects sperm from desiccation• spermatophores - sperm containers

–released to environment–inserted into female genital opening

mating behavior• internal fertilization

– protects sperm from desiccation• copulation

–direct transfer of sperm–requires accessory sex organs

»penis»vagina or cloaca

development• site of development varies

–oviparous• egg bearing• external development

–viviparous• live bearing• internal development in uterus

–ovoviviparous• development in eggs retained within body

human reproductive system• male organs produce & deliver sperm

– semen• sperm

–produced in testes–stored in epididymis–transported through vas deferens–combined with fluids–ejaculated through urethra during

copulation

human reproductive system• male organs produce & deliver sperm

– semen• bulbourethral fluid

–pH buffer & lubricant• seminal fluid

–fructose, prostaglandins, mucus/protein• prostate fluid

–modifies uterine environment–clots seminal fluid

male primary and accessory sex organsFigure 43.8

sperm production in seminiferous tubules

Figure 43.9

human reproductive system• sperm development

– within the scrotum– within seminiferous tubules

• developing sperm are engulfed by Sertoli cells

• developing sperm migrate from periphery to lumen of tubule

• mature sperm are released into lumen and travel to epididymis

• Leydig cells produce testosterone

SpermatogenesisFigure 43.9

human reproductive system

• hormonal control of sperm development

– gonadotropin-releasing hormone (GnRH)

• from hypothalamus, stimulates

–luteinizing hormone (LH)

»from anterior pituitary, stimulates

-testosterone release from Leydig cells

human reproductive system• hormonal control of sperm development

– spermatogenesis is controlled by • follicle-stimulating hormone (FSH) &• testosterone

–on Sertoli cells

control of

male sex hormone production

Figure 43.10 [remember to study this figure and

explanatory slides associated with it]

oviduct cilia

human reproductive system• female sex organs

– ovary produces eggs– eggs are released into oviduct– oviduct transports egg to uterus– embryo implants in uterine wall– cervix retains uterine contents during

pregnancy; dilates during birth– sperm enter, baby departs through vagina

female primary and accessory sex organs

Figure 43.11

human reproductive system• female reproductive cycles

– ovarian cycle ~28 days• production of a secondary oocyte

–surrounded, supplied by follicle cells–growth, maturation, first meiotic

division–release from ovary

• increased hormone production–follicle cells of corpus luteum–produces estrogen & progesterone

stages of the ovarian cycleFigure 43.12

meiosis

human reproductive system• female reproductive cycles

– uterine cycle• growth of endometrium

– ~days 5-19 of ovarian cycle–prepares for implantation

• breakdown of endometrium– ~end of ovarian cycle–expelled from body through vagina

human reproductive system• hormonal control of reproductive cycles

– GnRH • from hypothalamus• stimulates

– gonadotropins (LH & FSH)• from anterior pituitary• stimulate

– estrogen• from ovary

human reproductive system• hormonal control of reproductive cycles

– ~ 5 days before start of ovarian cycle• anterior pituitary increases gonadotropins• follicle matures & increases estrogen• estrogen stimulates endometrial growth• estrogen feedback reverses to boost LH • LH causes release of secondary oocyte

human reproductive system• hormonal control of reproductive cycles

– corpus luteum secretes estrogen & progesterone• maintain endometrial growth• inhibit gonadotropin release to• inhibit follicle development

– without fertilization, corpus luteum degenerates

Figure 43.13

hormonal control of ovarian and

uterine cycles

Figure 43.14 [remember to study this figure

and explanatory slides associated with it]

human reproductive system• hormonal control of reproductive cycles

– with fertilization• human chorionic gonadotropin (hCG)

–is produced by embryo–sustains corpus luteum

• gradually, embryo derived cells–produce estrogen & progesterone–prevent anterior pituitary from releasing

gonadotropins