Animal Reproduction

Asexual vs. Sexual Reproduction Asexual Reproduction: The creation of new individuals whose genes all come from one parent without the fusion of egg and sperm Sexual Reproduction The creation of offspring by the fusion of haploid gametes to form a diploid zygote

Asexual Reproduction Fission The separation of a parent into 2 or more individuals Budding new individuals arise from outgrowths of existing ones

Asexual Reproduction Fragmentation The breaking of the body into several pieces, some or all of which develop into complete adults through regeneration Regeneration The regrowth of lost body parts

Advantages of Asexual Reproduction Isolated animals can produce offspring without locating mates Creates numerous offspring rapidly Perpetuates successfully genotypes perfectly

Parthenogenesis Parthenogenesis is a process in which an unfertilized egg develops into an individual Individuals produced by parthenogenesis are therefore haploid Male honeybees produced by parthenogenesis, females from fertilized eggs Occurs in aphids, daphnia, rotifers, and many plants (and others – some reptiles, birds, fish, but no mammals)

Hermaphroditism Hermaphrodites are individuals who possess both male and female reproductive organisms Solution for organisms who rarely encounter members of the opposite sex must still mate with a member of the same species, but can act as the male OR the female

Fertilization Fertilization is the union of sperm and egg cells External fertilization – Eggs are released by the female into a wet environment, where they are fertilized by the male Requires a moist environment Amniotic egg as evolutionary adaptation Internal fertilization – Sperm are deposited in/near female reproductive tract, where fertilization occurs

Gamete Production & Delivery Gametes are produced through the process of meiosis See gametogenesis diagram The simplest organisms do not have gonads, which are the organs that produce gametes in most animals Many nonmammalian vertebrates have a common opening to the outside, known as the cloaca, which is part of the digestive, excretory, and reproductive systems

Female Reproductive Anatomy Ovaries Female gonads Contain follicles – egg cell surrounded by follicle cells (nourishment & protection for the egg) One follicle matures and releases its egg during each menstrual cycle  ovulation Cells of the follicle also produce estrogen The egg travels to the uterus during ovulation via the oviduct (fallopian tube)

Female Reproductive Anatomy The uterus is a thick, muscular organ that expands to hold the fetus during pregnancy The cervix is the opening from the uterus to the vagina The vagina is the “sperm repository” and serves as the birth canal

Male Reproductive Anatomy Testes Male gonads Contain seminiferous tubules (produce sperm) and Leydig cells (produce testosterone) Testes are held in the scrotum, which maintains a lower-than- body temperature

Male Reproductive Anatomy Sperm pass from the testes through the coiled tubes of the epididymis During ejaculation, the sperm move through the vas deferens, enter the urethra (drains urinary and reproductive system), and exit through the penis The prostate gland, seminal vesicles and bulbourethral glands produce the other components of semen (other than sperm)

Female Reproductive Cycles Ovarian Cycle Follicular Phase: Follicles are growing and oocytes (eggs) are maturing Ovulation: Egg is released Luteal Phase: Follicular tissue is transformed into the corpus luteum (important for regulating hormone levels during pregnancy)

Female Reproductive Cycles The Uterine (Menstrual) Cycle Menstrual Flow Phase The endometrium (lining of the uterus) disintegrates and is released (menstruation) Caused by the disintegration of the corpus luteum Proliferative Phase Estrogen signals the endometrium to thicken Secretory Phase After ovulation (uterine cycle), endometrium continues to thicken and nutrient fluid is released

Human Oogenesis Primary Oocytes Stop at prophase I of meiosis At puberty, FSH (follicle- stimulating hormone) stimulates a primary oocyte to complete meiosis I and start meiosis II (usually one per month) Stops at metaphase II of meiosis Meiosis is completed when a sperm penetrates the oocyte

Human Spermatogenesis Spermatogonia differentiate into spermatocytes and finally into spermatids The acrosome is the tip of a haploid sperm cell that help the sperm cell penetrate the egg FERTILIZATION

Animal Development

A Body-Building Plan for Animals How does a zygote become an animal? 2 competing views: Preformation: the egg or sperm contains an embryo - a pre-formed, mini infant – that simply becomes larger during development (humunculus) Epigenesis: The form of an animal emerges gradually from a relatively formless egg Epigenesis has emerged as the favored explanation among embryologists

Fertilization brings together the sperm (n) and egg (n) into a zygote (2n) activates the egg Induces metabolic activity (cellular respiration, protein synthesis, etc.) There are 2 reactions that ensure that polyspermy (fertilization of an egg by multiple sperm) Acrosomal Reaction Cortical Reaction

The Acrosomal Reaction The acrosome is a specialized vesicle at the tip of the sperm The acrosome releases hydrolytic enzymes that break down the jelly coat of the egg and allow the sperm to enter Lock-and-key mechanism – benefits? The contact of the acrosome with the cell membrane of the egg cell changes the membrane potential of the cell membrane (depolarization) Result: no other sperm cells can enter This is the fast block to polyspermy (<1 sec after binding of sperm to egg)

The Cortical Reaction A signal transduction pathway causes calcium to be released, which causes cortical granules to release their contents This causes the vitelline layer to be transformed into the fertilization envelope (impenetrable) This is the slow block to polyspermy (20 sec after binding of sperm to egg)

Cleavage Cleavage is a succession of rapid cell divisions that follow fertilization The cells virtually skip G 1 and G 2, so little growth (if any) occurs Cleavage basically splits the cytoplasm of one large cell (zygote) into many smaller cells (blastomeres)

Cleavage Morula (“mulberry”) is formed First 5 – 7 cellular divisions Blastocoel (fluid-filled cavity) begins to form Blastula is a hollow ball of cells

Gastrulation Gastrulation is the process by which the cells of the blastula are rearranged to form a 3-layered embryo These 3 layers are known as germ layers Ectoderm (outer) – develops into epidermis & nervous system Endoderm (inner) – develops into primitive gut Mesoderm (middle) – develops into most everything else

Gastrulation The blastopore (indentation/groove through which cells move) first appears during gastrulation Protostome Deuterostome These 3 germ layers will develop into all of the tissues and organs of the adult animal

Organogenesis Organogenesis involves the development of organs from the 3 germ layers (endo-, meso-, ectoderm) Involves cell differentiation and localized morphogenetic changes in tissue and cell shape A primitive notochord forms during organogenesis Notochord is replaced by vertebral column in vertebrates

Cellular Potency Totipotency Cells that retain the zygotes potential to form all parts of the animal They can become ANYTHING Stem cells Determination The progressive restriction of a cell’s developmental potential Analogy: Choosing a college major…

The Genetics of Development All stages of development are regulated by genes Gene activation and expression ultimately control an organism’s development