Animal Reproduction AP Biology Chapter 46 Campbell 6e Ppt courtesy of Tracy Jackson http://home.att.net/~tljackson/neville.html
Reproduction • There are 2 modes of reproduction in the animal kingdom. • Asexual = creation of individuals whose genes come from one parent by mostly mitotic cell division. • Sexual = creation of individuals from the fusion of gametes to form a zygote.
Asexual Reproduction • Fission, budding, and fragmentation are some of the mechanisms that allow organisms to reproduce asexually. • Fission = the division of bacterial cells. • Budding = new individuals splitting off of the parent. • Fragmentation = breakage of the organism into several parts to form new organisms. (accompanied by regeneration of new body parts.)
It is advantageous in a constant environment because a large number of offspring can be produced in a short time. • Flatworms can divide into 2 halves; each half grows into a separate organism. • Cnidarians undergo budding where a new individual grows from and then breaks off of the parent individual. • Echinoderms can be cut to form new individuals. • Insects- parthenogenesis (unfertilized egg develops.)
Sexual Reproduction • In sexual reproduction new individuals are produced by the fusion of haploid gametes to form a diploid zygote. • Sperm are male gametes, ova (ovum singular) are female gametes. • Meiosis produces cells that are genetically distinct from each other; fertilization is the fusion of two such distinctive cells that produces a unique new combination of alleles, thus increasing variation on which natural selection can operate.
Sperm and eggs are sometimes produced by the same individual and sometimes produced by different individuals. • The sexes of vertebrates are separate but some vertebrates can change sex.
Variation • Sexual reproduction promotes variation because offspring inherit genes from two different parents. • In addition, when an individual produces gametes, crossing-over and independent assortment mix genes from that individual’s parents. Gametes are haploid; they contain some genes from the individuals mother and some from the father. • Variation is advantageous to species in fluctuating environments.
Reproductive Cycles • Reproductive cycles in many vertebrates and invertebrates are related to changes in day length. • Day length is a reliable indicator of season for timing circannual events such as reproductive behavior and migration. • In many species, the pineal gland secretes melatonin in the dark. Increasing daylength (decreased melatonin levels) triggers reproductive behavior.
“Morphisms” • Sexually reproducing animals are either • Monoecious = male and female in same organism • Dioecious = male and female in different organisms. • Dioecious species are either sexually • Monomorphic (look the same) • dimorphic. (look different)
Monoecious • Even though monoecious organisms have both male and female reproductive systems they rarely fertilize their own eggs. • Either the two systems mature at different times or the events of sperm transfer and fertilization are separated.
They do have one advantage that dioecious organisms do not have. When dioecious organisms mate, only one parent produces new offspring. That's not the case with monoecious animals. When monoecious meet, they both can receive sperm from their partners and will later fertilize their own eggs with the sperm they received when they mated. • The result is that when monoecious animals mate, both parents produce young. Monoecious organisms are often sessile or slow moving, and there is an advantage to having both animals produce young when there is a chance encounter.
Dimorphism • The sexes of most sexually reproducing species differ in both primary sexual characteristics (sex organs) and secondary sexual characteristics associated with mating and bearing young. • Males are characterized by brighter colors and more elaborate structures such as horns. Bright colors may increase the attractiveness of males to females, while horns may serve as weapons for fighting with other males for control of a female.
Dimorphism • Dimorphism is most pronounced in insects where the morphologies of the penis and vagina are precisely matched for each species, apparently preventing incorrect matings between closely related species.
Mechanisms of Sexual Reproduction • Fertilization plays an important role in reproduction--- can occur in two ways. • Internal • External
External Fertilization • During external fertilization, many gametes are released into the water by each sex at the same time and place. • This type of fertilization requires water because animal sperm must swim to the eggs. Water also protects the gametes from drying out. • Species which have external fertilization are either aquatic or return to water for reproduction.
Internal Fertilization • Internal fertilization is practiced by species that lay shelled eggs or have a period of internal embryonic development. • This type of fertilization enables animals to reproduce in a terrestrial environment because it enables sperm to swim to the egg and it prevents gametes from drying out. • The male often has a copulatory organ (a penis) for transferring sperm. • Males of some aquatic animals (ex: sharks, skates, rays) have specialized pelvic fins which allow the sperm to be passed to the female.
Eggs and Birth • Animals can be grouped into those which • Give birth to living offspring and • Those which lay eggs that eventually hatch into offspring. • Those animals which give birth to live offspring are called live-bearing or viviparous. • Those animals which lay eggs are called egg-laying or oviparous. The difference is in the place where the offspring develops before it is born.
Human Reproduction • Human reproduction employs internal fertilization, and depends on the integrated action of hormones, the nervous system, and the reproductive system. • Gonads are sex organs that produce gametes. • Male gonads are the testes, which produce sperm and male sex hormones. • Female gonads are the ovaries, which produce eggs (ova) and female sex hormones.
Male Reproductive System • Testes are suspended outside the abdominal cavity by the scrotum, a pouch of skin that keeps the testes close or far from the body at an optimal temperature for sperm development. • Seminiferous tubules are inside each testis, and are where sperm are produced by meiosis. About 250 meters (850 feet) of tubules are packed into each testis. • Spermatocytes inside the tubules divide by meiosis to produce spermatids that in turn develop into mature sperm.
Spermatogenesis • Sperm production begins at puberty at continues throughout life, with several hundred million sperm being produced each day. • Once sperm form they move into the epididymis, where they mature and are stored.
Male Hormones • The anterior pituitary produces follicle-stimulating hormone (FSH) and luteinizing hormone (LH). • Action of LH is controlled by the gonadotropin-releasing hormone (GnRH). • LH stimulates cells in the seminiferous tubules to secrete testosterone, which has a role in sperm production and developing male secondary sex characteristics. • FSH acts on cells to help in sperm maturation. Negative feedback by testosterone controls the actions of GnRH.
Semen • Sperm pass through the vas deferens and connect to a short ejaculatory duct that connects to the urethra. • The urethra passes through the penis and opens to the outside. • Secretions from the seminal vesicles add fructose and prostaglandins to sperm as they pass. • The prostate gland secretes a milky alkaline fluid. • The bulbourethral (Cowper’s) gland secretes a mucus-like fluid that provides lubrication for intercourse. Sperm and secretions make up semen.
The Female Reproductive System • The female gonads, ovaries, are located within the lower abdominal cavity. • The ovary contains many follicles composed of a developing egg surrounded by an outer layer of follicle cells. • Each egg begins oogenesis as a primary oocyte. • At birth each female carries a lifetime supply of developing oocytes, each of which is in Prophase I. A developing egg (secondary oocyte) is released each month from puberty until menopause, a total of 400-500 eggs.
The Ovarian Cycle • After puberty the ovary cycles between a follicular phase (maturing follicles) and a luteal phase (presence of the corpus luteum). These cyclic phases are interrupted only by pregnancy and continue until menopause, when reproductive capability ends. The ovarian cycle lasts usually 28 days.
During the first phase, the oocyte matures within a follicle. At midpoint of the cycle, the oocyte is released from the ovary in a process known as ovulation. • Following ovulation the follicle forms a corpus luteum which synthesizes and prepares hormones to prepare the uterus for pregnancy. • The secondary oocyte passes into the oviduct (fallopian tube or uterine tube). The oviduct is connected to the uterus.
Structures • The uterus has an inner layer, the endometrium, in which a fertilized egg implants. • At the lower end of the uterus the cervix connects the uterus to the vagina. • The vagina receives the penis during intercourse and serves as the birth canal.
External Genitals • The female external genitals are collectively known as the vulva. • The labia minora is a thin membrane of folded skin just outside the vaginal opening. • The labia majora cover and protect the genital area.
Hormones and Cycles • The ovarian cycle is hormonally regulated in two phases. The follicle secretes estrogen before ovulation; the corpus luteum secretes both estrogen and progesterone after ovulation. • Hormones from the hypothalamus and anterior pituitary control the ovarian cycle. • The ovarian cycle covers events in the ovary; the menstrual cycle occurs in the uterus.
Menstrual Cycle • Menstrual cycles vary from between 15 and 31 days. • The first day of the cycle is the first day of blood flow (day 0) known as menstruation. • During menstruation the uterine lining is broken down and shed as menstrual flow. • FSH and LH are secreted on day 0, beginning both the menstrual cycle and the ovarian cycle. Both FSH and LH stimulate the maturation of a single follicle in one of the ovaries and the secretion of estrogen.
Rising levels of estrogen in the blood trigger secretion of LH, which stimulates follicle maturation and ovulation (day 14, or midcycle). • LH stimulates the remaining follicle cells to form the corpus luteum, which produces both estrogen and progesterone. • Estrogen and progesterone stimulate the development of the endometrium and preparation of the uterine inner lining for implantation of a zygote. • If pregnancy does not occur, the drop in FSH and LH cause the corpus luteum to disintegrate. The drop in hormones also causes the sloughing off of the inner lining of the uterus by a series of muscle contractions of the uterus.
Credits Most information from this site: http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookREPROD.html Other sites http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20102/Bio%20102%20lectures/Animal%20Reproduction/animal.htm http://www.saburchill.com/chapters/chap0031.html http://tidepool.st.usm.edu/Crswr/103animalreproduction.html