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Needham Notes Lesson 26 Lesson Outline: Exercise #1 - Basic Functions Exercise #2 - Phylogenetic Trends Exercise #3 - Case Studies to Compare • Reproductive Strategies- Energy Partitioning • External versus Internal Fertilization • Sexual Dimorphism o Functional Characteristics o Aids to Identification o Copulatory Organs • Timing - Copulation, Ovulation, Fertilization, Development Objectives: Throughout the course what you need to master is an understanding of: 1) the form and function of structures, 2) the phylogenetic and ontogenetic origins of structures, and 3) the extend to which various structures are homologous, analogous and/or homoplastic. At the end of this lesson you should be able to: Describe the advantages and disadvantages of internal and external fertilization Describe sexual dimorphism and the selection pressures that lead to it Describe the trends seen in the design of copulatory organs Describe the various forms of reproductive strategy for delaying development of the fertilized egg and the selective advantage of them References: Chapter 15: 351-386 Reading for Next Lesson: Chapter 16: 387- 428 Exercise #1 List the basic functions of the urogenital system: The urinary system excretes the waste products of cellular digestion, ions, amino acids, salts, etc. It also plays a key role in water balance along with numerous other structures in different species living in different environments (i.e. gills, skin, salt glands). The primary function of the system is to give rise to offspring, - to reproduce. Exercise #2 Describe the evolutionary trends that we see in the urogenital systems of the different vertebrate groups: The phylogenetic trends that we see throughout the chordates were covered in detail in lectures (lecture 31 and 32) and are summarized schematically in the next figures: Exercise #3 – Comparisons – Case 1 Reproductive Strategies - Energy Partitioning Some would argue that the primary reason that organisms exist is to reproduce and make more organisms. We are the sperm and eggs way of making more sperm and eggs. As a consequence, all energy that does not go into growth and survival goes into reproduction in most animals. Given the limited amount of energy available for this purpose, every species is confronted with the question of how best to partition that energy. For example, the two extremes in energy partitioning are seen by comparing mating smelt and humans. In the case of the fish, all energy goes into making gametes. The males and females get together once a year on a spawning beach and when conditions are right, they all release their eggs and sperm into the water synchronously. No energy goes into mating, courting, sexual dimorphism, childcare, etc. - just producing gametes. In mammals such as humans, on the other hand, large amounts of energy are spent on sexual dimorphism, mating, courtship, pregnancy and child rearing. One strategy is based on quantity. If you release enough gametes, some will meet, some eggs will get fertilized and some of the fertilized eggs will survive. The other strategy is based on quality. Put less energy into making gametes and more energy into making sure that the sperm and egg met, and that the embryo will develop and reach sexual maturity. There are numerous strategies in between. The diversity of reproductive structures, physiology and behaviour seems to exceed that of any other system. This is in part due to the freedom of reproductive structures and processes from the demands of other organs and systems. There is a definite autonomy of the reproductive system; it exists for the future of the species and not for the individual that houses it. Case 2 External versus Internal Fertilization In most water dwelling vertebrates, fertilization is external. Eggs and sperm are shed simultaneously from the body into the water where fertilization occurs. If the female uterus houses the embryo, or if a shell seals an egg, then sperm must fertilize the egg before it descends the oviduct. In these instances, fertilization must be internal. The disadvantage of external fertilization in an aquatic environment is the possibility of dispersion of the gametes before fertilization can occur combined with the release of the gametes into a harsh environment (especially for freshwater fish). Many mechanisms exist to minimize these effects: - juxtaposition of genital openings during release of gametes. - mass spawning - broadcast fertilization (the ultimate in promiscuity) common in fish that release a large number of small eggs. - nest building, with courtship and fertilization. The disadvantage of external fertilization in a terrestrial environment is the problem of desiccation of the gametes. Internal fertilization usually takes place in the genital tract. It appears to have evolved independently in every group. An example where it does not occur in the genital tract is in Haplochromis where it takes place in the mouth chamber. The female spawns and takes her eggs up into her mouth. The male has egg shaped spots on his tail. The female tries to swallow these which induces sperm release and the female takes up the sperm "accidentally". Internal fertilization provides a better, controlled environment for fertilization, but to ensure fertilization, ova and sperm must still meet at the right time. Mechanisms to ensure this include: - ova and sperm may be viable for prolonged periods - copulation can occur anytime (but the female must nourish the sperm). sperm in fish - 4-10 months turtles - up to 4 years snakes - overwinter lizards - up to 6 years birds - up to 45 days rodents - up to 156 days - copulation and ovulation may be timed together. - induced ovulation. - copulation on or during estrus - frequent copulation throughout the reproductive season. Case 3 Sexual Dimorphism These are external indications of gender. They fall into two categories - functional structures essential for uniting gametes or nurturing the young and - characteristics to aid individuals in identifying the sex or sexual condition of other individuals of the same species. Functional Characteristics In animals with internal fertilization these include: - various types of intromittent or copulatory organs in males. - ovipositors in female fish, brood patches in female birds, mammary glands in female mammals. Aids to Identification These include sight, sound and smell - which one usually being a function of environment. Visual: - colour. - shape. - movement patterns (appropriate movement responses start the behaviour chain leading to copulation in practically all vertebrates. Sound: - little used in aquatic species. - well developed in frogs and birds. Odor: - particularly used in mobile, terrestrial vertebrates. - allows them to identify gender in the absence of the individual and helps bring the sexes together. Allow sexual and species recognition at a distance. Allows animals to be more active and mobile. Copulatory Organs If fertilization is internal, there must be a way to transfer sperm from the male to the female cloaca or vagina. In many vertebrates copulation simply involves momentary apposition of the male and female cloacae to transfer sperm. Often, however, the male possesses an intromittent organ for inserting the sperm into the female reproductive tract. In sharks and many fish, this may occur in the form of specialized pectoral or anal fins. You have seen the claspers on the male shark in lab. In some fish the anal fin is designed in a similar fashion and is referred to as a gonopodium. During copulation, one clasper or gonopodium is inserted into the female and the terminal cartilage is spread by muscle to hold it firmly in place. Sperm are flushed along a groove in the clasper or gonopodium into the female by water from a siphon sac in the male. Fertilization in most frogs is external. Usually it occurs in water. Most salamanders, on the other hand, reproduce on land and the males produce a spermatophore that consists of a gelatinous capsule containing sperm. Following a courtship display, the male deposits the spermatophore in front of the female and the female gathers the spermatophore up with the lips of her cloaca. Turtles, crocodiles, birds and mammals all possess a single penis while lizards and snakes possess hemipenes. In all cases these structures are erectile. Thus they are flaccid and often retracted into the cloaca when not in use but become engorged with blood making them erect and stiff when needed. In many reptiles and amphibians, sperm are stored in a spermatheca or pocket in the cloaca. The female secretes nutrients to nourish the sperm and some species can store sperm for over 10 years. This decouples copulation and sperm transfer from fertilization allowing courtship and mating to occur opportunistically and fertilization and egg deposition to occur when conditions are right. In birds, the intromittent organ consists of little more than swellings of the edges of the male cloaca while in others there is a true penis that is quite elaborate. All mammals have a single penis although in male marsupials the tip is forked - to fit into the two lateral vaginas of the females. In many mammals the penis is strengthened by bone - the baculum or os penis. In all cases, these structures serve to enhance sperm transfer and ensure that the sperm reaches the oviduct of the female. Case 4 Timing - Copulation, Ovulation, Fertilization, Development Many strategies exist to match the timing of copulation to the timing of ovulation by the female. These range from situations in which the female changes in sexual characteristics around the time of ovulation to stimulate males to mate - to the situation in which males always want to mate but females are only receptive when they are ovulating. A more precise strategy is one in which the act of copulation induces the female to ovulate. This occurs in rabbits for instance. If copulation does not occur at the time of ovulation, then sperm must be stored until ovulation occurs. We've discussed this already in amphibians and reptiles. In bats, fertilization may also be delayed (as in amphibians and reptiles).
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