Animal Physiology II Lecture Notes PDF

ANIMAL PHYSIOLOGY II SC/BIOL 3070 - 4.0 Reproduction Suggested readings: Sherwood pgs. 757-765, 770-779 and 783-794 Reproduction 1 Outline Learning Objectives: Introduction and energy investment in reproduction Modes of reproduction: Asexual vs. Sexual Modes of Fertilization and Embryonic Developmen...

ANIMAL PHYSIOLOGY II SC/BIOL 3070 - 4.0 Reproduction Suggested readings: Sherwood pgs. 757-765, 770-779 and 783-794 Reproduction 1 Outline Learning Objectives: Introduction and energy investment in reproduction Modes of reproduction: Asexual vs. Sexual Modes of Fertilization and Embryonic Development Mammalian Male and Female Reproductive Systems Reproduction 2 Introduction - individual organisms do not live forever ➥ continuity of a species relies on individuals reproducing - a significant proportion of the energy expenditure of an animal is related to reproductive activities ➥ energy is utilized for the mode of reproduction (gamete production, mating, etc.) and is also used in parental care of the offspring Reproduction egg production vs. parental care? 3 Energy investment in reproduction - the most favorable mode of reproduction for a species is that yielding the greatest number of offspring reaching sexual maturity Two opposing strategies of energy investment in reproduction: 1) r-selected species - animals produce small-sized offspring at start of development ➥ this allows production of large numbers of offspring (e.g. sea urchins can release 100 million eggs in a single spawning!) - little or no parental care or protection, which greatly reduces the number of offspring reaching reproductive maturity (e.g. only 6 out of 1 million mackerel spawn survive to reproductive age!) Reproduction 4 Energy investment in reproduction - the most favorable mode of reproduction for a species is that yielding the greatest number of offspring reaching sexual maturity Two opposing strategies of energy investment in reproduction: 2) K-selected species - animals produce larger-sized offspring at start of development ➥ produce fewer numbers of offspring (e.g. rarely more than 8 to 10 offspring) - considerable energy expended in parental care Reproduction 5 Energy investment in reproduction - the most favorable mode of reproduction for a species is that yielding the greatest number of offspring reaching sexual maturity Two opposing strategies of energy investment in reproduction: - Some species show characteristics of both strategies (e.g. cichlid fishes: produce hundreds of larvae but invest a lot of time and energy in parental care; females may stop their own feeding for weeks in order to protect their young!) Reproduction 6 Modes of reproduction - both asexual and sexual reproduction occur in the animal kingdom ➨ asexual: production of offspring whose genes all come from one parent; there is no fusion of gametes - offspring is a clone (i.e. genetically identical to parent) ➨ sexual: production of offspring by fusion of gametes from male (sperm) and female (eggs) - involves fertilization and genetic variation - each parent supplies half the genetic material Reproduction 7 Asexual reproduction → Normally does not involve meiosis, haploid formation or fertilization Advantages Disadvantages - allows individuals to reproduce - variability only due to mutation - requires less energy (e.g. for finding a mate, producing eggs and sperm) - lack of variation means less survival after environmental change - reproduction occurs much faster - types of asexual reproduction: 1) budding 2) fragmentation 3) parthenogenesis Reproduction 8 Types of asexual reproduction 1) budding ➥ new individuals arise from existing ones due to outgrowths or “buds” (i.e. cell division at a particular site) - animals that reproduce by budding include hydra, corals, some sponges and flatworms Reproduction 9 Types of asexual reproduction 2) fragmentation ➥ when the body of an organism breaks into two or more parts, which develop into new adult organisms ➥ must be accompanied by regeneration of lost body parts - fragmentation is common in echinoderms (starfish), hydra, flatworms, some marine annelid worms, sponges and corals Reproduction 10 Types of asexual reproduction 3) parthenogenesis ➥ development of an unfertilized egg into a new individual that is a clone of the parent - animals that are parthenogenic include insects (e.g. bees, wasps, ants) and some rotifers ➥ some species of fishes, sharks, amphibians and lizards also reproduce by complex forms of parthenogenesis ➥ if a diploid set of chromosomes is needed, the egg will often duplicate by mitosis and then fuse to give the egg two sets of chromosomes Reproduction 11 Types of asexual reproduction 3) parthenogenesis: e.g. honeybees ➥ male drones are produced parthogenetically from unfertilized eggs ➥ female eggs (queen and worker bees) are produced sexually using sperm from male (drone) bees Drone Queen Worker (male) (female) (female) ➥ males are haploid and females are diploid Reproduction 12 Types of asexual reproduction 3) parthenogenesis: e.g. whiptail lizard species - all-female species that reproduces by parthenogenesis - lizard on top plays the role of a “male” - they switch sex roles every few weeks during the breeding season ➥ mediated by hormones - as estrogen levels rise, ovaries grow and the lizard behaves female-like - after ovulation, estrogen levels drop and progesterone levels rise to elicit male-like behaviour Reproduction 13 Alternating Asexual and Sexual Reproduction - some animals can alternate between asexual (parthogenetic) and sexual reproduction ➥ termed heterogamy or cyclical parthenogenesis - triggered by unfavorable climates, depletion of food resources, lack of males - when environment is favorable, asexual reproduction is used ➥ allows animals to quickly increase their population by exploiting the optimal climate and rich supply of resources - when food sources are depleted or the climate is hostile, these animals switch to sexual reproduction ➥ ensures a mixing of the gene pool, and variations found in the offspring may be better suited for survival - occurs in aphids, certain wasps, most rotifers, water fleas (i.e. Daphnia) Reproduction 14 Alternating Asexual and Sexual Reproduction - some animals can alternate between asexual (parthogenetic) and sexual reproduction ➥ termed heterogamy or cyclical parthenogenesis Life Cycle of Daphnia - in asexual cycle, females produce diploid eggs that develop directly into females - triggered by an environmental stimulus, the same females may also produce diploid eggs that develop into males, which produce haploid sperm ➥ in the sexual cycle, haploid sperm fertilize the haploid eggs ➥ fertilized eggs can be enclosed in a protective shell and undergo diapause until conditions improve Reproduction Environmental stimulus 15 Modes of reproduction - both asexual and sexual reproduction occur in the animal kingdom ➨ asexual: production of offspring whose genes all come from one parent; there is no fusion of gametes - offspring is a clone (i.e. genetically identical to parent) ➨ sexual: production of offspring by fusion of gametes from male (sperm) and female (eggs) - involves fertilization and genetic variation - each parent supplies half the genetic material Reproduction 16 Sexual reproduction → involves the fusion of a haploid gametes to produce a diploid zygote Three general stages of sexual reproduction: 1) gamete formation 2) fertilization 3) embryonic development Advantages Disadvantages - increases variation in offspring - use significant energy to produce gametes and find a mate - increases the reproductive success in a changing environment - may need to change usual activity or location to ensure gametes are brought together at right time of the year - increases the rate of adaptation - reproductive behaviours may attract predators - eliminates harmful genes from a population - may lead to deadly competition between conspecifics for mates Reproduction 17 Three stages of sexual reproduction 1) gamete formation - gametogenesis: production of gametes ➥ occurs through meiosis which produces cells that are genetically distinct from each other - genetic diversity is created by chromosomal cross-over (recombination) during meiosis, leading to a novel set of genes that can be passed onto offspring ➥ sperm are male gametes ➥ egg or ovum (ova, plural) are female gametes Reproduction 18 Three stages of sexual reproduction 2) fertilization - fusion of distinctive male and female gametes to initiate the development of a new individual organism - provides a unique combination of alleles, thus increasing variation Reproduction 19 Three stages of sexual reproduction 2) fertilization Two modes of fertilization: (a) external fertilization: - fusion of gametes occurs externally (i.e. outside specialized organs within the bodies of parents) ➥ occurs in many aquatic invertebrates, most fishes and amphibians where parents discharge gametes into the water ➥ often courtship rituals to trigger simultaneous gamete release - demands relatively large numbers of sperm cells and eggs to be shed ➥ can result in more zygotes ➥ lower survival rate Reproduction 20 Three stages of sexual reproduction 2) fertilization Two modes of fertilization: (b) internal fertilization: - sperm deposited in or near the female reproductive tract, and gametes fuse inside the female body ➥ occurs in the majority of terrestrial animals including some snakes, but also some fish and frogs - requires copulation (i.e. process in which male introduces sperm into the female’s body) and complex reproductive systems - protection of developing eggs (e.g. from excessive heat/cold, drying, predators, etc.) Reproduction 21 Three stages of sexual reproduction 3) embryonic development Three strategies of embryonic development: (a) ovipary: embryonic development occurs in an egg outside the body using resources within the egg (“egg-laying”) - fertilization may be: ➥ external (e.g. fishes) ➥ internal (e.g. birds, reptiles) - levels of parental care can range from none to intense - oviparous animals include fish, amphibians, reptiles, some snakes, all birds, monotremes (e.g. platypus), most insects Reproduction 22 Three stages of sexual reproduction 3) embryonic development Three strategies of embryonic development: (b) vivipary: embryonic development occurs inside the mother and derives significant resources from the mother ➥ “live-bearing” – give birth to live young ➥ fertilization is always internal - viviparous animals include placental mammals, and some sharks, reptiles, fish, amphibians and insects Reproduction 23 Three stages of sexual reproduction 3) embryonic development Three strategies of embryonic development: (c) ovovivipary: embryonic development occurs in an egg that remains in the mother’s body until it hatches ➥ embryo derives significant resources from egg yolk rather than the mother ➥ fertilization is always internal - ovoviviparous animals include many fish and invertebrates, and some sharks, reptiles, snakes Reproduction 24