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Reproduction and Development Asexual Reproduction: reproduction without gametes Common in protozoa and in invertebrate animals Binary fission: division of a parent into two or more individuals Budding: offspring are outgrowths of parents Fragmentation: animal breaks into two or more parts each fragment can become a individual RESULT: offspring are identical to parent (no genetic variation) 1 Sexual Reproduction: reproduction with gametes Present in all groups of animals Bisexual reproduction: involving the union of gametes from two genetically different parents RESULT: offspring are a new combination of both parents Fertilization depends on mechanisms that help sperm meet eggs of the same species Things to consider: Sperm must be able to find an egg Gametes need a moist environment (to prevent drying out and help sperm swim) Two approaches: External fertilization: release of gametes into the surrounding environment Requirement: timing! ensure sperm encounters eggs Internal fertilization: Sperm is deposited in or near the female reproductive tract and fertilization occurs in it Requirement: cooperative behavior between male and female structural adaptations for delivering sperm Solution: use of environmental cues, pheromones, and/or courtship behavior (involves species recognition , female choice, male-male competition ) Advantages and disadvantages of each type of fertilization ? 2 Why do so many animals reproduce sexually rather than asexually? sexual reproduction asexual reproduction Investment of energy and time Genetic variation Environment type favored changing vs stable environment colonization vs crowded Reproductive patterns Animals may reproduce exclusively asexually or sexually, or they may alternate between the two modes Hermaphroditism : An animal that has both sexual organs Beneficial to animals that are sessile , burrowing or endoparastitic In reef fish and oysters, sequential hermaphrodites changes its sex with age and size Wrasses: female first, male live in harems In clown fish male first 3 Reproductive patterns Parthenogenesis: development of an embryo from a unfertilized egg (can be diploid or haploid) Beneficial when environment is stable In aphid and Daphnia, as a response to seasons or stress During environmentally stable times eggs ___________ During environmental stress eggs _____________ In bees, wasps and ants, role in social organization Fertilized eggs queens and sterile workers Unfertilized eggs males drones Reproductive patterns Oviparous (egg-birth) eggs are laid outside of body for development fertilization may be internal or external may show various degrees of parental care Ovoviviparous (egg-live-birth) eggs are retained inside but embryos derive all nourishment for yolk inside the egg common earthworms, insects, sharks and snails and in some reptiles Viviparous (live-birth) eggs develop inside deriving their nourishment directly from the mother mostly in lizards, snakes, mammals and sharks but also in scorpions 4 Animal development: Fertilization Animal development: Cleavage (one layer embryo) 5 Animal development: Gastrulation (two-layers) Animal development: Formation of a third layer of cells and a complete gut and in some a second cavity 6 Animal development: the fate of cells in Germ layers Embryo development provides information about how animal groups are related 7