Reproduction lecture 1 (2).docx

CONCEPT 1: Animals reproduce both asexually and sexually In sexual reproduction, the fusion of haploid gametes forms a diploid cell, the zygote. The animal that develops from a zygote can in turn give rise to gametes by meiosis. The female gamete, the egg, is large and nonmotile, whereas the male...

CONCEPT 1: Animals reproduce both asexually and sexually In sexual reproduction, the fusion of haploid gametes forms a diploid cell, the zygote. The animal that develops from a zygote can in turn give rise to gametes by meiosis. The female gamete, the egg, is large and nonmotile, whereas the male gamete, the sperm, is generally much smaller and motile. In asexual reproduction, new individuals are generated without the fusion of egg and sperm. For most asexual animals, reproduction relies entirely on mitotic cell division Examples of asexual selection Binary and multiple fission- Splitting of cells into two or more Budding of hydra- new individuals arise from outgrowths of existing ones Asexual reproduction-fragmentation- the breaking of the body into several pieces, followed by regeneration, regrowth of lost body parts Parthenogenesis, an example of asexual in which an egg develops without being fertilised. In the case of honeybees, males (drones) are fertile haploid adults that arise by parthenogenesis. In contrast, female honeybees, including both the sterile workers and the fertile queens, are diploid adults that develop from fertilised eggs. Parthenogenesis is a form of asexuaI reproduction where individual's develop from unfertilized eggs. This behavior has been observed in both pIants and anima Is. Many invertebrates reproduce parthenogen ica Ily; the colony hierarchy seen in bees and and is st ructured by whether individuals form from asexual or sexual means, and whether they have a full or half set of chromosomes. However,. parthenogenesis is extremeIy rare in vertebrates. It has been confirmed in a handful of sharks. snakes, lizards, and birds. There are no known naturaIly parthenogenic mammaIs, though researchers have induced parthenogenesis in mice and rabbits. In some species, parthenogenesis can be induced by environmental conditions - aphids switch to parthenogenesis in the summer, enabling them to reproduce extremely quick y with no resources lost to non-reproductive (male) units. Sometimes an absence of males will aIso trigger parthenogenesis allowing, the population to stay afloat until mates are available. This has been witnessed a handful of times in zoos. Despite these advantages, parthenogenesis can reduce genetic diversity, potentially leaving the populatio1n vulnerable to changing conditions. When eukaryotes reproduce sexually, offspring receive genetic material from two parents. Meiosis and fertilization shuffle and reshuffle genes, generating lots of genetic diversity. The offspring of sexually reproducing organisms are never identical to either their parents or their siblings (except for identical twins). Genetic variation increases the likelihood of a population’s adapting to new or changing environmental conditions. Clownfish are protandrous –males early in dev, females later Some sea slugs are simultaneous hermaphrodites Wrasse are protogynous – females when smaller, males when larger, can switch Animals exhibit reproductive cycles Cycles are controlled by hormones, whose secretion is in turn regulated by environmental cues. Animals reproduce only when sufficient energy sources are available and when environmental conditions favour the survival of offspring. Ovulation, the release of mature eggs, occurs at the midpoint of each cycle. The pineal gland responds to light. Seasonal variation in light triggers different levels of melatonin production, which is linked to breeding hormones and behaviour. This mechanism controls seasonal breeding. Animals that reproduce asexually also exhibit reproductive cycles Daphnia reproduce asexually when environmental conditions are favourable and sexually during times of environmental stress. As a result, the switch between sexual and asexual reproduction is roughly linked to season. Courtship behaviour in an asexual animal During the breeding season, one female of each mating pair mimics a male. Each member of the pair alternates roles. An individual adopts female behaviour when the level of the hormone oestradiol is high, and it switches to male-like behaviour when the level of the hormone progesterone is high. Why did sexual reproduction evolve? Sexual reproduction may enhance the reproductive success of parents when environmental factors, such as pathogens, change relatively rapidly. In contrast, asexual reproduction is expected to be most advantageous in stable, favourable environments because it perpetuates successful genotypes precisely. CONCEPT 2: Fertilisation depends on mechanisms that bring together sperm and eggs of the same species In species with external fertilisation, the female releases eggs into the environment, where the male then fertilises them. In species with internal fertilisation, sperm deposited in or near the female reproductive tract fertilise eggs within the tract. Internalfertilization Externalfertilization Malegametefi1seswithfemale gameteinsidethebody. Malegametefi1seswithfemale gameteoutsidethebody. Malereleasesitsspenns inthe femalebody. Bothmaleandfemalebody releasesoutsideinmaedimnlte water. Afterfe1til ationzygote development occurninsidethe body. e.g.Human,cattleetc. Afterfe1til ation,zygote development occursoutsidethe body. e.g.Frog A moist habitat is almost always required for external fertilisation to prevent the gametes from drying out and to allow the sperm to swim to the eggs. Many aquatic invertebrates shed their eggs and sperm into the surroundings, and fertilisation occurs without the parents making physical contact. Timing is crucial! Individuals clustered in the same area release their gametes into the water at the same time, a process known as spawning. InternaI Fert'iIi'zation Internal fertiliization occurs inside the femalieparent Reptiles (lay many eggs) Birds (lay only a few eggs) Mammals development takes place inside the female body Mating animals may make use of pheromones, chemicals released by one organism that can influence the physiology and behaviour of other individuals of the same species. Concept summary Animals reproduce both asexually and sexually Animals exhibit reproductive cycles Fertilisation depends on mechanisms that bring together sperm and eggs of the same species 6 point summary