Animal Reproduction Lecture Notes PDF

 Learning Objectives Distinguish between sexual and asexual reproduction Identify different modes of asexual reproduction Discuss adaptations for reproduction and fertilization Distinguish between oviparous, ovoviviparous, and viviparous  Animal Reproduction...

 Learning Objectives Distinguish between sexual and asexual reproduction Identify different modes of asexual reproduction Discuss adaptations for reproduction and fertilization Distinguish between oviparous, ovoviviparous, and viviparous  Animal Reproduction Sexual reproduction is the creation of an offspring by fusion of a male gamete (sperm) and female gamete (egg) to form a zygote Asexual reproduction is creation of offspring without the fusion of egg and sperm  Modes of Asexual Reproduction Budding is a simple form of asexual reproduction found only among invertebrates – New individuals arise from outgrowths of existing ones  Modes of Asexual Reproduction Fission is splitting into two daughters of equal size  Modes of Asexual Reproduction Fragmentation is breaking of the body into pieces, some or all of which develop into adults – Fragmentation must be accompanied by regeneration, regrowth of lost body parts  Modes of Asexual Reproduction Parthenogenesis: egg develops without being fertilized Parthenogenesis is mainly observed in invertebrates, but is observed in some vertebrates (rarely) Queen or worker (sterile female)  Sexual Reproduction Sexual reproduction is the creation of an offspring by fusion of a male gamete (sperm) and female gamete (egg) to form a zygote Human sexual reproduction Evolution of Reproduction Strategies Genetic recombination during sexual reproduction produces offspring of varied phenotypes, which can increase reproductive success when environmental factors change Asexual reproduction is expected to be most advantageous in stable, favorable environments  Reproduction Cycles Reproductive cycles are controlled by hormones and environmental cues (changing seasons) Ensuring they expend resources to reproduce only when sufficient food is available and when environmental conditions favor the survival of offspring.  Reproduction Cycles Reproductive cycles are controlled by hormones and environmental cues Ovulation is the release of mature eggs at the midpoint of a female cycle. Can occur at specific times to increase probability of offspring arriving during productive conditions Growing Corpus Degenerating Maturing follicle luteum corpus luteum follicle Follicular phase Ovulation Luteal phase Day 0 5 10 14 15 20 25 28  Reproduction Cycles Because seasonal temperature is often an important cue in reproduction, climate change can decrease reproductive success  Reproduction Adaptations For some animals, finding a partner for sexual reproduction may be challenging (e.g., sessile animals) Hermaphroditism (=monoecious): individual has male & female reproductive organs (compared with dioecious = individuals have one set of reproductive organs) Caryophyllia corals: deep sea species contain male and female organs  Reproduction Adaptations For some animals, finding a partner for sexual reproduction may be challenging (e.g., sessile animals) Simultaneously hermaphroditism: animals can produce sperm and eggs at the same time Two hermaphrodites can mate, and some hermaphrodites can self-fertilize  Reproduction Adaptations For some animals, finding a partner for sexual reproduction may be challenging (e.g., sessile animals) Simultaneously hermaphroditism: animals can produce sperm and eggs at the same time Two hermaphrodites can mate, and some hermaphrodites can self-fertilize  Reproduction Adaptations Sequential hermaphroditism: animals that start out as one sex and switches to the opposite sex at a point in their life Oysters: male → female (depending on size)  Reproduction Adaptations Sequential hermaphroditism: animals that start out as one sex and switches to the opposite sex at a point in their life California sheephead: Live in harems with one male and many females; when male dies, largest female turns into the male  Fertilization Adaptations Fertilization, the union of egg and sperm, can occur externally or internally External fertilization: eggs shed by the female are fertilized by sperm in the external environment – moist habitat is required to allow sperm to swim to the egg and to prevent the gametes from drying out In internal fertilization, moist habitats are not necessary  External Fertilization Spawning –release their gametes into the water at the same time In some cases chemical signals trigger spawning; in other environmental cues are responsible  Actinopterygii Almost all are oviparous and shed eggs: – Into the water column (cod, herring) and then “crop- dusted” with sperm gametes (broadcast spawning) – Lay in a nest within sediment (salmon) or on hard substrate (garibaldi) NOAA Fisheries  Internal Fertilization In internal fertilization, sperm are deposited in or near the female reproductive tract, and fertilization occurs within the tract Internal fertilization requires behavioral interactions and compatible copulatory organs  Internal Fertilization In internal fertilization, sperm are deposited in or near the female reproductive tract, and fertilization occurs within the tract Internal fertilization requires behavioral interactions and compatible copulatory organs  Internal Fertilization Viviparous: (“Live bearing”) Embryos develop within and obtain nutrients from the mother. Ovoviviparous: Eggs are retained inside females body, but receive no nutrition from the mother during development. Oviparous: Eggs that hatch after they have been laid.  Ovoviviparous Ovum = “egg” vivus = “alive” parous = “bear” Combination of Viviparous + Oviparous  Internal Fertilization Internal fertilization is typically associated with production of fewer gametes but higher zygote survival Internal fertilization is also often associated with mechanisms to provide protection of embryos and parental care of young  Ensuring the Survival of Offspring Internal fertilization: Better zygote survival from eggs being fertilized internally (sheltered from potential predators and fluctuating environmental conditions) – Oviparous animals have shells and internal membranes that protect against water loss and physical damage during the eggs’ external development  Ensuring the Survival of Offspring External fertilization: – Lay many eggs, increase probability that some survive – Guard eggs  Learning Objectives Discuss development from zygote to birth, including fertilization, cleavage, gastrulation, and organogenesis Discuss the process of gastrulation Identify the three germ layers (ectoderm, mesoderm, endoderm)  Animal Development In sexual reproduction, development begins at fertilization – union of haploid gametes (sex cells) to form a diploid zygote Human sexual reproduction  Animal Development In sexual reproduction, development begins at fertilization – union of haploid gametes (sex cells) to form a diploid zygote Once fertilization is complete, cell divides through cleavage – multiple cleavage events lead to blastula (cells surrounding blastocoel) Fig. 47.6  Animal Development Fertilization –> Cleavage Eggs and zygotes of many animals (except mammals) have a polarity – defined by distribution of yolk; important for axis formation Animal Pole – less yolk Vegetal Pole – more yolk Fig. 47.22 Animal Development In non-amniotes (e.g., frogs) cleavage planes follow pattern relative to poles  Animal Development Fertilization –> Cleavage In some amniotes (e.g., chicken), a small disk forms on top of a large yolk cell.  Animal Development Fertilization –> Cleavage –> Gastrulation Gastrulation: reorganization of the blastula into a two-layered or three-layered embryo called a gastrula. – Involves movement of cells inward through a blastopore – Results in formation of 3 germ layers ECTODERM (outer layer) MESODERM (middle layer) ENDODERM (inner layer)  Fertilization Zygote (diploid) Cleavage Eight-cell stage (haploid + haploid) Cleavage Blastocoel Blastula Cross section of blastula Gastrulation Cross section Blastocoel of gastrula Endoderm Embryonic Ectoderm Germ Layers Blastopore Archenteron Figure 32.2-3 Gastrulation Gastrulation in frogs The blastula has a dorsal and ventral side, left and right side, and anterior and posterior end. The cell movements begin on the dorsal side, opposite where the sperm entered the egg.  Gastrulation In chicks – blastula sits on a large yolk mass Two layers – epiblast (upper) and hypoblast (lower) – During gastrulation, epiblast moves towards midline then inward towards yolk – primitive streak (functions as blastopore) – Epiblast gives rise to 3 layers Fig. 47.11  Gastrulation Human eggs are small, storing little food reserves. Fertilization takes place in the oviduct. Development begins while the embryo moves down the oviduct to the uterus  Gastrulation The cells of the blastula continue to divide resulting in two layers of cells the embryoblast and Trophoblast (collectively called the Blastocyst) when it arrives in the uterus  Reproduction in Eutherians Egg implants into uterine wall – trophoblast stops immunological rejection of embryo in Eutherians (absent in marsupials = short gestation/altricial young) Trophoblast: cells that form the outer layer of the blastocyst and develop into the placenta.  Gastrulation Once in the uterus, Trophoblast expands and blastocyst implants (~7 days after fertilization).  Gastrulation Extraembryonic membranes start to form (10- 11 days) and gastrulation begins (13 days)  Amniotic Egg Major function is protection (from drying) Contains four specialized membranes: the amnion, the chorion, the yolk sac, and the allantois  Gastrulation Gastrulation results in 3-layered embryo with 4 extraembryonic membranes: amnion, chorion, yolk sac, and allantous  Organogenesis Fertilization –> Cleavage –> Gastrula –> Organogenesis Organogenesis: the development of the organs  Organogenesis Fertilization –> Cleavage –> Gastrula –> Organogenesis Localized interactions of the germ layers to form organs. E.g., neurulation in frogs – genesis of brain and spinal cord – Notochord forms from the mesoderm – Neural plate forms from the ectoderm  Organogenesis E.g., neurulation in frogs – Neural plate curves inward to form the neural tube (forms central nervous system) – Neural crest cells develop along the neural tube in vertebrates and migrate to various parts of the embryo (form nerves, parts of teeth, skull bones, etc.) Fig. 47.14  Fate of Cells Adult variation of cells in 3 germ layers Fig. 47.9  Summary Asexual reproduction can occur by budding, fission, fragmentation, or Parthenogenesis. Sexual reproduction (fusion of gametes to form a zygote) is costly, but results in genetic diversity. In human reproduction, ovulation releases an egg from an ovary, where it can be fertilized in the oviduct, then implants in the uterine wall due to the trophoblast expansion (absent in marsupials), then gastrulation results in 3-layered embryo with 4 extraembryonic membranes: amnion, chorion, yolk sac, and allantous