Reproduction is the creation of a new individual or individuals from previously existing individuals. In animals, reproduction can occur in two primary ways: Asexual & Sexual Reproduction
i. Asexual means without sex and requires only one parent ii.in asexual reproduction, one individual produces offspring that are genetically identical to itself. These offspring clones are produced by mitosis. iii.asexual reproduction appears in bacteria and unicellular eukaryotes, and in many invertebrate phyla, such as sponges, cnidarians, flatworms, annelids, and echinoderms.
1. Budding a. A new individual arises as an outgrowth (bud) of the parent, which will eventually detach and grow into a fully formed individual. b. Budding is an unequal division of an organism (as opposed to binary fission which is equal division). a. Budding is especially prominent in cnidarians, including Hydras
2. Gemmules a.gemmulation is the formation of a new individual from an aggregation of cells surrounded by a resistant capsule, called a gemmule. b.gemmules are internal buds; a specialized mass of cells that can develop into offspring. c.in many freshwater sponges, gemmules develop in the fall and survive the winter in the dried or frozen body of the parent. In spring, the enclosed cells become active, emerge from the capsule, and grow into a new sponge.
3. Fragmentation a.occurs in multicellular organisms when the body of the organism (parent) breaks into two or more parts, each of which will form a new organism b.occurs in Planarians and Sea Anemones
4. Fission followed by regeneration a.when a piece of a parent is detached, it can grow and develop into a completely new individual b.occurs in some cnidarians, flatworms, echinoderms, and some segmented worms (annelids).
5. Parthenogenesis a.parthenogenesis means virgin origin and it is the process in which an unfertilized egg develops into a complete individual. b.can occur in wasps, bees, crustaceans, and ants. Animals that have no sex chromosomes often reproduce by this process. Some reptiles and fish are also capable of reproducing in this manner.
Animals that remain in one particular place and are unable to look for mates would need to reproduce asexually. Numerous offspring can be produced without "costing" the parent a great amount of energy or time. A disadvantage of this type of reproduction is the lack of genetic variation. Offspring are genetically identical to parent and to each other. Because all of the organisms are genetically identical, they therefore share the same weaknesses. If the stable environment changes, the consequences could be deadly to all of the individuals. a. As a result, organisms that reproduce asexually are found in stable environments that experience very little change to which they are very well suited.
1.Yeasts, protists, plants and bacteria are capable of asexual reproduction as well. 2.Bacterial asexual reproduction most commonly occurs by a kind of cell division called binary fission. Since the cells produced through this type of reproduction are identical, they are all susceptible to the same types of antibiotics.
i. Sexual reproduction is the production of offspring formed by the union of gametes from two genetically different parents; Egg of one parent is fertilized by the sperm of another. 1. Most animals are dioecious, which means having separate sexes.
1. Individual animals that have both male and female reproductive organs are called monoecious, or hermaphrodites. a. Many sessile, burrowing, or endoparasitic invertebrate animals are hermaphroditic. b. Some hermaphrodites can self-fertilize, for example tapeworms. c. Some hermaphrodites cannot self-fertilize and so must exchange sperm to fertilize each other s eggs, for example some snails. Mating Helix aspersa (garden snails). Turbellarians mating by penis fencing. Each has two penises on the undersides of their heads which they use to inject sperm.
d. The advantage is that with every individual producing eggs, a hermaphroditic species has the potential to produce twice as many offspring as could a dioecious species in which half the individuals are nonproductive males. i. Earthworms are simultaneous hermaphrodites; during mating each partner passes sperm from genital pores along grooves to seminal receptacles of its mate.
e. Some fish are sequential hermaphrodites, in which a genetically programmed sex change occurs within an individual organism. In many species of reef fish (such as wrasses) an animal begins life as either a female or a male (depending on the species) but later becomes the opposite sex. i. Clownfish are sequential hermaphrodites, meaning that they develop into males first, and when they mature, they become females. If the female clownfish is removed from the group, such as by death, one of the largest and most dominant males will become a female.
i. Usually produce haploid gametes in specialized organs called gonads 1. Testes produce sperm, and ovaries produce eggs 2. Haploid gametes are produced from diploid (2n) cells by meiosis a. Sperm Small, motile, male sex cell. b. Ovum (Egg) Larger, not motile, female sex cell that contains food reserves.
1. Produces genetically variable offspring, with different combinations of parental genes 2. The genetic diversity introduced by sexual reproduction can allow a species to survive changing environmental conditions. This is a major benefit to the survival of the species. 1. The organism must use energy to produce gametes 2. The organism may have to change its usual pattern of activity to ensure these gametes are brought together at the right time of the year. 3. Some reproductive behaviors may attract predators, not only a reproductive mate. 4. Reproduction in some species leads to deadly competition between males.
The costs of sexual reproduction however clearly outweigh the costs as reflected by the fact that nearly all eukaryotic organisms reproduce sexually.
a. Development begins with fertilizationthe union of male and female gametes to form a zygote. b. Can be internal or external i. Egg and sperm are released from animals; Union of sperm and egg takes place outside the bodies of the parents. 1. External fertilization is common in animals that live in water; Release of sperm and eggs into the water is called spawning. 2. Release is often synchronized using environmental cues (e.g., seasons, tides)
i. Union of sperm and egg takes place inside the female s body 1.Important adaptation to life on land 2.Often involves copulation, a sexual union to facilitate the reception of sperm by a female 3.Males typically have a penis for depositing sperm into the vagina of females
a. Once the egg is fertilized, activation of the egg occurs and rapid changes result. Some changes are: i. Increase in cytoplasmic Calcium (Ca 2+ ), which hardens the egg lining (preventing additional sperm from entering) and increases rates of cellular respiration and protein synthesis. ii. The sperm nucleus within the egg cell begins to swell. After about 20 minutes, the nucleus of the sperm and egg fuse, forming the diploid zygote and the beginning of a new life.
a. After fertilization, rapid cell divisions (mitosis) occur. This process is called cleavage. i. During cleavage the embryo divides repeatedly to convert the large, unwieldy cytoplasmic mass into a large cluster of small, maneuverable cells called blastomeres. ii. Cells undergo the S (DNA synthesis) and M (Mitosis) stages but skip the G phases so virtually no growth occurs. Therefore the cells become smaller with each division and the whole embryo is no larger than the original zygote. iii.the zygote continues to divide into a small, solid ball of blastomere cells called a morula. iv.after more divisions, some of the inner cells begin to compact outward and the intercellular fluid accumulates in the open space, which forms the next developmental stage called the blastula.
iii. Formation of this blastula stage, with one layer of germ cells, occurs in all multicellular animals. iv. In most animals, development continues beyond the blastula to form one or two more germ layers in a gastrula stage. a. The blastula is a hollow, fluid-filled ball of cells. i. In mammals, the cluster of cells is called a blastocyst; it forms in about 5 days after fertilization. ii. The cells are arranged around a central fluid-filled cavity called a blastocoel.
a. Gastrulation converts the spherical blastula into a more complex configuration and forms a second germ layer. i. As cells on one side of the blastula push inward, the opening to the gut is created. ii. The central cavity is known as the archenteron or primitive gut, and the opening to the archenteron is called the blastopore.
a. Now with two cell layers (called germ layers), the entire structure is called a gastrula. i. The layer of cells on the outer surface of the gastrula is called the ectoderm. 1. These will continue to grow and divide, eventually forming the skin and nervous tissue of the animals, as well as any other body coverings (fur, scales, etc.). ii. The layer on the inner surface of the gastrula is called the endoderm. 1. The endoderm cells also continue to grow and divide, eventually developing into the animal s digestive tract and digestive organs.
a. Animals with two germ layers are called diploblastic. i. Some organisms stop development at this stage. 1.Animals with only ectoderm and endoderm are limited to a tissue level of organization. ii.at this stage, the opening leads to what is called a blind or incomplete gut. 1.Certain animals, sea anemones and flatworms, for example, have a blind gut, usually called the gastrovascular cavity. 2.Anything consumed by an animal with a blind gut must either be completely digested, or the undigested parts egested through the mouth. 3.However, most animals have a complete gut with a second opening, the anus, and therefore continue developing to form this structure, as well as a third germ layer.
a. Most animals have a third germ layer and are triploblastic. b. The third layer, called the mesoderm, lies between the ectoderm and the endoderm. c. Animals with three tissue layers develop an organ level of organization. d. The mesoderm layer will eventually develop into skeleton and muscles (muscoskeletal system), circulatory system, excretory system, reproductive system, and in some animals, the respiratory system.
a. There are two major groups of triploblastic animals: a. The groups are identified by four developmental characters: i. Radial or spiral positioning of cells as they cleave (divide) 1. In radial cleavage, the cells align directly over each other 2. In spiral cleavage, the cells are not aligned directly over each other, but instead are at an angle.
ii. Regulative or mosaic cleavage of cytoplasm 1.In mosaic development, cell fate is determined by the components of the cytoplasm found in each blastomere. a.an isolated blastomere can t develop. 2.In regulative development, the fate of a cell depends on its interactions with neighbors, not what piece of cytoplasm it has. In regulative development, a blastomere isolated early in cleavage is able to from a whole individual (e.g. twins).
iii. Fate of the blastopore to become mouth or anus 1.Protostome means first mouth. The blastopore becomes the mouth; if there is a second opening, it will become the anus. 2.Deuterostome means second mouth. The blastopore becomes the anus and the mouth develops as the second opening.
i. Schizocoelous or enterocoelous formation of a coelom. 1.The coelom is a body cavity found in many triploblastic organisms that is completely surrounded by mesoderm. 2.This coelom can form in one of two ways: Schizocoelous forms as mesoderm splits. Enterocoelous forms as an out-pocketing of the gut.
a. Mollusks, annelids, and arthropods are considered protostomes. b. Echinoderms and vertebrates are considered deuterostomes.
a. A coelom, or true body cavity, is a body cavity completely surrounded by mesoderm. b. Some protostomes do not develop a coelom. i. Animals without a coelom are called acoelomates. ii. Flatworms, such as planarians, develop to an early gastrula stage and form a mesoderm layer. However, the mesoderm completely fills the blastocoel and a coelom never forms. c. In other protostomes, mesoderm lines only one side of the blastocoel, leaving a fluid-filled blastocoel next to the gut. d. The fluid-filled cavity surrounding the gut, only partially lined with mesoderm, is called a pseudocoelom; animals with this are called pseudocoelomates.
a. A single cell generally contains all of the genes necessary to construct the entire body. b. Differentiation (or specialization) in the cells of a developing embryo occurs when certain specific genes become activated. c. As development proceeds, a signal activates certain genes, which in turn produce other signals which activate new genes, which produce still more signals etc. d. Cells in developing embryos continue to differentiate and become specialized to perform different functions. e. As embryos continue to develop over time, most animals become juveniles that look like miniature adults. f. Most animals continue to grow and develop into adult stage. g. Eventually the adult will reach sexual maturity, mate, and the cycle begins again.
i. Symmetry is a term that describes the arrangement of body structures. i. Different kinds of symmetry enable animals to move about in different ways. 1.An animal, like a sponge, that is irregular in shape has no symmetry or an asymmetrical body plan. 2.Animals with no symmetry are often sessile organisms that do not move from place to place.
1.Animals with radial symmetry can be divided along any plane, through a central axis, into roughly equal halves. 2.Radially symmetrical animals tend to be sessile or sedentary (i.e., attached to a substrate or less motile). 3.Radial symmetry is an adaptation that enables an animal to reach out in all directions from one center; good for sensing food and predators.
1.An organism with bilateral symmetry can be divided down its length into similar left and right halves. 2.Animals with bilateral symmetry tend to be active and to move forward at an anterior end. a.animals with bilateral symmetry have various degrees of cephalization concentration of sensory tissues at the anterior end. b.they can find food and mates and avoid predators because they have sensory organs and good muscular control.
a.in bilateral animals: i. the anterior, or head end, often has sensory organs. ii.the posterior is the tail end. iii.the dorsal side is the upper surface. iv.the ventral side is the lower surface. b.in animals that are upright: i. The back is the dorsal surface. ii.the belly is the ventral surface.
i. About 95% of all animals are invertebrates. 1.Of the 1.5 million identified animal species, approximately only 42,500 have a backbone.