|Unlike algae, fungi, and plants, members of only a few groups of animals reproduce asexually. When it occurs, asexual reproduction in animals usually occurs in one of three ways: parthenogenesis, polyembryony, and budding (or splitting). Parthenogenesis is a modification of sexual reproduction, in which eggs develop without fertilization. Polyembryony is the internal production of numerous, multicellular propagules. On release, these develop into the next stage of the life cycle. Budding or splitting is the production of new individuals externally by the differentiation of "parent" cells or tissues. This kind of reproduction is very common in plants, and is evident in such familiar forms as strawberries (which send out runners) and redwood trees (which send up shoots from the base of the tree). Although less common in animals, budding and splitting lead to some rather spectacular colonies, including coral reefs.
(A) Polyembryony and budding in Parasitic Flatworms: Many Chances in the Lottery of Life.
Two classes in the phylum Platyhelminthes consist of important parasites, and each practices a different form of asexual reproduction.
(1) Polyembryony in Trematoda
Flukes are members of the flatworm class Trematoda. Most members of this class have complicated life cycles with two or more hosts. A prominent feature of their life cycles is polyembryony. Review the life cycle of the Asian liver fluke (Chlonorchis sinensis), as illustrated below. How many stages reproduce asexually by polyembryony? Find a redia on a prepared slide. Can you find any cercaria developing inside? If redia and cercaria are produced in the numbers shown in the diagram below, how many cercaria would come from each egg (assuming that the miracidium finds a snail and that stages within the snail experience perfect survival)?
See lab on Symbiosis for detail of the Chlonorchis life cycle (V & C fig. 7.37).
|(2) Budding in Cestoda
Tapeworms are members of the flatworm class Cestoda. They do not practice polyembryony, but do practice a strange form of budding. Examine (Rust, fig. 60; V & C figs. 7.38-7.41), and compare it with whole specimens and slides. Where does budding occur in tapeworms? In the "end," what is the product of this budding?
In general, what is the function of asexual reproduction in these parasites? In answering this question, it is useful to view them simply as very specialized animals. What is the habitat of a parasite? Is this habitat permanent or temporary, and why would this be important in their reproductive patterns? Compared to a free-living flatworm that lives under rocks in a lake, how likely is the propagule of a parasitic flatworm to find suitable habitat?
|(B) Asexual Reproduction in Cnidaria: Colony Formation and Alternation of Form.
Budding and Splitting are very common in the phylum Cnidaria (jellyfish, sea anemones, corals, hydroids, and relatives). You will examine the role of asexual reproduction in the alternation of forms and the formation of colonies in Cnidarians.
(1) Alternation of polyp and medusa forms in Cnidarians.
Examine the diagrammatic life cycle of a cnidarian, (Rust, fig. 55; V & C fig. 7.14), and the preserved material showing the life cycle of Obelia. The cnidarian body form is typically a two-layered cup. The mouth and gut are in the middle of the cup, surrounded by food gathering tentacles. But this cup can be oriented in one of two ways: with the mouth down or with the mouth up. The mouth down form is called a medusa, and is exemplified by the familiar jellyfish. It swims in the open water. The mouth up form is called a polyp, and is exemplified by the sea anemone. It is usually attached to the bottom. The basic life cycle of Cnidarians is an alternation between the asexually reproducing polyp stage and the sexually reproducing medusa stage. Polyps produce medusae asexually, by budding, and medusae mature to produce gametes that unite and develop into another polyp. (Note that both the polyp and medusa stages are diploid, so this is different from the alternation of haploid and diploid generations in plants and fungi.) This basic life cycle is most evident in the class Hydrozoa, of which Obelia is a member. In other classes of Cnidarians, one or the other stage is emphasized. In the Anthozoa, the medusa stage is absent, and the polyp may reproduce sexually. Corals and sea anemones are members of this group. In the Scyphozoa, the polyp stage is reduced or absent. Jellyfish are members of this group. See examples of Anthozoans and Scyphozoans.
|(2) Replication and colony formation by budding in Cnidarians.
As indicated in both the generalized life cycle and the life cycle of Obelia, polyps often produce other polyps asexually. Examine slides of Hydra budding. This is an asexual means of replication in this solitary animal. Budding or splitting occurs in other Cnidarians, and sometimes leads to the formation of colonies of genetically identical individuals. Examine the displays of anemone colonies, coral colonies, gorgonians, and colonial hydroids like Obelia. These colonies are the result of asexual budding of polyps. In these groups, the formation of large colonies is similar to the morphogenesis of a single individual, leading to large size and specialized form. The form of these colonies is often related to the capture of light or food. For example, some aspects of colony shape in corals are related to the capture of light (remember that corals receive much of their nutrition from endosymbiotic algae). Gorgonians and hydroids feed on animals suspended in the water, and the shape of the colony serves to place the feeding polyps out into water currents. Colonies sometimes compete aggressively for space, with polyps of different clones digesting one another. The polyps of some sea anemone colonies are differentiated into "warrior" and "reproductive" clones. See the display of anemone clone wars. Differentiation of colony members is sometimes well-developed. Obelia is an example of a colony with differentiated polyps, some specialized for feeding and others for reproduction. The champions in this game, however, are the Siphonophores. Examine the Portuguese man-o-war; it is not a single individual, like a jellyfish, but a colony of differentiated clones, each of which serves a specialized function.
|(C) Parthenogenesis in Aphids.
Parthenogenesis occurs widely in a few groups of animals, such as aphids and tiny rotifers, but rarely or never in other groups. Aphids are small insects that suck sap from plants; as such they are garden and crop pests. They have complex life cycles that alternate between asexual, parthenogenetic reproduction and sexual reproduction. Examine the aphid life cycle, and observe the live aphids. What is the advantage of parthenogenesis during summer? Why reproduce sexually in the autumn? What are the advantages and disadvantages of the winged and wingless forms?