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Learning Goals
By the end of this reading you should be able to:
- Compare and contrast the advantages of internal and external fertilization
- Describe adaptations of animals related to fertilization
- Discuss the differences between oviparity, ovoviviparity, and viviparity
The Evolution of Reproduction
Once multicellular organisms evolved and developed specialized cells, some also developed tissues and organs with specialized functions. An early development in reproduction occurred in the Annelids. These organisms produce sperm and eggs from undifferentiated cells in their coelom and store them in that cavity. When the coelom becomes filled, the cells are released through an excretory opening or by the body splitting open.
Reproductive organs evolved with the development of gonads that produce sperm and/or eggs. These organs contained cells that went through meiosis, an adaption of mitosis, which reduced the number of chromosomes in each reproductive cell by half while increasing the number of cells through cell division. The process of meiosis also contributed to the genetic diversity between gametes, through crossing over and independent assortment of chromosomes.
Complete reproductive systems developed in insects, where separate sexes are present. Sperm are made in testes and then travel through coiled tubes to the epididymis for storage. Eggs mature in the ovary. When they are released from the ovary, they travel to the uterine tubes for fertilization. Some insects have a specialized sac, called a spermatheca, which stores sperm for later use, sometimes up to a year. This allows fertilization to be timed with environmental or food conditions that are optimal for offspring survival.
Vertebrates also have complete reproductive systems and separate sexes. Within the vertebrates, there are different adaptations within groups related to reproductive structures. Non-mammals, such as birds and reptiles, have a common body opening, called a cloaca, for the digestive, excretory, and reproductive systems. Coupling in these groups usually involves positioning the cloaca openings opposite each other for the transfer of sperm. Mammals, on the other hand, have separate openings for the systems in the female, In addition, female mammals also have a uterus that supports developing offspring. The uterus has two chambers in species that produce large numbers of offspring at a time, while species that produce one offspring, such as primates, have a single uterus.
Sexual reproduction starts with the combination of a sperm and an egg in a process called fertilization. Sperm transfer from the male to the female during reproduction ranges from releasing the sperm into the watery environment for external fertilization to the joining of the cloaca in birds or the development of a penis for direct delivery into the female’s vagina in mammals for internal fertilization.
External Fertilization
External fertilization usually occurs in aquatic environments where both eggs and sperm are released into the water. After the sperm reaches the egg, fertilization takes place. Most external fertilization happens during the process of spawning where one or several females release their eggs and the male(s) release sperm in the same area, at the same time. The release of the reproductive material may be triggered by water temperature or the length of daylight. Nearly all fish spawn, as do crustaceans (such as crabs and shrimp), mollusks (such as oysters), squid, and echinoderms (such as sea urchins and sea cucumbers). Broadcast spawning can result in a greater mixture of the genes within a group, leading to higher genetic diversity and a greater chance of species survival in a hostile environment. For sessile aquatic organisms like sponges, broadcast spawning is the only mechanism for fertilization and colonization of new environments.
Pairs of organisms that are not broadcast spawners may exhibit courtship behavior. This allows the female to select a particular male. The trigger for egg and sperm release (spawning) causes the egg and sperm to be placed in a small area, enhancing the possibility of fertilization.
External fertilization in an aquatic environment protects the eggs from drying out. However, the presence of the fertilized eggs and developing young in the water provides opportunities for predation resulting in a loss of offspring. Therefore, millions of eggs must be produced by individuals, and the offspring produced through this method must mature rapidly, in order to ensure that at least some of the offspring survival. In general, the survival rate of eggs produced through broadcast spawning is low.
Review Question:
Quick Review: External Fertilization
Internal Fertilization
Internal fertilization occurs most often in land-based animals, although some aquatic animals also use this method. Internal fertilization has the advantage of protecting the fertilized egg from dehydration on land. The embryo is isolated within the female, which limits predation on the young. Internal fertilization enhances the fertilization of eggs by a specific male. Fewer offspring are produced through this method, but their survival rate is higher than that for external fertilization.
There are three ways that offspring are produced following internal fertilization:
Oviparity: The fertilized eggs that result from internal fertilization are laid outside the female’s body and develop there, receiving nourishment from the yolk that is a part of the egg. This occurs in most bony fish, many reptiles, some cartilaginous fish, most amphibians, two mammals (duck-billed platypi, echidnas), and all birds. Reptiles and insects produce leathery eggs, while birds and turtles produce eggs with high concentrations of calcium carbonate in the shell, making them hard.
Ovoviviparity: fertilized eggs are retained in the female, but the embryo obtains its nourishment from the egg’s yolk and the young are fully developed when they are hatched. This occurs in some bony fish (like the guppy Lebistes reticulatus), some sharks, some lizards, some snakes (such as the garter snake Thamnophis sirtalis), some vipers, and some invertebrate animals (like the Madagascar hissing cockroach Gromphadorhina portentosa).
Viviparity: the young develop within the female, receiving nourishment from the mother’s blood through a placenta. The offspring develops in the female and is born alive. This occurs in most mammals, some cartilaginous fish, and a few reptiles.
For each of the ways that offspring are produced, there is a trade-off in offspring survival and energy expended by the parents. Oviparous organisms often produce a much larger number of fertilized eggs because the survival of the offspring is lower. Viviparous organisms in contrast produce fewer offspring and expend larger amounts of energy in protecting and nourishing the offspring prior to live birth.
Review Question:
Summary
Sexual reproduction starts with the combination of a sperm and an egg in a process called fertilization. This can occur either outside the bodies or inside the female. Both methods have advantages and disadvantages. Once fertilized, the eggs can develop inside the female or outside. If the egg develops outside the body, it usually has a protective covering over it. Animal anatomy evolved various ways to fertilize, hold, or expel the egg. The method of fertilization varies among animals. Some species release the egg and sperm into the environment, some species retain the egg and receive the sperm into the female body and then expel the developing embryo covered with shell, while still other species retain the developing offspring through the gestation period.
Review Questions
REVIEW: Offspring of internal fertilization
REVIEW: Offspring of Internal Fertilization
Thinking About It
Attributions:
Materials based on :https://cnx.org/contents/GFy_h8cu@10.99:m2mqXRb6@4/Fertilization
