A Sexual Experience?

Katie Owens
Product Management Coordinator

January 2017

Reproduction is a fundamental characteristic of all living organisms. The ability of a species to replicate and multiply is essential for continuous existence. Among animals, there are many diverse mechanisms and factors that promote successful reproduction. There are 2 primary forms of reproduction: asexual and sexual. Both have their disadvantages and advantages within an environment. While the majority of the animal kingdom reproduces sexually, many reproduce asexually—and some organisms have the ability to perform both ways!   

Sexual reproduction

Sexual reproduction is the combination of genetic material from 2 different sexes. By joining gametes (sex cells) from the male and female, a diploid cell (zygote) is formed. This is the process of fertilization.  The mechanism of fertilization, reproductive structures, and development of offspring varies across species. Because the offspring obtain a combination of traits from two different parents, genetic recombination occurs and creates diversity within populations. Over time, traits that promote greater reproductive success will be favored and harmful ones will be eliminated. This allows species to adapt and survive in changing environments. 

Although most animals reproduce sexually, the time and energy involved can be disadvantageous.  Choosing and courting a mate while competing with other animals is time consuming and labor intensive. When populations are low in a certain area, it can be difficult to find a partner, and competition increases. Bringing offspring to life is also more complicated for sexual organisms (for example, the period of pregnancy in humans). They produce fewer offspring less often than do asexual organisms. Fortunately, many animals have been able to overcome these challenges in order to continue their existence. 

Asexual reproduction

Asexual reproduction involves a single organism producing genetically identical offspring without the fusion of gametes from two parents. The ability to reproduce without a partner is advantageous in areas where populations may be low or the organism is isolated from its own species. Asexual reproduction is rapid and requires low amounts of energy, allowing organisms to quickly produce large quantities of offspring. This helps to prevent intruders and competition from taking over an area. Unfortunately, excess reproduction can cause overcrowding and competition for food within a species. Adapting to changes in the environment can be very difficult since genetic diversity does not exist among asexual species.

Budding, regeneration, and parthenogenesis are the three major types of asexual reproduction among animals.

Budding occurs when cells undergo repeated mitosis to form an outgrowth or “bud.” This portion of the parent develops into identical offspring that can detach. Cells will undergo mitosis and form certain structures before the individual breaks loose. In the animal kingdom, budding can be seen in certain species of cnidarians such as hydra. 

Some animals can reproduce by way of regeneration, the reproduction of a complete organism from fragments of the parent body. When a starfish loses an arm due to injury or predation, it can result in the growth of a new and functional organism. 

The asexual process of parthenogenesis is the development of offspring from a female’s unfertilized egg. Invertebrates including some rotifers, insects, and crustaceans may reproduce parthenogenically depending on the time of year or conditions. Typically, organisms that can perform parthenogenesis also reproduce sexually. Asexual reproduction can be advantageous to species that have to remain in an environment where there is a low population of mates. Philodina are a rare example of animals that reproduce exclusively by parthenogenesis.

1. Some animals reproduce both sexually and asexually, like Daphnia pulex, the most common species of water flea. Under what conditions would asexual reproduction be favorable?
2. Although asexual reproduction is generally simpler and more efficient, most species in the animal kingdom reproduce sexually. What could be the advantage that has allowed for the evolution of sexual reproduction?
3. Complete the following Venn diagram with similarities and differences between asexual and sexual reproduction.
   
   
   
   
4. Assign the correct form of reproduction (sexual, budding, regeneration, or parthenogenesis) to the following examples.
   
   
   1. Planaria, a genus of flatworms, can become 2 fully functioning individuals when the parent has been bisected.
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   2. Jellyfish eggs can attach to surfaces where they grow into polyps. Polyps form into branches that have identical DNA from the original and then can detach as a new organism.
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   3. A queen honeybee will lay hundreds of thousands of eggs but only some are fertilized. The fertilized eggs become diploid female workers or queens, and the unfertilized become haploid male drones.
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   4. Many species of penguins will mate with the same partner each season, forming lifelong partnerships. Some have a specific mating season while others can create offspring 2 or 3 times per year.
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   5. Aphids are typically pests of crops. As winter approaches, males and females mate to produce fertilized eggs. In the spring, the eggs will hatch, releasing females without wings. These females can produce several generations of aphids without fertilization from males. The following winter, when it becomes necessary to move to a new plant, males and females will once again breed offspring with wings (2 types).
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