Reproduction in Animals: Sexual or Asexual? Help

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By — McGraw-Hill Professional
Updated on Aug 30, 2011

Introduction to Reproduction in Animals: Sexual or Asexual?

Sexual reproduction occurs in animal organisms and in non-animal organisms such as in the World of Plants. Even in reproducing pine trees, there are sperm cells from the male (pollen) cones that fertilize egg cells (ova) from the female (seed) cones, thereby resulting in a zygote. The lowly fungi were likewise shown to engage in sexual reproduction – the fusion of a male and female gamete (sex cell) to create a zygote. And this zygote in turn develops into an embryo, within many different types of organisms.

Asexual Reproduction In Animals

But we also need to remember that asexual reproduction is likewise common! In this reproductive strategy, there are no gametes or sex cells involved. Asexual reproduction, you may remember, is important in the spreading of the black bread mold, Rhizopus. (Review Figure 8.4, if desired.)

Certain animals (living organisms that are neither plants nor fungi) also use asexual means to reproduce themselves. Not surprisingly, the animals engaging in asexual reproduction are generally among the most primitive. Consider, for example, the invertebrates. Certain types of starfish reproduce asexually by means of fragmentation (breaking of their body into fragments), followed by regeneration (the re-growing of lost body parts). When such a starfish has one of its arms fragmented (broken off), an extensive series of mitoses (my- TOH -seez) or cell divisions occur within the broken arm fragment. One such broken arm can thus asexually reproduce (via mitosis) a whole new starfish!

Sea anemones can asexually reproduce by means of fission ( FIH -shun) – “the process of” (-ion) “splitting or cleaving” (fiss). In this case, one approximately round sea anemone stretches into two identical individuals, creating a type of dumbbell-shaped pattern. The two duplicate individual sea anemones then split or cleave apart.

Finally, some invertebrates such as tunicates asexually reproduce by budding. A new individual simply grows as a small bud off the body of a parent. When the growing bud becomes large enough, it separates from the parent body.

Basically, all of the above examples of asexual reproduction have two main features in common:

1. The major mechanism for reproduction is simply an extensive series of mitoses (cell divisions involving no reduction in the number of chromosomes present).

2. A group of clones ( KLOHNS ) – identical copies of a single parent organism – is created. Like a branch or “twig” ( clon ) growing from the trunk of the same tree, a clone has an exact copy of the genes and chromosomes of its parent.

External Fertilization: Sexual Reproduction In Lower Vertebrates

In animals engaging in sexual reproduction, the mature ovum of the female is fertilized by a sperm cell from the male. In certain types of lower (more primitive) vertebrates, such as fish and amphibians, there is external fertilization of the ovum “outside of” the body of the female.

Consider two mating frogs. The larger female frog is mounted by a smaller male, who clasps her body with his forelimbs. This helps stimulate the female to release a mass of over 100 eggs into the pond water! Nearly simultaneously, the male frog releases a jet of sperm cells, which fertilize many of the ova externally, right in the surrounding water. After a time of development, the fertilized ova develop into swimming tadpoles.

Internal Fertilization: Sexual Reproduction In Higher Vertebrates

When one examines the mating of higher vertebrates such as mammals, sexual reproduction generally occurs via internal reproduction. In this type of reprodution, the ovum is fertilized by mature sperm cells that have traveled “within” (intern) the female body.

Practice problems for these concepts can be found at:  Urine and Sex in Animals Test

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