Hi, and welcome to this review of asexual reproduction! In this video, we’ll go over the four main forms and see what organisms reproduce using these methods.
Asexual reproduction means “not sexual” so organisms that reproduce this way don’t require a mate or sex cells to reproduce. Asexual reproduction involves only one parent organism, so the offspring that come from that parent will be genetically identical to the parent and all the other offspring that organism produces. Along with not requiring any gametes to fuse, asexual reproduction does not require fertilization or the mixing of genetic material, and meiosis isn’t necessary, which seems to indicate that asexual reproduction saves time and energy! That sounds pretty good, right? It’s generally a lot more simplified and resource-conserving than sexual reproduction, so why don’t more organisms reproduce this way?
For one thing, asexual reproduction doesn’t allow for any genetic variation. If an asexual organism can only produce clones of itself and environmental changes or a disease threatens their existence, all of the population may be wiped out, leading to extinction of that particular organism. In less extreme cases, no genetic variation means less diversity, which means the organism isn’t able to adapt or evolve over time. However, there is one way that an asexual species can adapt. Whether a species is sexual or asexual, no one’s safe from genetic mutations; but remember that not all mutations are disease-causing. So if an asexual organism incurs a mutation during the replication of its DNA, it might benefit the species and actually help an asexual organism to adapt to its environment after all. Because of this, it’s better to say that organisms that reproduce asexually adapt to their environment at a much slower rate than organisms that reproduce otherwise.
There are certain advantages we can’t overlook. As a whole, reproducing asexually uses a lot less energy. The process of cloning oneself is a lot simpler than the process of fertilization. During fertilization, genetic information from each parent crosses over and combines to form offspring that continue to grow or gestate for a varying amount of time (depending on the organism). Asexual organisms also don’t have to waste precious time trying to find a mate. With all of this free time, the population can increase more rapidly and establish themselves in their habitat much faster.
The actual process of asexual reproduction depends on the organism. Binary fission occurs when a unicellular organism splits into two pieces to create two new organisms. This process is common in lots of prokaryotes like bacteria, but the poster child for binary fission is actually a eukaryotic unicellular organism–an amoeba. When amoebas undergo binary fission, the DNA that is contained within the nucleus is simultaneously replicated while the cell begins dividing. This is true for any organism that undergoes binary fission.
Another very similar process to binary fission is a process called budding. Budding occurs when the parent organism produces a small bud at one particular site that eventually detaches and becomes its offspring. Some plants and simple animals reproduce this way. For example, freshwater hydra will begin replicating cells in a concentrated area on the parent’s body creating a protrusion that will bud off and become an individual. While hydra are multicellular organisms, budding can also be utilized by some unicellular organisms, such as yeast. The parent yeast cell will extend its cytoplasm out to form a bud and then replicate its DNA to be inserted into the budding offspring. Once this happens the cell will undergo cytokinesis to break the newly formed bud free from the parent. Now the offspring can continue to grow until it decides to reproduce this way.
At this point you might be wondering, “What’s the difference between binary fission and budding?” In binary fission, one organism will divide itself equally to form two daughter organisms causing the parent’s identity to become lost. One cell will divide itself equally to form two offspring. However, during budding, one cell will produce a smaller outgrowth at one particular site. That bud alone is considered the offspring while the original cell is called the parent.
Multicellular organisms aren’t limited to budding alone. Another process some organisms use is called fragmentation. During fragmentation, a piece of the parent organism will detach and form the offspring. Let’s look at a couple of examples. If a sea star gets into a tussle and loses one of his five arms, that severed fragment can, under certain circumstances, subdivide its cells and form a whole new sea star. Fragmentation is a lot like budding in this way, but there are a couple of important differences. Budding involves a new organism being produced from an outgrowth on the parent, while fragmentation involves a portion of the parent’s body breaking off to form one or more organisms. Remember that fragmentation is only for living things that are multicellular whereas budding can occur within unicellular organisms, like yeast, or multicellular organisms, like hydra.
The last process we’ll talk about today might be the most interesting one of them all–parthenogenesis. Biologists recently discovered that some organisms like the komodo dragon can reproduce sexually and asexually depending on the state of the environment. Female komodo dragons are known to mate with males to produce their offspring, but researchers have recently discovered that females can produce an embryo that develops from an unfertilized cell. So here’s what happens. Under normal conditions, females will produce haploid eggs, meaning they contain only half of the genetic information they need until they’re fertilized by a male. But under environmental stress or a decreasing population size that limits the number of available mating males, the females will produce a fully viable diploid egg by cloning the available chromosome, which means the diploid egg has two complete sets of chromosomes. For komodo dragons, females have one W and one Z chromosome (WZ) while the males have two Zs (ZZ). When the female goes through parthenogenesis she has eggs that are either W or Z, so she will clone the chromosome in each egg so that they have the genotype WW or ZZ. WW is not viable, so only the ZZ male genotype survives. The coolest part is that entering parthenogenesis is not permanent–the female can go back to reproducing sexually if she finds a mate or if the environmental stress is removed.
Alright, now that we’ve covered everything, here are a couple of review questions to test your knowledge.
Which processes allow for unicellular asexual reproduction?
- Binary Fission
The correct answers are A and B. Both binary fission and budding allow for certain unicellular organisms to reproduce. Examples of each include bacteria and yeast, respectively.
Which processes allow for multicellular asexual reproduction?
- Binary Fission
The correct answers are B, C, and D. Some multicellular organisms can reproduce via budding. In this video our example was hydra. As for fragmentation and parthenogenesis, all both methods allow for specific multicellular organisms to asexually reproduce this way.
I hope this review was helpful! Thanks for watching, and happy studying!