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Polyploidy in Plants & Animals: Speciation & Cells

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  • 0:00 Polyploidy
  • 0:43 Triploids
  • 1:43 Polyploidy in Plants
  • 2:19 Polyploidy in Animals
  • 4:04 Speciation
  • 4:57 Lesson Summary
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Lesson Transcript
Instructor: Amanda Robb
In this lesson, we'll go over what polyploidy is and what cells look like during polyploidy. We'll also cover specific examples of speciation in plants and animals.

Polyploidy

You probably know that you were made from your mom and dad. You got 23 chromosomes from your mom, and 23 from your dad. Any organism that gets more than half from mom and half from dad would die. . . or would they?

Normally, humans have two sets of chromosomes, one from your mom and one from your dad. We call this diploid, since di- means two and -ploid means having to do with chromosomes, or pieces of DNA.

Think of it like a deck of cards. Each deck has four suits, so a normal deck of cards would have four sets of chromosomes. We are diploid, so our bodies only get two suits of cards. Any organism with more than two sets of chromosomes is called polyploid.

Triploids

Before we understand polyploidy, we need to understand a little about how our bodies make new cells, particularly sperm and eggs, or gametes. All of our other cells are diploid, so when gametes are made they need to be haploid, or have only one set of chromosomes, so when the sperm and egg fuse, the new organism can be diploid again.

However, during this process, sometimes something goes wrong. The most common occurrence is when one new gamete gets two copies of the chromosomes. This can happen when females make eggs, or when males make sperm. When an egg with two sets of chromosomes fuses with a normal haploid sperm, the resulting cell has three sets of chromosomes, or is triploid.

Now, every cell in that new organism will be triploid. For most animals, this is extremely detrimental and the organism won't survive. Plants tend to tolerate polyploidy better, and even flourish with such intense genetic changes. Let's look at some examples of polyploidy in plants and animals.

Polyploidy in Plants

Scientists have hypothesized that two thirds of flowering plants are polyploids. Most ferns and grasses are polyploids, and even crops that we enjoy everyday, such as potatoes, apples, sweet potatoes and strawberries.

Bananas make for an interesting example. Bananas are triploids, and typically triploid organisms do not make the sperm or eggs needed to reproduce, making them effectively sterile. That means you can't get banana seeds to make more bananas. Farmers cut off shoots from the side of the plant before it produces fruit and dies, and propagate the bananas asexually.

Polyploidy in Animals

There are many fewer species of polyploid animals than plants. The exact reason for this is not entirely known. Some scientists think that it might have to do with the increased complexity of animal body plans compared to plants. Others suggest polyploidy may interfere with gamete formation, cell division, or regulation of the genome. However, there are some exceptions in fish, reptiles, and insects.

A specific example of polyploidy in action is the triploid trout. In Idaho, the cutthroat trout is the state fish, but other fish, like the rainbow trout were introduced to the area. Conservationists wanted to prevent the rainbow trout from breeding with the cutthroat trout, to preserve their species. The solution? Create a triploid rainbow trout that can't reproduce using polyploidy.

So how did scientists manage this? It turns out that fish reproduce slightly differently than humans. Eggs start as diploid, and sperm as haploid. When the cells fuse, the new cell kicks out the third set of chromosomes, so the new fish can be diploid. However, if the last set of chromosomes isn't kicked out, the new fish will become triploid, and unable to reproduce. Scientists took advantage of this by exposing the fish eggs to warm water in a hatchery, preventing the extra chromosomes from being kicked out. The now triploid and sterile fish could then be introduced to the river without fear of them mating with the local cutthroat population.

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