Heterosis in agriculture

Published on December 1, 2013   53 min

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Other Talks in the Series: Agricultural Genetics

0:00
Hello. My name is Nathan Springer. I am a professor in the Department of Plant Biology at the University of Minnesota. My lab studies maize genetics. And we have done some research on the phenomena of heterosis. Today, I will be talking about heterosis in agriculture.
0:20
It is useful to begin with an outline of what I will talk about today. First, I want to discuss what is heterosis, what is the meaning of this word, and what are we describing when we use this word? Second, what is the genetic basis of heterosis? Heterosis is a complex phenomenon. And it would be useful to better understand the genetic processes that might contribute to heterosis. Third, what are the implications of heterosis for agriculture? Heterosis is widely used in agricultural systems throughout the world. And it has some strong implications for both how seed is produced to go into fields, as well as the yield and the types of plants that are utilized.
1:06
So what is heterosis? Heterosis refers to the phenomenon in which hybrid offspring exhibit characteristics that lie outside the range of the parents. This was initially described in 1908 by George Shull. You'll often hear the term hybrid vigor and heterosis used interchangeably. Heterosis is a phenomena that was formally described by George Shull when inbred lines were crossed. But this phenomena can be observed any time two related individuals are crossed or two unrelated individuals are crossed. The two pictures on this page show examples of heterosis in maize. The picture on the left shows four ears of corn. The ears of corn on the far left and far right of this image are from the inbred lines B73 and Missouri 17. These are two commonly used lines of corn. The ears in the middle are F1s. They are the result of crossing together B73 and Missouri 17. You'll notice that the F1 ears are substantially larger and have more seeds than either parent. Similarly, you can see hybrid vigor for plant growth characteristics in the picture on the right. Once again, the two parents are shown growing on the two edges of this image, and the rows in the middle are F1s. You'll see that the F1s have grown faster and result in more vigorous plants then either parent.
2:36
This image illustrates the importance of heterosis in yield. On this graph, we're looking at the average corn yields in kilograms per hectare for the period of time from 1865 until the early 2000s. What you'll notice is that early in this process, from about 1865 to the mid-1920s, corn yields did not grow up on a yearly basis. Instead, they stayed relatively flat. However, in the early 1930s, double-cross hybrids and heterosis started to be implemented. Later, we'll discuss the difference between a double-cross and a single-cross hybrid. But as you notice, they're highlighted by the red arrows. The advent of using double-cross with single-cross hybrids led to a major gain in the amount of yield per unit area. This image and a description of the process of improving corn yields comes from Forest Troyer's review in 2006, titled "Adaptedness and heterosis in corn and mule hybrids." It is worth pointing out that this increasing yield has been necessary to continue to provide enough caloric intake for the world's growing population. Whether we're able to continue to increase our productivity in agriculture will be an important determinant in our ability to continue to feed the world.
4:00
Heterosis plays a very prevalent role in agriculture. For some species, such as maize, virtually all of the acreage planted in the developed world is now planted using hybrid seed. There are other species that traditionally have been grown as single lines. But the acreage is shifting more and more towards hybrids. For example, this plot shows the area and percent of acres that are grown using hybrid rice. In many systems, the prevalence of hybrids is dictated by the ease of making hybrid seed, as well as the gain in yield provided by hybrids relative to inbred varieties.
4:42
The observation and quantification of heterosis is most readily observed when crossing two pure-breeding homozygous lines. This is because heterosis is distinct from the concepts of segregation and transgressive variation. In order to explore these concepts, let's consider the structure of the genome in different generations. We're going to use red and blue colors to illustrate the different types of chromosomes. If we have two parental lines that are both homozygous, these parental lines can be crossed together to generate an F1. The F1 generation will be heterozygous at all positions in the genome in which the two parents vary. If an F1 plant is self-pollinated or crossed to another F1 plant, this will result in F2 plants. F2 plants will have portions of their genome that's homozygous for one parent or the other and portions of their genome that remain heterozygous. If F2 plants are selfed for several generations, they can result in a recombinant inbred line. A recombinant inbred line will be homozygous throughout the genome and will have a shuffling of the chromosomal segments that were present in parent 1 and parent 2. Now, let's look at the phenotypes that these types of lines display. If we look at the picture of the plants, we can see the F1 plant, which shows heterosis relative to the two parents, B73 and Missouri 17. However, transgressive segregation can be illustrated by looking at the recombinant inbred lines. Here we show examples of several families of recombinant inbred lines that have short plants, intermediate plants, or tall plants. It's worth noting that some of these recombinant inbred lines have phenotypes that are shorter than either parent or taller than either parent. This is likely because of segregation for alleles for tall and short that were present in the two parents. In the plot on the right, you'll see a bell curve showing the population of plants. So the two parents have similar heights, just under 240 centimeters. The recombinant inbred lines show a distribution of height, so some individuals taller and some shorter. But much of the population has a height similar to that of the two parents. The F2 plants have a similar distribution. But it is shifted slightly to the right for taller plants. This is because in the F2 plants, not only do we have segregation, but part of the genome was still heterozygous. And, therefore, we have both heterosis and segregation for alleles.
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Heterosis in agriculture

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