Exercise 15.1

Want a Big Tomato? Timing Matters!

(This exercise is based on Cong, B., J. Liu, and S. D. Tanksley. 2002. Natural alleles at a tomato fruit size quantitative trait locus differ by heterochronic regulatory mutations. PNAS 99: 13606–13611.)

(Note: The reference above links directly to the article on the journal’s website. In order to access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)

INTRODUCTION

Evolutionary biologists have long been fascinated with the domestication of animals and plants, as these provide striking examples of how human-induced selection (conscious or unconscious) can lead to dramatic changes in the target organisms. Darwin, who was a pigeon breeder, started his The Origin of Species with examples of change induced by such artificial selection, using this type of change as a metaphor for what natural selection could achieve over a much longer extent of time. After completing The Origin, Darwin wrote a two-volume treatise on changes in animals and plants that were produced under domestication.

In recent years, evolutionary geneticists have uncovered several of the actual genes that were involved in the domestication of animals and plants. One such gene is the fw2.2 locus in tomatoes (Lycopersicon esculentum); the allele found in most domesticated varieties causes the fruit of the tomato to be larger in comparison to those with the allele of the wild progenitor. Previous studies in Stephen Tanksley’s lab at Cornell University show that the large-fruit allele is responsible for about a 30% increase in fruit size over that in plants with the small-fruit allele of fw2.2 but with the same genetic background.

By what mechanisms does this genetic difference lead to changes in the sizes of tomatoes? Although the DNA sequence differs between the large-fruit and the small-fruit alleles, the differences are not in the protein-coding regions of the gene. Instead, the differences are likely in regulatory regions upstream of the coding sequence. Moreover, biologists have postulated that regulatory changes often underlie large morphological changes, especially those that occur relatively rapidly. For these reasons, Bin Cong and other members of Tanksley’s lab investigated whether alleles of fw2.2 were expressed differently.

Cong and his colleagues had available Near Isogenic Lines (NIL) that differed only with respect to the allele at the fw2.2 locus: one line carried the large-fruit allele and the other the small-fruit allele. Before looking at the gene expression of this allele, let’s examine the effects of this allelic difference on phenotypic traits.

QUESTIONS

Figure 1 The effects of the fw2.2 locus on various traits. The dark line shows the results for the line with the “large-fruit” allele and the red line shows that for the “small-fruit” allele. Panel A shows the weight of the fruit in greenhouse conditions, B shows the fruit weight in field conditions, C and D shows the fruit length and width respective (in greenhouse conditions), E shows the fruit shape index (greenhouse), F shows the thickness of the pericarp, G shows the number of cell layers in the pericarp, and H shows the area of the placental tissue. The x-axis is days post anthesis, the full opening of the flower. (Cong, B., Liu, J., and S. D. Tanksley. 2002 Natural alleles at a tomato fruit size quantitative trait locus differ by heterochronic regulatory mutations. PNAS 99: 13606—13611. doi:10.1073/pnas.172520999 — http://www.pnas.org/content/99/21/13606.full © [2002] National Academy of Sciences, U.S.A.)

 

Question 1. Do the large-fruit lines achieve larger-sized fruit in both the greenhouse and in the field? If so, under what condition is the difference bigger? If not, under what condition is the size difference seen?

 

Question 2. What is the average weight of a large-fruit tomato fruit grown in the field after 30 days post anthesis?

 

Question 3. Does the allelic difference at fw2.2 influence both fruit length and fruit width, just fruit length, or just fruit width? Does this allelic difference affect fruit shape? If so, in what way?

 

Question 4. Does the allelic difference affect the number of cell layers?

 

Question 5. Does the allelic difference affect the area of the placental tissue? If so, in what way?

Figure 2 The expression pattern of the fw2.2 gene over time (days post anthesis). Values for the large-fruit and small-fruit lines are in the light-colored and dark-colored bars, respectively. (Cong, B., Liu, J., and S. D. Tanksley. 2002 Natural alleles at a tomato fruit size quantitative trait locus differ by heterochronic regulatory mutations. PNAS 99: 13606—13611. doi:10.1073/pnas.172520999 — http://www.pnas.org/content/99/21/13606.full © [2002] National Academy of Sciences, U.S.A.)

 

Question 6. At 4 days post anthesis, in which line is the fw2.2 expression higher? At 10 days post anthesis?

 

Question 7. Describe the pattern of difference in gene expression between the large-fruit and the small-fruit lines?