Exercise 8.1

Geographic Variation for Flowering Time in a Species of Wheat

(This exercise is based on Matsuoka Y., S. Takumi, and T. Kawahara. 2008. Flowering time diversification and dispersal in central Eurasian wild wheat Aegilops tauschii Coss.: Genealogical and Ecological Framework. PLoS ONE 3(9): e3138.)

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INTRODUCTION

The timing of flowering in angiosperms (flowering plants) is an important reproductive trait that often has marked fitness consequences. Individuals that flower either earlier or later than the optimum period may face much lower reproductive success. Within a species, variation in flowering time usually involves both genetic and environmental inputs. Adaptation to local environmental conditions may lead to the evolution of geographic clines, wherein flowering times vary as a function of latitude, longitude, and/or altitude.

Yoshihiro Matsuoka at Fukui Prefectural University in Japan and colleagues examined geographic variation in Aegilops tauschii, a species of wild wheat whose range extends from southeastern Europe through the Middle East and much of Asia. Matsuoka and his associates took plants (accessions) from across the range and grew them in common greenhouses. They then determined the flowering time for each of the accessions.

QUESTIONS

Question 1. What is gained from the researchers growing the plants in a common setting (the greenhouses) instead of growing each accession in its native environment?

Use the information in Figure 1 to answer questions 2 through 6.

Figure 1 The locations, lineages, and flowering time phenotypes of the accessions. The researchers also genotyped the chloroplasts of each accession and found that the plants clustered into four different lineages (HG7, HG9, HG16, HG 17). Early-flowering plants are in red, intermediate-flowering plants are in green, and late-flowering plants are in blue. Circles, triangles, stars, and asterisks denote, respectively, accessions of the HG7 lineage, HG9 lineage, HG16 lineage, and HG17 lineage. The dashed yellow line indicates the southern limit of the temperate desert vegetation zone at the Last Glacial Maximum.

Question 2. What is the predominant flowering phenotype from accessions that come from near the southern edge of the Caspian Sea?

Question 3 What are the geographic range and the phenotype of accessions of the HG17 lineage?

Question 4. At the time of the Last Glacial Maximum, much of northern Asia was in the temperate desert vegetation zone. Is late flowering common or rare in the plants that currently live south of this zone?

Question 5. Based on this map, what latitudinal and longitudinal trends seem most likely?

Question 6. Is the HG16 lineage more common in the east or in the west?

Use the information in Figure 2 to answer questions 7 through 10.

Figure 2 The relationship between flowering time for latitude and longitude.

Question 7. What clines are observed?

Question 8. Based on the regression line, for every degree increase in latitude, what is the expected change in the flowering time?

Question 9. Based on the regression line, for every degree increase in longitude, what is the expected change in the flowering time?

Question 10. Suppose the latitudinal cline is due to temperature acting as the selective force. What do you expect would happen to the mean flowering time, given global warming? Why?