Erica (Zhijun) Lei
BI164 Lab Report 1
Due Thursday, 8 March 2018
1
Effects of Artificial Selection on Traits of Brassica rapa
Abstract
The purpose of the paper was to determine if there has been a positive shift in the value of the trichome in Brassica rapa plants and a change in other unrelated traits across generations. We counted the trichome number on the petiole of the first true leaf of all the plants in the base population (BP), took the top 10% with the highest trichome number and had bred them with artificial selection 1st round (AS1) plants. We then repeated the process to produce AS2 plants. From BP to AS2, the trichome number and the bud number increased, while there was no significant change in leaf numbers. It is suggested that future researchers could look into genotypes rather than only morphology to study if evolution occurred.
Introduction
To obtain certain features of the plant for either production or consumption, humans select plants according to their desired characteristics, and then only use them to breed. People find the selected plants tend to pass the similar features onto their progeny, and the desired traits would be even more prominent. The whole process is known as the artificial selection.
In the market, we often see the family of Brassicaceae, whose cultivated varieties mature rapidly. Brassica rapa has been used extensively in biology to study the process of artificial selection. In this experiment, we would conduct an artificial selection of B. rapa and observe how plant physiological features change over time. Our purpose is to examine whether we can alter the phenotype and genotype of a plant through artificial selection, and at the same time, find out if other unrelated traits change across generations as well.
It was hypothesized that our experiment provided evidence for successful artificial selection of trichome number in Brassica rapa and that other traits were also affected by this selection. Specifically, true leaf number and bud number will both decrease.
Methods
We were provided with 10 of an initial base population (BP) of a rapid-cycling cultivar of B. rapa which had no known artificial selection for trichome number previously. Because counting every trichome number on the plant is time-consuming, and the hairiness of one part of a plant is positively correlated with hairiness on every part, we counted the trichome on the petiole of the first true leaf. Next, the top 10% of the BP generation expressing an extreme count of trichome were selected to breed the next generation. These BP plants generated B. rapa Artificial Selection 1st Round, abbreviated as AS1. We discarded the remaining 90% of BP plants. Repeating the same selection and breeding process, we then had the B. rapa AS2 generation. After recording the trichome number of AS2 plants, we identified the top 10% hairiest AS2 cultivar plants to breed seeds for AS3 generation for future experiments. The rest were disposed. To determine if selection of trichome number also affected other traits, we chose to count the number of leaves and number of buds on all BP and AS2 plants and recorded the data.
All plant individuals used in this experiment were cultivated in the same greenhouse, at 23 Celsius degree, using the same potting soil, the same feeding regime, and under constant light of 200 mE.
Finally, we used R to compute statistical data, and we also used R to perform a Wilcoxon rank sum test for each variable to see if there is a significant difference in any of the traits between BP and AS2 samples.
Results
Trichome Number
Figure 1: Boxplots of trichome number on the petiole of the first true leaf of AS2 plants and BP plants (nAS2 = 158, nBP=159).
Out of 159 BP plants and 158 AS2 plants, we observe that the trichome number of BP has a maximum of 31, a median of 1 and a minimum of 0. The trichome number of the AS2 B. rapa has a maximum of 53, a median of 15 and a minimum of 0.
Figure 2: Histograms of trichome number on the petiole of the first true leaf of AS2 plants and BP plants (nAS2 = 158, nBP=159).
The Wilcoxon test result suggests that the trichome number of B. rapa AS2 is significantly higher than the trichome number of B. rapa BP (W = 21980, p-value < 0.001).
True Leaf Number
Figure 3: Boxplot of true leaf number of the AS2 samples and the BP samples (nAS2 = 158, nBP=159).
Since we aimed to find if any other traits were inadvertently altered over the course of artificial selection, we recorded the number of true leaves on each plant in the BP and the AS2. In the BP, the maximum number of true leaf on one B. rapa is 10; the median is 5; the minimum is 2. In the AS2, the maximum number of true leaf on one B. rapa 8; the median is 4; the minimum is 2.
Figure 4: Histograms of trichome number on the petiole of the first true leaf of the AS2 samples and the BP samples (nAS2 = 158, nBP=159).
The Wilcoxson test resulted in no significant difference in true leaf numbers of two groups (W = 11873, p-value=0.3811).
Bud Number
Figure 5: Boxplot of bud number of the AS2 samples and the BP samples (nAS2 = 158, nBP=159).
The other trait we investigated on was bud number. For the BP, the observed maximum value is 36, the median is 18, and the minimum is 0. Whereas for the AS2, the values have a maximum of 43, a median of 21, and a minimum of 2.
Figure 6: Histograms of bud number of the AS2 samples and the BP samples (nAS2 = 158, nBP=159).
After running the Wilcoxson test, we interpret the result as that there is a significant difference between the bud number in AS2 plants and BP plants, so the AS2 B. rapa has a greater number of buds than the BP B. rapa (W = 15710, p-value = 0.0001105)
Discussion
The trichome number and the bud number of the AS2 population were higher than the trichome number and the bud number of the BP, whereas the true leaf number is not significantly different between two populations. The evidence is partly consistent with our hypothesis, and we failed to predict the change of true leaf numbers and bud numbers.
The median trichome number of the BP was 1, whereas for AS2 it was 15. A previous study about the artificial selection of B. rapa also produced a similar pattern, in which the trichome number of B. rapa in the BP was 20.8 ± 13.4, and the trichome number increased to 93.9 ± 28.7 in the final generation of artificial selection (Ågren and Schemske 1992). Furthermore, their study showed that the increase of trichome number of B.rapa is genetically controlled and could be significantly influenced by artificial selection.
Inconsistent with our hypothesis, the number of true leaves did not change over time. A possible explanation may be that the change in leaf number is too small within two generations. It could also be the case that a certain type of allele that controls the leaf number of B. rapa is majorly dominant and hard to be influenced by the change in other traits such as trichome number. Another possibility may be that the leaf-number controlling allele is located at a different chromosome from where the trichome-number controlling allele is located. Consequently, the expression in leaf number does not change simultaneously with the increasing trichome number. In a study involving three B. rapa genotypes growing under greenhouse conditions for 4 weeks, even though at the end, the leaf areas, shapes, patterns and margins all had some degrees of variation, the leaf number remained similar in three genotypes (Zanewich et al. 1990).
The bud number yielded an unintended increase during the artificial selection process, which is also inconsistent with our hypothesis. However, this increase seems contradictory to the principle of resource allocating, the idea that individuals have limited resources for functions of reproduction, growth, and defense, so there should be a trade-off relationship among these functions (Bazzaz et al. 1987). Thus, in our case, since the trichome number increased, the defense functions were strengthened, so the reproduction functions expressed by bud number should be weakened. Nonetheless, because leaves, stem growth, and other support structures should all be taken into account this dynamic balance of apparatus, the increase of bud number did not violate the principle of resource allocating. One possibility is that the AS2 generation alters how plants produce hormones and sugars, henceforth changing endogenous signaling molecules that are mediating the transition from vegetative to the reproductive phase (Galvão and Schmid 2014).
We do not have enough evidence to presume that evolution occurred in our experiment. A different frequency of specific phenotypes between generations cannot indicate how the allele frequency changes, whereas we have to track how allele frequency changes to determine if the evolution occurred. To study evolution, future studies need to know the genotypes of each generation and may need to look at other variables of plant physiology to determine the allele that controls trichome number fits which genetic model.
References
Ågren, J., and D. W. Schemske. 1992. Artificial selection on trichome number in Brassica rapa. Theoretical and Applied Genetics 83:673–678.
Bazzaz, F. A., N. R. Chiariello, P. D. Coley, and L. F. Pitelka. 1987. Allocating Resources to Reproduction and Defense. BioScience 37:58–67.
Galvão, V. C., and M. Schmid. 2014. Regulation of Flowering by Endogenous Signals. Advances in Botanical Research 72:63–102.
Zanewich, K. P., S. B. Rood, and P. H. Williams. 1990. Growth and development of Brassica genotypes differing in endogenous gibberellin content. I. Leaf and reproductive development. Physiologia Plantarum 79:673–678.