Lab Report: Phenotypic and Genetic Variation in Brassica rapa - Measuring Trichome Density, Exercises of Evolutionary biology

Download Lab Report: Phenotypic and Genetic Variation in Brassica rapa - Measuring Trichome Density and more Exercises Evolutionary biology in PDF only on Docsity! 1 Phenotypic and Genetic Variation in Rapid Cycling Brassica Parts I & II Overview: The objective of this lab is to reacquaint you with the concept of variation in a population. During this lab you will characterize a population of Brassica rapa with respect to leaf trichome density using basic descriptive statistics that describe variability. These descriptive techniques are traditionally applied to phenotypic traits. You will also estimate the genetic variance of our Brassica population as well, but this will require a second generation of plants. The goal of the first lab is to sow the seeds of the first generation, or the Parentals. The response of a population to natural selection depends upon two important factors: the strength of selection (S) and the heritability of the character under selection. This relationship was concisely described by R.A. Fisher as: R = h2S. This is also known as the Breeder’s equation. R = response to selection (The amount that the mean trait in a population changes after selection. This is the difference in the value of the mean trait between the Parental and the offspring.) h2 = heritability = Vg/Vp (The ratio of genetic variation to total phenotypic variation. The total phenotypic variation must be considered, since some of that variation could be environmental variation (which is also genetically determined, but we won’t go that far with it). Vg = Genotypic variation Vp = Phenotypic variation S = Selection differential (The difference of the means from the original population and the population of selected breeders.) During the next 4 labs, you will explore this relationship, and indeed, test it by measuring the variables that constitute this equation. After your data has been collected, you will prepare a lab report that summarizes the experiment and addresses whether the Breeder’s equation support by your data. The System: We will use one of the genetic stocks from the well-known laboratory rat of the plant world, Wisconsin Fast Plants TM. The workhorse of the Fast Plant industry is Brassica rapa, or field mustard, which comes to us from the mountainsides of Nepal. Mustards in general are weedy, and many species are considered invasive throughout much of the temperate parts of the world. You know them from their cultivated cousins in the same genus, Brassica, as broccoli, cabbage, cauliflower, brussel sprouts, and of course the spice, mustard. You might recall from Ecology class that the distinctive odor and flavor of the cabbage family is due to a class of compounds called glucosinolates, or mustard glycosides. These compounds effectively deter all insect herbivores but the specialized cabbage moths and cabbage loopers, among a few others. In addition to these chemical defenses, some Brassica have physical defenses on their leaves called trichomes. Trichomes are outgrowths of the leaf epidermis that take the form of a hair or scale. On mustard leaves, these will be in the form of short, spiny hairs that have been demonstrated to deter leaf-eating caterpillars. Lab 1 – We will plant seeds of Brassica rapa. Docsity.com 2 Lab 2 – We will measure trichome density on each plant and use descriptive statistics to describe the variation in trichome density. Recall that the ability of a population to respond to natural selection depends upon the following four critical elements (with a bit of help from Darwin): 1. Variation in the population – what type or variation is necessary for natural selection to operate? 2. Heritability – traits possessed by the parents are passed on to the offspring. 3. Struggle for existence – variability in traits may generate differential survival. 4. Differential reproduction – not all individuals produce the same number of offspring. What you will measure: For each plant you will count the number of trichomes on the right edge (leaf margin), excluding the petiole, of the first true leaf (above the cotyledons). On this leaf you would count 6 trichomes Trichome density does change as the plants age; leaves produced early in the life of a plant produce more trichomes than those produced later. (Why should the first leaves produced be more protected than leaves produced later?) But!, once a leaf is formed, the number of trichomes does not change (unless they break off). By adopting a uniform convention of trichome measurement, the data can be pooled. Remember, there is greater statistical power with a larger sample size. Docsity.com