Analysis of Phenotypic Variation in Pillbugs (Armadillidium vulgare)

1. Lab Overview and Background

Collecting data from living organisms, animals in particular, can be quite challenging, but often a necessity in many fields of biology. In this lab, you will practice measuring different types of traits in live pillbugs (Armadillidium vulgare) with the intention of addressing the following questions:

Is there variability in pillbug traits? Is the amount of variability consistent between traits?

As you will learn shortly, some traits have little to no variation whereas others have a great deal of variability. In evolution and ecology fields, we are concerned with variation for numerous reasons, one of them being changing environments.

1.1 What is phenotypic variation?

Each organism is characterized by a set of phenotypic traits. Those observable traits include the anatomical, physiological, biochemical, and behavioural features of the organism. Phenotypes are the result of the interaction between an organism’s genetic makeup (genotype) and its environment. Some phenotypes will directly impact whether an individual is likely to survive and reproduce.

Depending on the trait, genotype and environment have differing degrees of influence on the phenotype:

• • Some traits are completely determined by the genetic make-up of the organism (e.g., eye colour and blood type in mammals).
  • In complex organisms, such as plants and animals, many traits are under the influence of several genes (=quantitative traits; e.g., >180 genes involved in human height). For such traits, we observe a continuous range of phenotypes, representing the many possible alleles from several loci.
  • Phenotypic variability in complex traits may also result from phenotypic plasticity. In this case, the organism possesses a genotype whose expression is going to be affected by the environment (i.e., is plastic). For example, available nutrients and water affect plant growth, and exercise affects the cardio-vascular capacity of mammals.

  In summary, heritable and non-heritable (environmentally influenced) traits produce a range of differences among individuals in a population. The amount of variation in the heritable traits in a population reflects the potential of that population to experience evolutionary change.

  1.2 Why is phenotypic variation important to study?

  Heritable phenotypic variation is the raw material for adaptions – this is because natural selection operates on the phenotype of the organisms. Understanding how to measure variability in traits and how to properly analyze and present these types of data is an important first step in studies of natural selection and evolution in natural populations. If we were wanting to then discover whether a population is undergoing evolution through natural selection, we would need to determine whether the phenotypic variation is related to variation in reproductive success in the population, and that the variation has a heritable component.

  1.3 A reminder on cellular respiration

  Energy is essential for life. To get energy, chemicals are converted into useable energy through cellular respiration (equation 1). The energy from cellular respiration is used for many processes such as formation of macromolecules, mitosis (growth), and moving substrates into and out of the cell.

  Equation 1: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Heat + Energy (ATP)

  The ultimate function of respiration is to move oxygen into body tissues where it will be used in cellular respiration, and to remove carbon dioxide, a product of cellular respiration, out of the body. Organisms do this in a variety of ways. For example, insects obtain oxygen for cellular respiration through spiracles, whereas pillbugs, a crustacean, use gills). We will measure cellular respiration using PASCO CO₂ sensors.

  1.4 The model organism

  In this lab, we will be using pillbugs (Armadillidium vulgare; Figure 1) as a model organism to study behavioural and physiological phenotypic traits. Pillbugs are interesting in that they are a land-based crustacean (i.e., closely related to lobster, shrimp, etc.). These organisms obtain oxygen through gills, like aquatic organisms. Having gills provides challenges to land-based organisms because gills extract oxygen from water. With the dry air on land, pillbugs have several adaptations that have allowed them to succeed: they choose specific environments that have high moisture content (e.g., forest floors, around decomposing logs), are active mostly at night (why would this be advantageous?), and they have physical structures that allow them to bring moisture up to their gills.

  

  Figure 1. Armadillidium vulgare (pill bug) (photo taken by Jody Rintoul)

  Pillbugs play an important role in their native habitats – they are detritivores, which means they feed on dead material. They are important in speeding up the decomposition process because they fragment plant and animal matter into smaller pieces that can then be further broken down by bacteria and fungi.

  1.5 Writing species names

  When writing species names in biology, we use the binomial nomenclature method. Each species name has a genus and specific epithet, which is where “binomial” comes from. For example, Armadillidium is the genus and vulgare is the specific epithet for the species of pillbug that we are using in this lab. There are specific rules to follow when writing species names:

  • The genus is capitalized whereas the specific epithet is not
  • When typing, both words are italicized. If hand-written, both are underlined. This convention specifies that the names are of Latin origin.

  In addition to the naming rules, we also have specific guidelines on using the species name in-text.

  • The first time the species in named in a document, write the name in full. After that, you can abbreviate the genus to using only the first letter of it, still italicizing the 2 parts (e.g., A. vulgare). If, however, there are 2 different species that you are writing about that have different genera with the same first letter, you want to write the name in full every time.
  • If starting a sentence with the species name, write it out in full
  • Sometimes we don’t know the species name of the organism we are working with. In that case, we write the genus and then add an abbreviation for species (sp.) after the genus. The genus is still italicized, but “sp.” is not (e.g., Armadillidium sp.). If you are referring to more than one species within a genus, you can indicate this by adding another “p” to your species abbreviation (spp.). Again, the abbreviation is not italicized (e.g., Armadillidium spp.).