Foxglove (Digitalis purpurea), photo by Jim Riley
Introduction to Wildflower Genetics: Understanding Plant Variation
Anthony J. F. Griffiths and Fred R. Ganders
Reprinted from Wildflowers Genetics: A Field Guide for British Columbia and the Pacific Northwest, with permission.
What is Genetics?
Genetics is usually defined as the study of inheritance. Inheritance itself is the process of handing on, from one generation to the next, the characteristic traits that make up a living plant or animal. To be more precise, it is not the characteristics themselves that are handed down from parent to offspring but the instructions or blueprints for those characteristics. When we say, for example, "She has her mother's hair," what we really mean is : "The instructions for growing hair like that were derived from her mother".
The 'instructions' that we have just referred to are embodied in minute components of every cell called genes. The characteristics of a plant or animal are to a large degree dictated by the genes it bears. It is genes that make a dogwood tree a dogwood tree and not a red alder tree. Each gene has different forms, which are responsible for the main differences found within and between species. For example, the sea blush, Plectritis congesta, which grows on Vancouver Island and in the Gulf Island region of British Columbia, has two kinds of fruits. Wherever this species grows, some plants have fruits with wings and some have fruits with no wings. This difference is due to a difference in one kind of gene, which we could call a fruit-wing gene.
The existence of genes was discovered in the nineteeth century by Gregor Mendel, an Austrian monk, who performed experiments with the garden pea in a monastery garden about the size of your average Vancouver backyard. Since then, genetics has grown into a highly sophisticated experimental discipline embracing chemistry, mathematics and many areas of biology.
By and large genetics as a science can be divided into two major areas, transmission genetics and moleclar genetics. Transmission genetics is concerned with pinpointing the existence of specific genes and following thir patterns of transmission from one generation to the next. This book is concerned mainly with transmission genetics.
Molecular genetics is an exciting area of research whose goal is to understand the structure and function of genes at the molecular level. A great deal is now known about this subject, thanks to a large battery of sophisticated techniques...This knowledge can greatly enhance our understanding and appreciation of the way in which hereditory blueprints in cells--the genes--determine how a plant or animal looks and behaves. The colour, shape, size, and other characteristics of an organism are under genetic control. The success of life on this planet is the success of the blueprints of life, the genes. Some biologists say that a plant or animal's body is simply a vehicle for serving the genes, existing only to protect them against environmental stress and to propagate them through time.
Variation: the Raw Material of Genetics
The starting point of genetics is the observation of variation. If all the plants of one species in your walk are identical, you cannot begin a genetic discussion of them or perform genetic analysis on them. All geneticists, from Mendel to professional geneticists today, start their studies with members of a single species that differ with respect to at least one aspect of their makeup. For example, one member of a pair of foxgloves might have red petals and the other member white petals. Or one member of a pair of foxgloves might have smooth stems and the other member hairy stems.
Causes of Biological Variation
I Environmental Causes
- Caused by physical environment (light, temperature, moisture, minerals, exposure, etc.)
- Caused by biological factors (parasites or any other species that interact with the species under study).
II Developmental Variation
- Difference between juvenile stage and mature stages
- Variation within single plant
- Mistakes in development
III Genetic Variation
- Caused by nuclear genes
- May give well-defined classes of types, or discontinuous variation
- May give continuous range of variation
- Caused by cytoplasmic genes--usually inherited maternally
A variant plant may be different simply because the environment it is growing in is itself different. In general, the environment may be divided into physical components (tempterature, light, moisture, minerals, exposure, etc.) and biological components (parasites or any other species that interact with the species under study). Thus, environmental variation may have a physical or a biological basis. For example, there may be better light or more moisture, the city work crews may have sprayed the area with herbicide, or the soil may be very shallow where your variant plant is growing (physical basis). Or your plant may be infected with some parasite, whereas other individuals around it are not (biological basis). Environmental differences may exist even a few centimeters apart.
Environmental variation can generally be identified if the variation disappears when both variant and normal plants are transplanted to a common environment. Therefore this is a test that should be performed [to aid in determining the cause of variation.]
Let's look at some specific examples taken from British Columbia.
Nutrients and Water:
In the springtime, the sea blush (Plectritis congesta) is a common sight around the southern coastline of the Strait of Georgia, where large populations form bright carpets of pink. Off the Sunshine Coast there is a small unnamed island approximately 100 m long where the sea blush grows profusely into very large plants, over 1/2 m tall, with very thick stems and lush foliage. On Mill Hill near Victoria, the sea blush is found to be much smaller, usuallly around 10 cm in height. Yet when seeds from both these locations were planted in one growth chamber at the University of British Columbia, the plants were indistinguishable. Evidently the soil on the small island (we have named it Plectritis Island) is very rich in nutrients, possibly a result of an accumulation of sea-bird droppings.
Here again we can use sea blush as an example. The pink petal colour of the sea blush is due to a chemical called anthocyanin. The synthesis of anthocyanin in the plant's cell is profoundly influenced by temperature, with more produced at lower temperatures. In a warm growth chamber the petals are very pale pink, and in a cold chamber they are dark magenta. Temperature also affects the production of anthocyanin in the leaves of another Georgia Strait coastal plant, the blue-eyed Mary (Collinsia parviflora). In some areas many plants of this species have dark blotches of purple anthocyanin pigment in the epidermis of the leaves. We have collected such plants from nature and transplanted them into a greenhouse with no coolling systems, only to return a few days later and observe that the large intense blotches had completely disappeared.
Light also affects pigmentation. Stonecrop (Sedum) is often found to be green when growing in the shade but pigmented with red or purple hues when growing in the sun. The effect of light and shade (and perhaps concomitant variation in temperature and humidity) is often seen in the shape, size and thickeness of tree leaves, giving rise to the terms sun and shade leaves. Sun leaves are those exposed to full sun, while shade leaves are found inside the canopy of the tree, or shaded by neighbouring trees. Shade leaves are typically thinner and larger than sun leaves.
Aquatic versus aerial habit
Several species of buttercups grow in or near water. As a result, one plant may have some stems immersed in water and other stems growing in the air. This difference can have profound effects on the form of the leaves.
An example of a variant caused by a parasite is the gold-spot form of the piggyback plant (Tolmiea menziesii). This form is probably caused by a virus infection. At any rate, the variant can be propogated by cuttings and is in great demand as house plants. Witches'-broom, found on a variety of plants, are usually caused by parasitic infection of a localized area, often by dwarf mistletoe. Galls, or plant tumours, are likewise caused by parasitic infection, often by insect larvae.
Development, the process whereby a fertilized egg is converted into a mature organism, is a complex process under sophisticated biological control. Nearly always development proceeds through characteristic stages, often called juvenile stages, in which individuals appear different from the mature form. A good example is seen in the red huckleberry (Vaccinium parvifolium). The juvenile leaves of this plant are dark green, toothed, thick, and evergreen. In contrast, the adult foliage is lighter green, smooth, and toothless, and is shed each year.
Another good example of developmental variation is the striking variation of leaf shape within a single plant of pepperweed (Lepidium perfoliatum), a common plant in the drier areas of British Columbia. Different plants of this species may be missing certain types of leaf patterns. On large plants the lower leaves are compound, finely divided into narrow leaflets. The upper leaves are simple, smooth, and heart-shaped, and they surround the stem where they are attached. Small, stunted plants may have few if any finely divided leaves and consequently look quite different. This may be a matter of chance or of environmental influences on the normal development of the plant. It is not an inherited difference.
Because development is under such fine control, mistakes can occur. These may occur simply by chance, or they can be triggered by some environmental agent. We are all familiar with the tragic birth defects produced by the drug thalidomide. Mistakes occur in plant development too. One example is the familiar four-leaf clover....
These kinds of development mistakes can happen spontaneously. A good example of this is the occurrence of three cotyledons in plant seeds. Flowering plants are divided into two major groups: the monocts have one leaf on the seedling, and the dicots have two. For unknonwn reasons, very few flowering plants normally have more than two. Nevertheless, in seed samples, whether collected from nature or from seed companies, tricot plants are regularly seen. Initially, the plant takes on a trilateral symmetry, which may persist in the mature plant or in some cases may lead to obvious disarray in the subsequent leaves and branches....Presumably when the embryo is assembling itself inside the seed there is a crucial stage at which the number of cotyledons is decided. Most of the time this stage is passed successfully but all processes have errors. The remarkable fact is that such errors are so rare. Considering how many things could go wrong in the development of a sophisticated and complex structure such as a higher plant (or human being), one might anticipate that errors would be more common than they are.
Developmental variation is not inherited; the variants never pass on their defects to their progeny. In fact this characteristic is one of the keys to the identification of developmental variation. Another key is the presence of both abnormal and normal conditions in the same plant, as in clover plants that have both four- and three-part leaves; this situation cannot be hereditary.
The key to the identification of gene-caused variaton is that it is inherited from generation to generation. In addition, some standard inheritance pattern can be recognized. First we will consider the variant alleles of nuclear genes that have major effets [in plants].
We have already mentioned that in blue-eye Mary (Collinsia parviflora) plants with spotted leaves are reguarly found growing beside unspotted plants. We want to know if this variation is genetic, and, if so, whether it is controlled by a major gene and which allele is dominant.
Using the procedure outlined below, we carried out an analysis of this variation in blue-eyed Mary. We discovered the spotted type in a seed sample obtained from Jack Point near Nanaimo on Vancouver Island, in a population that was subsequently wiped out by construction of a new seaport. Luckily, the spotted type is quite common along the coast. Before those seeds grew in our greenhouse, however, nobody had ever reported the spotted type, so we had to do the genetic analysis.
Self-pollinate the two types and examine the progeny. We found that the unspotted plants had all spotted progeny and the unspotted plant had all unspotted progeny. This shows that variation is inherited and must have a genetic basis. (Notice that in this case the parental types bred true for their own appearance, meaning that all the progeny resembled their parental type. This is not always the case, as we shall see.)
Cross-pollinate the two types and examine the progeny. We found that the progeny are all spotted. This suggests that a dominant spotting allele is at work.
Self-pollinate the hybrid plants and examine the progeny. We found that 3/4 of the progeny are spotted and 1/4 are unspotted. What does this prove? It proves that the variation was caused by an allele difference of a single gene of major effects and confirms that the allele for spotted leaves is dominant.
This publication is available for purchase form the UBC Botanical Garden gift shop.