By Dr. Malcolm Manners
Question: which of the following mutations is most likely responsible for color sports and which for climbing sports?
I'll try to explain sports, but it's a challenge, without going too deeply into the genetics and biochemistry of it. There are at least 6 possible kinds of sports.
First, realize that the genetic code exists in the form of long chains of DNA, made up of repeating "letters" of nucleotides: A (Adenine) T (Thymine) C (Cytosine) G (Guanine) These are bound to a sugar-based backbone. The chain is double (the double helix), with "T" of one side always bonding to an "A" of the other side; "C" always bonding to "G." It can be imagined as looking rather like a ladder, with the sugar chain making up the vertical portions, and the -A-T- and -C-G- pairs making up the rungs. In reading the DNA, 3-letter words (codons) form the language of the process. Each codon represents one specific amino acid, which may be incorporated into a protein (the major way genetic information is actually converted to useful "stuff" in the organism). So, the system consists of a "language" with a 4-letter alphabet and 3-letter words. Not very complicated, compared to a human language, but by having an almost unlimited number of words in a "sentence" (code for one protein), the possibilities are almost infinite. Now to the possible "sports:"
1. DELETION. In this case, one of the letters is left out. This can affect quite a long stretch of the code, since, by deleting a letter, you respell the words that follow. e.g.: Assume a sequence consists of these 4 codons (words): cat tac agt tag If the first "a" were lost, it becomes: ctt aca gtt ag? The gene is now read incorrectly.
2. ADDITION. In this case, an extra letter is inserted. This mutation may also affect the reading of many codons afterward: Again, start with "cat tac agt tag" Add a letter "G" after the "a" in "cat:" The sequence now becomes caG tta cag tta g??
3. SUBSTITUTION. In this case, one letter is traded for another. Only the one codon will be misread; it has no effect on following codons. BUT that one codon change can remarkably affect the protein produced. Sickle-cell anemia is due to a single letter substitution, yet can be devstating to its victim. A substitution might look like this: cat tac agt tag might have the "a" in cat replaced, to become: cgt tac agt tag.
4. The 4th kind of sport is fundamentally different from the first three. In this case, there is no net change in the code; rather, a chunk of existing code moves from one area of the DNA chain, to another spot, and reinserts itself. This is known as a "TRANSPOSON," or nicknamed "jumping gene." Even though there is no new genetic material being added nor any subtracted, the chain is now different. Rather like converting the previous sentence to read: "Even though there is genetic no new material being added ..." All the words are there, but they aren't in the same order anymore. The reading changes. Actually, it would likely be a long chain of codons that would get moved, rather than just one codon; but perhaps you get the picture. Transposons, discovered by Barbara McClintock, are now realized to be quite important in many mutations.
5 and 6. The final two types of sports assume that there has been a mutation, due to one of the first 4 types, but that it did not affect every cell in the plant. Plant systems have 3 developmental layers of tissue -- L1, L2, and L3. If the cells in only one of the layers mutate, we now have what is known as a "chimera;" a plant with 2 different genetic codes. Good examples are callico cats (different code for black vs. orange hair) and Thompson pink grapefruit (genetically white grapefruit peel, pink pulp, and seeds which always produce white-fruited seedlings). Variegeted plants are also generally chimeras -- one layer knows how to make chlorophyll; another layer does not. Ok, now the 2 kinds of sports that can happen due to chimeric situations:
5. A periclinal chimera has one whole layer mutated, but not the other two layers. Therefore there is a ring of mutated tissue. The Thompson grapefruit is a good example of that type. The change occurs in cylindrical rings. Thornless blackberries are another good example. Only the layer that makes the epidermis lacks the information needed to make the prickles. If you take root cuttings, the epidermis may be made by a different layer, and you get prickly plants.
6. A sectorial chimera crosses the layers, to make a pie-piece-shaped wedge in the cylinder of the stem. In that case, any bud on the mutated wedge will be different from the non-mutant portion. I have a 'Chicago Peace' bush like that -- on some branches, I'll get a flower that is 3/4 'Chicago Peace', 1/4 'Peace'. The line of demarcation is very precise, and makes for a fascinating flower. As for how any of these types of mutation may occur, there are numerous possible causes, including radiation (there is substantial natural radiation in the environment that could do it), chemical mutagens (far less likely in a plant, but possible), or just a random "mistake" made by the plant in the process of replicating the DNA.
I hope this grossly simplified explanation helps, and that the molecular geneticists among our readers won't crucify me for having been too simplified.
Malcolm M. Manners, Ph.D. Professor, Department of Citrus and Environmental Horticulture Florida Southern College, Lakeland, FL
Departmental website: http://www.flsouthern.edu/citrus/