Academic journal article The American Biology Teacher

When a Fly Has to Fly to Reproduce: Selection against Conditional Recessive Lethals in Drosophila

Academic journal article The American Biology Teacher

When a Fly Has to Fly to Reproduce: Selection against Conditional Recessive Lethals in Drosophila

Article excerpt

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Teachers of evolutionary biology at the high school and college levels often need hands-on, inquiry-based laboratory exercises that allow students to observe and explain fundamental evolutionary processes. In its National Science Education Standards, the National Research Council has emphasized the fundamental importance of inquiry-based teaching approaches that incorporate real-life experiments (Olson and Loucks-Horsley, 2000). In many situations, these are preferable to traditional instruction or use of simulations. In evolutionary biology, a strong obstacle to developing such educational activities is the difficulty of observing microevolution in action within a semester. Both available technology and time constraints attract more and more college and high school teachers to various simulation activities, computer-based or otherwise. While some of these activities are attractive and efficient (e.g., Dubowsky & Hartman, 1986; Soderberg & Price, 2003; Rodriguez & Dvorsky 2006), a live experimental design has unique value. A number of such exercises (both field and laboratory) have been described in the recent literature (McComas, 1991; Hilbish & Goodwin, 1994; Salata, 2002; Vondrasek et al., 2004; Coleman & Jensen, 2007), yet there is a shortage of educational experimental designs that allow students to observe natural selection in action. One of the reasons why it has been difficult to bring microevolutionary experiments into classrooms is the obvious tradeoff between choosing an allele that is strongly selected against, such as a lethal allele, and choosing one that is easy to visualize. Alleles in the latter category are often subject to weaker selection, which means that a change in allele frequency takes longer to detect. For example, observing selection against a visible marker in Drosophila in an educational setting can be quite straightforward (Salata, 2002), but most visible mutations in Drosophila experience only weak stabilizing selection (e.g., Yampolsky et al., 2005), so it may take more generations to observe a significant change in allele frequency than is possible within a teaching-lab framework. Selection against lethal mutations--or sterility mutations, which, from the evolutionary standpoint, are identical--is strongest and therefore fastest. But both lethal mutations, most of which act early in embryonic development, and sterility mutations are hard to observe phenotypically. A good compromise in addressing this dilemma is to use conditional mutations, that is, mutations that cause death (or sterility) under some conditions but are viable (fertile) under other conditions.

In designing such experiments, it is most straightforward to use microorganisms, given their short generation time and readily available conditional lethal mutations. For example, auxotroph mutants are easy to maintain and observe on selective media and are strongly selected against on restrictive media (Welden & Hossler, 2003; Krist & Showsh, 2007). However, a disadvantage of using microorganisms for this purpose is that technical details of handling and propagating the cultures may obscure the evolutionarily essential components of the experiment. Thus, it is desirable to develop a selection experiment that uses a more familiar, eukaryotic diploid organism that offers the advantages of conditional lethal mutations. Drosophila, which has been the mainstay model organism in genetics for almost a century, is a quickly developing, easy-to-culture organism with a generation time of as little as 9 days. Culturing and handling require no special equipment, and stocks, culture components, and laboratory manuals are readily available (Ashburner et al., 2005; Carolina Biological Supply Company, 2008). Auxotrophic mutants are available in Drosophila (e.g., ade2; Johnstone et al., 1985), but they are hard to visualize and require special minimal media for selection to operate. …

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