Investigating Evolution with Living Plants

nvestgating Evoluton with Living Plants

Mark A. Schlessman

I F, like me, you're a botanist teaching in a biology department, your plant biology courses may con- stitute only a small fraction of your teaching. And probably much of your teaching on plants is done in the context of introductory biology or other courses that are not solely devoted to plants. You may be looking for ways to illustrate broadly applicable biological principles with plants. If you are a high school teacher, you may be searching for inexpensive exper- Downloaded from http://online.ucpress.edu/abt/article-pdf/59/8/472/48231/4450361.pdf by guest on 28 September 2021 imental activities on plants or evolution. Perhaps you're just looking for investigative labs that use live plants, illustrate important biological principles, include some quantitative data analysis, and require little equipment. Here I describe two such labs. Each of them deals with the evolution and consequences of inbreeding, or mating among relatives. Each can be adapted to a variety of class sizes, course contents, and Figure 1. Monkey flower plants (Mimulus, Scrophulari- student backgrounds. aceae): right, M. guttatus; left, M. laciniatus. The ruler is 15-mm long. Monkey Flower Business: The Evolution of Flower Size in Mimulus eses for the evolution of small flowers in M. laciniatus: Background the "smaller pollinator" hypothesis and the "selfing" Because the reproductive success (Darwinian fit- hypothesis. ness) of an individual plant often depends on its The "smaller pollinator" hypothesis is based on the success at attracting pollinators, floral characters are idea that floral structure has evolved to promote relatively constant within species. (This is one reason efficient pollination. Essentially all general biology why flowers are usually necessary to identify plants.) On the other hand, differences in floral morphology among species help prevent nonproductive interspe- cific pollinations, and it is not unusual to find that closely related species have distinctly different flowers. Such differences may evolve for a variety of reasons. In this lab, students evaluate alternative hypotheses for the evolution of different floral sizes in two species of monkey flowers, the genus Mimulus in the snapdragon family, Scrophulariaceae. Mimulus consists of about 100 species, most of which are native to North America. Recently, Mimulus has received considerable attention from evolutionary biologists interested in plant reproductive biology and speciation (Yoon 1995; Morell 1996). For this activity I have used M. guttatus, which has relatively large flowers, and M. laciniatus, which has much smaller flowers (Figures 1 & 2). I ask students to evaluate two hypoth- Figure 2. Monkey flowers with corollas (right) separated Mark A. Schlessman is Professor of Biology at Vassar Col- from calyces and pistils (left). Mimulusguttatus above, M. lege, Poughkeepsie, NY 12604-0187. E-mail schlessm@ laciniatusbelow. All viewed from the side, except M. guttatus vassar.edu. corolla, which is seen from above.

472 THE AMERICAN BIOLOGY TEACHER,VOLUME 59, NO. 8, OCTOBER 1997 textbooks cover the relationshipbetween variation in extinct, but no new pollinator was available or work- floral structure and variation in kinds and behaviors able as a replacement. Would the plants become of pollinators, and also the differentmechanisms that extinct too? Perhaps. But what if the plants were able promote cross-pollination.Mimulus guttatus is excel- to self-fertilize,and a few individuals had some flow- lent for illustrating these concepts. The showy yellow ers that self-pollinated?Those flowers would produce flowers are bilaterally symmetrical, just as we are more fruits than flowers that could only be cross- (Figures 1 & 2). The enlarged bottom "lip" of the pollinated. If the offspring of self-pollinating plants corolla tube (the five fused petals) serves as a landing resembled their parents, sooner or later a new, self- platform for bumble bees. Red dots and yellow hairs pollinating species might evolve. Now, some students may help guide bees to nectar inside the flower. Some may object to this scenario on the grounds that self- botanists see a monkey's face in the pattern of red pollination and self-fertilizationresult in inbreeding, dots, hence the common name for Mimulus.As the bee and inbreeding is "bad" (see background on Brassica enters the flower, pollen on its back will be deposited lab below). You might respond by asking them to on the stigma, which protrudes beyond the two pairs consider which plant has the higher fitness, one that of stamens. In many monkey flowers, the two-lobed self-pollinates and self-fertilizes or one that produces stigmas are sensitive to touch. They will close whether no offspring at all. pollen is deposited or not, but may re-open if no How would the flowers of the new, self-pollinating Downloaded from http://online.ucpress.edu/abt/article-pdf/59/8/472/48231/4450361.pdf by guest on 28 September 2021 pollen is present. As the bee moves further inside, it species differfrom those of its cross-pollinatingances- contacts the anthers and receives new pollen on its tor? The flowers would probably be smaller, simply back. Although Mimulusis capable of self-fertilization because a smaller and narrower corolla tube would (i.e. it is genetically self-compatible),the spatial sepa- tend to bring the anthers closer to the stigma, which ration of stigma and anthers and the sensitive stigma would promote self-pollination.But would the reduc- lobes both promote cross-pollination.Closed stigma tion in floral size be equal for all floralparts? What are lobes prevent self-pollination as the bee backs out of the consequences, in terms of Darwinian fitness, of the flower. having smaller flowers? Smallercorollas might be less The smaller pollinator hypothesis assumes that M. attractiveto pollinators,but that is not likely to affect laciniatus(small flowers) is cross-pollinated in much the fitness of self-pollinating individuals. Smaller an- the same way as Mimulusguttatus (large flowers), but thers with fewer pollen grains would also have little that the former just has a smaller pollinator (i.e. a effect, as long as there were enough pollen grains to solitary bee versus a bumble bee). Under this hypoth- fertilize all of the ovules (potentialseeds) in the ovary esis, we can envision an ancestralpopulation in which of the flower. In contrast,smaller ovaries and calyces floral size varied and in which largerbees became less (sepals) might reduce fitness, because smaller ovaries abundant while smaller bees became more abundant. with fewer ovules would lower the number of seeds Those flowers that just happened to be the appropriate and smaller would afford less pro- size for effectivepollination by the smallerbees would produced, calyces be more likely to produce fruits than other flowers, tection for the developing fruits. Following this rea- thus selection would favor those plants with flowers soning, and noting that in a self-pollinating species that "fit" the smaller bees. In its simplest form, this relative sizes of floral parts do not have consequences hypothesis predicts that in order to retain the appro- for pollinator behavior and effectiveness, the selfing priate spatial and functional relationships among flo- hypothesis predicts that relative to M. guttatus, the ral parts, all the parts (sepals, petals, stamens and petals and stamens of M. laciniatus will be more carpels) of the M. laciniatusflower should be equally reduced than the sepals and ovaries. Lastly,under the reduced versions of those in M. guttatus(my students selfing hypothesis, the stigma of M. laciniatusshould re-named this the "Xerox?Mreduction hypothesis"). not protrudebeyond the anthers,but rathershould be This hypothesis also predicts that in both M. laciniatus located next to them in order to facilitateself-pollina- and M. guttatus the stigma should protrude beyond tion. the anthers in order to promote self-pollination. Thus, the selfing hypothesis differsfrom the smaller In contrast to the smaller pollinator hypothesis, the pollinator hypothesis in three ways. First, it assumes selfing hypothesis is based on a phenomenon that that while Mimulus guttatus is cross-pollinated, M. most high school and generalbiology textbooksdo not laciniatus has lost its pollinator and evolved self- address: the evolution of self-pollinating,self-fertiliz- pollination. Second, it predicts that some parts of the ing and consequently inbred species from cross-polli- M. laciniatusflower (petals and stamens) will be more nating, outbred ancestors.Some students may remem- reduced relative to those of M.guttatus than others(se- ber that Mendel's peas could self-pollinate and self- pals and ovaries). Third, it predicts that in M. lacini- fertilize, but you'll probably have to provide some atus,there will be no spatial separation between the background for this idea. Ask your students what stigma and anthers. would happen if a plant's pollinators became rare or In the lab, students use measurementsof floralparts

INVESTIGATINGEVOLUTION 473 1.0 [i fftus 7]

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0.0 a~~~~~~ calyx/corolla Figure 5. Class data from "Monkey Flower Business." Means plus 95% confidence intervals for calyx length/ corolla length ratios. Downloaded from http://online.ucpress.edu/abt/article-pdf/59/8/472/48231/4450361.pdf by guest on 28 September 2021

Figure 3. Illustrationof measurements of monkey flowers and Ritland(1989) should work well. Mimulusguttatus (after Ritland & Ritland 1989). Stamens are attached to the is available from commercial seed suppliers. (I used corolla and will remain with it when it is removed. Mea- Abundant Life Seed Foundation, P. 0. Box 772, Port surementstaken to the nearestmm are:a = calyx length,b = Townsend, WA 98368.) The less showy species are corolla length, c = ovary length, d = pistil length, e = more difficultto obtain. You can find your own, check distance from base of corolla to tips of longest stamens. Stigma-antherseparation relative to floral size = (d-e)/b. with native plant seed suppliers, or contact me for Students also measure the width of an anther of one of the some seeds. I sowed seeds in small peat pots contain- long stamens, estimating to the nearest 0.5 mm. ing commercialpotting soil and germinatedthem in a growth chamber with 14-hour days (but a growth (Figure 3) to test the predictions of the two hypothe- chamber is certainly not necessary!). Mimulus lacini- ses. They make three kinds of comparisons: sizes of atus bloomed in three to four weeks and produced the same floral parts, e.g. Mimulus laciniatus corollas flowers for at least three weeks. Mimulus guttatus versus M. guttatus corollas (Figure 4); ratios of sizes of bloomed in about five weeks and produced flowers different floral parts, e.g. calyx: corolla of M. laciniatus for two to three weeks. I recommend planting two or versus calyx:corolla of M. guttatus (Figure 5); and three groups of seeds at two-week intervalsto increase stigma-anther separation (Figure 6). Using these data, the likelihood that you'll have enough flowers when my students have concluded that the smaller pollina- you want them. Rulersand dissecting microscopesare tor hypothesis for the evolution of small flowers in M. the only necessary equipment. Dissecting points and laciniatus could be rejected. fine-pointed (watchmakers)forceps can be helpful. It is wise to have students practiceon one or two flowers Preparation & Execution before they startrecording measurements. Try it your- Mimulus is annual, so plants must be grown from self so you can show them the best way to proceed. I seed. Any of the species listed in Table 2 of Ritland give my students a handout on this lab at least three days ahead of time, and I ask them to prepare a chart 25- 25 ^ [ * ~~guttatus| 2020 -j i Le ~~~laciniatus|u~ 0.125T-acnitu

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corolla calyx ovary anthers -0.025 stigma/anther separation Figure 4. Class data from "Monkey Flower Business."~ Means plus 95%Oconfidence limits for corolla length, calyx Figure 6. Class data from "MonkeyFlower Business."Stig- length, ovary length, and anther width. ma-antherseparation relative to floral size.

474 THEAMERICAN BIOLOGY TEACHER, VOLUME 59, NO. 8, OCTOBER1997 contrasting the assumptions and predictions of the two hypotheses before they come to lab. I use this activity in a sophomore level course on Plant Structure and Diversity. My students take measurements (a total of at least 20 flowers for each species- each student measures only a few flowers), record their data on paper, enter them in a statistical software package, perform statistical analyses, and prepare tables and figures in one afternoon lab. We discuss their data in class the following morning, then they write up their work in the format of a research report for the American Journal of Botany. I always include a question that requires interpretation of similar data on the next exam.

Effects of Self- & Cross-Pollination in Downloaded from http://online.ucpress.edu/abt/article-pdf/59/8/472/48231/4450361.pdf by guest on 28 September 2021 Figure 7. Rapid-cyclingBrassica rapa in flower (left) and in Genetically Self-Compatible Versus Self- fruit (right).The white StyrofoaMTMpot on the left is 4.4-cm Incompatible Rapid-Cycling Brassica tall. Background Many students know that inbreeding, or mating sion. Students can test this prediction by comparing among relatives, is "bad" because it increases the the results of self- and cross-pollinations of regularly frequency of loci that are homozygous, and thus the outbred and inbred plants. expression of recessive, often deleterious alleles. But In many ways, the large- and small-flowered Mimu- there's much more to inbreeding depression than that. lus described above would be ideal for this activity. What if a population, such as a plant that is losing its You might want to try it with your students (see Carr pollinators, becomes inbred and remains so for a long & Dudash 1996). However, Mimulus has very small time? If the inbreeding depression is not so devastat- seeds that are relatively easy to count but hard to ing that the population becomes extinct, natural selec- weigh. For this and other reasons, instead of Mimulus tion will tend to eliminate the deleterious alleles. In I use rapid-cycling "stocks" of Brassica rapa (= B. fact, the deleterious recessive alleles that cause in- campestris). The commercial cultivars of this species breeding depression will remain in a population only include turnips, pak choi, and Chinese cabbage (Fig- if they frequently arise by mutation or if regular ure 7). outbreeding keeps them "hidden" from selection in Rapid-cycling Brassicas were developed by geneti- heterozygous genotypes. Thus, evolutionary geneti- cists working on the improvement of mustard family cists predict that regularly inbred populations will (Brassicaceae or Cruciferae) crops (Williams & Hill exhibit less inbreeding depression than regularly out- 1986). "Rcb's" have fairly large flowers that are easy to bred ones. work with, extremely short generation times, high Because they are often capable of self-fertilization, fruit production, and seeds that are large enough to flowering plants are excellent experimental organisms handle individually. You may be familiar with their for testing this prediction. Self-fertilization, mating educational offshoot, Wisconsin Fast PlantsTM,which with one's self, is the most extreme form of inbreed- are marketed by the Carolina Biological Supply Com- ing. It is genetically equivalent to a mating between pany. In contrast to Mimulus guttatus, which has identical twins. A quick Punnet Square will show that morphological features that limit self-pollination, if an Aa heterozygote self-pollinates, one-half of the Brassica exhibits self-incompatibility, a genetic mecha- offspring will be homozygotes (25% AA, 25% aa). nism that prevents self-fertilization if self-pollination Assuming that "a" is a recessive deleterious allele, one occurs. fourth of the offspring may exhibit inbreeding depres- Although the basic genetics of self-incompatibility sion due to homozygosity at this locus. Since individ- are fairly clear, its cellular and molecular mechanisms uals from a regularly cross-pollinating population are still poorly known. The phenomenon is somewhat should be heterozygous at many loci, self-pollinating analogous to self-recognition and non-self rejection by them should produce marked inbreeding depression. our immune system. If pollen is recognized as self, it On the other hand, individuals from a regularly will either not germinate at all, or the pollen tube will self-pollinating population should be mostly homozy- stop growing soon after germination. Pollen germina- gous for non-deleterious alleles, and self-pollination tion often requires hydration by stigmatic secretions, should produce a lower level of inbreeding depres- and uptake of nutrients from the style is essential for

INVESTIGATINGEVOLUTION 475 20 . * ~~~~crossed 1selfed i 15

0 0191 10

outcrosser selfer Figure8. Datafrom a groupproject on inbreedingdepres- sion in rapid-cyclingBrassica. Means plus 95%confidence intervalsfor seed set (numbersof seed per fruit). The differencebetween self- and cross-pollinatedfruits for the 11 12

outcrosser(self-incompatible stock CrGC 1-1) is statistically Downloaded from http://online.ucpress.edu/abt/article-pdf/59/8/472/48231/4450361.pdf by guest on 28 September 2021 significant(t test, P < 0.05),but for the selfer(self-compat- Figures 9-12. Procedure for self-pollinationof self-incom- ible stockCrGC 1-8) it is not. patible rapid-cyclingBrassica when the flowers are in bud. Figure 9. With watchmakers'forceps, gently part the sepals of a bud one to three days before it would open on its own. pollen tube growth. Self-incompatibilitymechanisms Figure 10. With forceps,crimp and lay back sepals to expose can block one or both of these processes. Self-incom- stigma and anthers. Figure 11. With forceps, remove a recently dehisced anther from another flower on the same patibility is governed by a supergene commonly plant. Figure 12. Place pollen from the recently dehisced known as the "S-locus," which may have several anther on the stigma of the bud. These figures were repro- alleles. In Brassica,pollen will not grow if it carriesan duced with the permission of Carolina Biological Supply S-allele that is also present in cells of the stigma. Thus, Company, Burlington, NC 27215 and Wisconsin Alumni S1 pollen will not grow on stigmas with the genotypes ResearchFoundation, Madison, WI 53707. SlSl, S1S2 or S1S3; but will grow on stigmas with these genotypes: S2S2, S2S3 or S3S3. This system is called "gametophytic" incompatibility, because the Brassicas,instructors' manuals, and activities booklets. incompatibilityphenotype of each pollen grain (male Their Wisconsin Fast Plants Manual contains a wealth gametophyte) corresponds directly to its genotype. of useful information, including clear diagrams of For this activity, I use two "stocks"of rapid-cycling floral morphology and detailed instructions on how to Brassica.One stock has the normal self-incompatible grow and pollinate the plants. phenotype and is thus cross-pollinatedand relatively Rapid-cycling Brassica will germinate within two outbred. The other is a specially developed self-com- days, and if grown under continuous light they will patible stock which is partially self-pollinated and flower after two to three weeks and set fruit in about presumably more inbred. Students cross- and self- five weeks. The flowers have two green sepals, four pollinate individuals of both stocks, then record three yellow petals, six stamens (four long and two short), or more of the following: fruit set, seed set, seed and a single pistil composed of two fused carpels (see weight, seed germination, and seedling growth. Fig- Figures 9-12 and the Wisconsin Fast Plants Manual). ure 8 summarizes one group's results for seed set. The Making effective hand-pollinations requires sound students concluded that these data supported the knowledge of floral structure and the process of floral prediction that regularly self-fertilizingplants should maturation, so it's a good idea to have your students exhibit less inbreeding depression than regularly familiarize themselves with Brassica flowers and prac- cross-fertilizingones. tice pollinating them before the actual experiment. Cross-pollinations can be made by removing an Preparation& Execution anther of the pollen donor with watchmakers' forceps Carolina Biological Supply Company markets a and dusting pollen on recipients' stigmas, or you can wide variety of rapid-cyclingBrassica rapa stocks (Wis- purchase or make "bee sticks" (see Wisconsin Fast consin Fast Plants), all of which are presumably self- Plants Manual) for this purpose. For cross-pollination incompatible.However, my students have found that of the self-compatible stock, the anthers of recipient these stocks occasionally self, so you may want to flowers should be removed before maturation so that obtain both stocks directly from the CruciferGenetics unintended self-pollination cannot occur. It's a good Cooperative (CrGC).Ask for CrGC stocks 1-1 (self- idea to emasculate the recipients of cross-pollen in the incompatible)and 1-8 (self-compatible).Carolina Bio- self-incompatible stock too, just to make sure. For logical also sells supplies for growing rapid-cycling self-pollination of the self-incompatible stock, you

476 THEAMERICAN BIOLOGY TEACHER, VOLUME 59, NO. 8, OCTOBER1997 need to circumvent the genetic incompatibility. I've concepts, and link lab activity with current research. done this by making self-pollinations of floral buds Each exercise also reinforces knowledge of floral biol- that have not fully opened (Figures 9-12). You might ogy by helping students think about the functions of also try using a salt solution to overcome genetic flowers, not just the names of floral parts. After incompatibility (Monteiro et al. 1988). writing their research reports, my students have done My students usually propose to self-pollinate sev- very well interpreting data from similar studies under eral flowers on one group of plants and cross-pollinate the pressure of an exam. several flowers on a different group of plants. A better Although they enjoy working with Mimulus flow- experimental design is to make both cross- and self- ers, especially M. guttatus, my students say they are pollinations on the same individuals. Flowers can be rushed to complete their measurements, data entry, marked with different colors of permanent ink, Wite- and analyses in one four-hour lab period. If you can, outTM, or thread. This design allows use of more introduce the plants and discuss the hypotheses in powerful statistical analyses (i.e. paired tests rather class before the lab, and save discussion of the data for than unpaired tests). the class after the lab. As given here, "Monkey Flower This activity is one of several semester-long group Business" will work for large classes as well as small. projects available to students in my junior/senior level In fact, my version is adapted from one used for a evolutionary biology course. The group usually di- large introductory class at the University of Maryland. Downloaded from http://online.ucpress.edu/abt/article-pdf/59/8/472/48231/4450361.pdf by guest on 28 September 2021 vides itself in two: one subgroup works with the With more time or smaller classes, the exercise can self-incompatible stock and the other handles the become open-ended when students generate their self-compatible stock. The students are given a mini- own hypotheses and tests. Students could determine mal amount of background information and are asked fruit and seed production when cross-pollination is to design their own experiments. They write up their prevented (e.g. by enclosing flowers or entire plants in results as a research report for the journal Evolution, fine mesh). Students might also want to compare the and give oral reports on their projects. results of self- and cross-pollinations, or ask whether fruits can be produced without any pollination at all Evaluation & Suggestions (i.e. remove the anthers before pollen is released and I am most enthusiastic about these activities because prevent cross-pollination). they allow me to demonstrate the value of plants as Students tell me that group research projects, such experimental organisms, illustrate some sophisticated as the Brassica activity described here, are the most lology NABTMembers Save 20%

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INVESTIGATINGEVOLUTION 477 valuable part of my evolution course. The Brassica Richards,A.J. (1986). Plant Breeding Systems. London: George activity can also be adapted to large labs; one simply Allen & Unwin. The numbers of flowers Wyatt, R. (1983). Pollinator-plantinteractions and the evo- needs to grow more plants. lution of breeding systems. In L. Real (Ed.), Pollination pollinated and the ways your students measure in- Biology(pp. 51-95). New York:Academic Press. breeding depression can be adjustedto your own time constraintsand equipment. If your students can weigh Monkey Flower Business (including research papers that seeds as well as count them, they may ask you about a instructors may wish to read and assign) trade-offbetween seed number and seed weight. This Bell, G. (1985).On the function of flowers. Proceedingsof the RoyalSociety of London, B Series,224, 223-265. could lead to furtherinvestigation of the effectsof seed Bradshaw,H.D., et al. (1995).Genetic mapping of floraltraits weight on germinationand seedling vigor. You might associated with reproductiveisolation in monkeyflowers also get into an interestingdiscussion on whether traits (Mimulus).Nature, 376, 762-765. such as fruit set, seed number, and seed weight are Carr, D.E. & Dudash, M. (1996). Inbreeding depression in characteristicsof the motherplant or its offspring. two species of Mimulus(Scrophulariaceae) with contrasting mating systems. AmericanJournal of Botany,83, 586- Several general biology texts have good discussions 593. of inbreeding, self-fertilization in plants, or both. Carr, D.E., et al. (1997). The relationshipbetween mating- Raven and Johnson(1989) mention genetic self-incom- system charactersand inbreeding depression in Mimulus patibility in plants, self-fertilization as a departure guttatus.Evolution, 51, 363-372. Downloaded from http://online.ucpress.edu/abt/article-pdf/59/8/472/48231/4450361.pdf by guest on 28 September 2021 from random mating that increases homozygosity, Coyne,J.A. (1995). Speciation in monkeyflowers.Nature, 376, 726-727. and the possible detrimental effects of inbreeding in Cruden, R.W. & Lyon, D.L. (1985). Patterns of biomass small, often rare or endangered, populations. Curtis allocation to male and female functions in plants with and Barnes (1980) give an extended discussion of differentmating systems. Oecologia,66, 299-306. mechanisms that promote outbreeding, including ge- Dole, J.A. (1992). Reproductive assurance mechanisms in netic "self-sterility." Levine and Miller (1992) also three taxa of the Mimulusguttatus complex (Scrophulari- aceae).American Journal of Botany, 79, 650-659. briefly mention self-incompatibility.Campbell (1993) Dudash, M.R., et al. (1997). Five generations of enforced briefly discusses the effects of inbreeding on allele selfing and outcrossing in Mimulusguttatus: Inbreeding frequencies in populations using wildflowers as an depression variation at the population and family level. example, and also explains self-incompatibility in Evolution,51, 54-65. plants. Purvis, Orians and Heller (1995) have a nice Lovet Doust, J. & Cavers, P.B. (1982).Biomass allocationin hermaphroditicflowers. CanadianJournal of Botany,60, section on plant reproduction and mention self-polli- 2530-2534. nation in several contexts. A discussion of the evolu- Macnair,M.R. & Cumbes, Q.J. (1989).The genetic architec- tion of self-pollination and self-fertilizationmay raise ture of interspecificvariation in Mimulus. Genetics,122, some very interesting questions, such as whether 211-222. regularly self-fertilizing organisms fit into our con- Macnair,M.R., Macnair, V.E. & Martin,B.E. (1989). Adaptive speciation in Mimulus:An ecological comparison of M. cepts of what a species is, and whether "not needing" cupriphiluswith its presumed progenitor,M. guttatus.New a structure is an appropriate explanation for the Phytologist112, 269-279. evolutionary reduction or loss of that structure. Macnair,M.R. & Cumbes, Q.J.(1990). The pattern of sexual resourceallocation in the yellow monkey flower, Mimulus guttatus.Proceedings of the RoyalSociety of London B, 242, References 101-107. General references on floral biology, pollination, and mat- Mossop, M.R., Macnair, M.R. & Robertson, A.W. (1994). ing among plants (includes general biology texts cited Within-populationvariation in sexual resource allocation above) in Mimulusguttatus. Functional Ecology, 8, 410-418. Barth, F.G. (1985). Insects and Flowers: The Biology of a Morell,V. (1996).Starting species with third parties and sex Partnership. Princeton,NJ: Princeton University Press. wars. Science,273, 1499-1502. Campbell, N. (1993). Biology, 3rd ed. Redwood City, CA: Pennell, F.W. (1951).Mimulus. In L. Abrams (Ed.),Illustrated Benjamin Cummings Publishing Co., Inc. Floraof the Pacific States Vol. 3 (pp. 688-731). Stanford,CA: Curtis, H. & Barnes, N.S. (1980). Biology, 5th ed. New York: StanfordUniversity Press. Worth Publishers, Inc. Ritland, C. & Ritland,K. (1989).Variation in sex allocation Faegri, K. & van der Pijl, L. (1980). The Principles of Pollination among eight taxa of the Mimulusguttatus species complex Ecology. New York: Pergamon Press. (Scrophulariaceae).American Journal of Botany,76, 1731- Levine, J.S. & Miller, K.R. (1992). Biology: Discovering Life. 1739. Lexington, MA: D.C. Heath & Co. Robertson, A.W., Diaz, A. & Macnair, M.R. (1994). The Percival, M. (1965). Floral Biology. New York: Pergamon quantitativegenetics of floral charactersin Mimulusgut- Press. tatus.Heredity, 72, 300-311. Purvis, W.K., Orians, G.H. & Heller, H.C. (1995). Lie: The Schoen, D. (1982). Male reproductive effort and breeding Science of Biology, 4th ed. Sunderland, MA: Sinauer Asso- system in an hermaphroditicplant. Qecologia,53, 255-257. ciates, Inc. Vickery, R.L. (1964). Barriers to gene exchange between Proctor, M. & Yeo, P. (1972). The Pollination of Flowers. New members of the Mimulusguttatus complex (Scrophulari- York: Taplinger Publishing Company. aceae). Evolution,18, 52-69. Raven, P.H. & Johnson, G.B. (1989). Biology, 2nd ed. St. Yoon, C.K.(1995, September 5). Fornew species, a few genes Louis: Times Mirror/Mosby College Publishing. are all that's needed. TheNew YorkTimes, pp. C1 & Cl1.

478 THEAMERICAN BIOLOGY TEACHER, VOLUME 59, NO. 8, OCTOBER1997 Rapid-cycling Brassica and inbreeding depression (including a research paper that instructors may wish to read and assign) Anon. Wisconsin Fast PlantsTM Manual. Burlington,NC: Caro- Anatomnyin (bay? lina Biological Supply Company. . Wisconsin Fast PlantsTmGrowing Instructions. Burling- ton, NC: Carolina Biological Supply Company. Holtsford, T.P. & Ellstrand, N.C. (1990). Inbreeding effects in Clarkiatembloriensis (Onagraceae) populations with different natural outcrossing rates. Evolution, 44, 2031-2046. Monteiro, A.A., Gabelman, W.H. & Williams, P.H. (1988). Use of sodium chloride solution to overcome self-incompatibility in Brassica campestris. HortScience, 23, 876-877. Williams, P.H. (Ed.) (1985). CrGC Resource Book. Madison, WI: Crucifer Genetics Cooperative. Williams, P.H. & Hill, C.B. (1986). Rapid-cycling populations of Brassica. Science, 232, 1385-1389. Sources of Seed, Supplies and Information on Rapid-

Cycling Brassicas Yes!iTh mindcantforget Downloaded from http://online.ucpress.edu/abt/article-pdf/59/8/472/48231/4450361.pdf by guest on 28 September 2021 Carolina Biological Supply Company, 2700 York Road, what~~ithehand hav learned! Burlington, NC 27215 (phone: 800-222-7112). TheOOLOIKTNSystm isa ne :teahn&'ernn The Crucifer Genetics Cooperative (CrGC) develops, ac- tehoog htcobns h esoa xprec quires and maintains seed stocks of mustard family crops of bai_ua ietei pta eiswt and their wild relatives, sponsors annual crucifer genetics th nls oeta fca.Suet ul workshops, and maintains a database of members' research interests. It can supply a wide variety of rapid- cycling stocks of at least seven Brassica species. Members of CrGC are mostly university and college researchers and educators; dues are $25 per year. Crucifer Genetics Coop- erative (CrGC). Dept. of Plant Pathology, University of Wisconsin-Madison, 495 Russell Labs, 1630 Linden Drive, Madison, WI 53706 (phone: 608-262-8638; e-mail: [email protected] or [email protected]). Fast Plants/Bottle Biology Notes is a newsletter primarily for teachers of grades 1-12. It contains "how-to" articles and information about teaching resources and workshops. To get on the mailing list, contact Wisconsin Fast Plants, 798:. is;5 iS'' ' anatomy:r-day9L016 .com)2 :Z University of Wisconsin-Madison, Dept. of Plant Pathol- LOLL\ C)v~Ei_ ~x 8 )5 - 597 RtOl .fl ) O ogy, 1630 Linden Drive, Madison, WI 53706 (phone: ()kr/O 90 )U 800-462-7417; e-mail: [email protected]).

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