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Asymmetry of floral structures and the possibility of self-pollination in Oerstedella exasperata (Orchidaceae) Lindsay Brownell Department of Biology, Davidson College ABSTRACT The purpose of this study was to investigate the ca uses of the pervasive asymmetry found in the neotro pical orchid Oerstedella exasperata Previous studies have found that high levels of a symmetry decrease reproductive success, and can be caused by a high amount of homozygosity within a population. This study tested the ability of O. exasperata to self-pollinate, a condition that indicates the possibility of a high level of homozygosity. In add ition, the relationships between three degrees of asymmetr y and floral color, floral mass, plant size and sun exposure were investigated to determine whether higher asymmetry is correlated with lower reproductive success or re duced overall plant fitness. Results showed an overall tr end suggesting that self-pollination is possible, w hich implies that O. exasperata s high level of asymmetry could be caused by homoz ygosity. Floral asymmetry was not related to any of the other examined factors, implying that it is a neutral trait that does not affect the overall fi tness of this species of orchid. These results contradict other studies findings that asymmetry is detrimental to plant hea lth, indicating that O. exasperata could be an exception to the general rule that sym metry indicates superior genetics in plants. RESUMEN El propsito de este estudio fue investigar las cau sas de la asimetra en la orqudea neotropical Oerstedella exasperata Estudios anteriores han reportado que niveles ele vados de asimetra bajan el xito reproductivo, y pueden ser causados por homozigosidad alta en una p oblacin. Este estudio prob la capacidad de O. exasperata de auto-polinizarse, una condicin que indica la posib ilidad de un nivel alto de homozigosidad. Tambin, las relaciones entre tres grados de asimetria y color floral, masa floral, tamao de la planta y exposicin al sol fu eron investigados para determinar si la alta asimetra se correlacion a con un bajo xito reproductiv. Los resultados mos traron una tendencia que surgiere que la auto-polinizacion ocu rre, lo que implica que el alto nivel de asimetra en O. exasperata puede ser causado por homozigosidad. La asimetra f loral no fue relacionado a nungn otro factor exami nado, lo implica que puede ser un caracter neutro que no afe cta la salud general de este especie de orqudea. E stos resueltos contradicen otros estudios que dicen que la asimetr a es mala para la salud de plantas, e indica que O. exasperata pueda ser una excepcin a la regla general que sime tra indica una gentica superior en plantas. INTRODUCTION Symmetry is a characteristic used by many organisms to evaluate the genetic quality of other individuals. Potential mates with relatively high l evels of fluctuating asymmetry have been shown to be less fit than more symmetrical individu als, and therefore tend to be selected against (Palmer and Strobeck 1986). Symmetry is also an imp ortant factor in plant-pollination systems, as flowers provide a visual signal to pollinators i n order to disperse their pollen. Various studies have shown that pollinators prefer symmetrical flow ers over asymmetrical ones, and that bees in particular show a preference for symmetrical flower s independent of pollinator rewards, floral color, odor and size (Mller 1995, Mller and Eriks son 1995, Mller and Sorci 1998). Giurfa et al. (1999) also note that flowers with lower levels of fluctuating asymmetry enjoy higher
visitation rates, which result in greater pollen re moval and deposition, as well as the receipt of pollen of superior quality. Given these findings, it is surprising that the Neo trpical montane orchid Oerstedella exasperata shows a persistently high level of fluctuating asym metry, both within and between individuals (Snayd 2007). However, Snayd (2007) has found that asymmetry does not have a negative effect on the reproductive success of O. exasperata as measured by pollinia removal. In addition, Snayd found that there was no correlation between the degree of bilateral asymmetry and the degree of fringe symmetry, which contradict s earlier studies (Mller 1995 and Neal et al. 1998) that indicate that asymmetry in flowering pla nts results in an overall decrease in fitness, which is the ability to survive and transmit genes to the next generation (Jenkins 1990). It is possible, therefore, that asymmetry is a selectivel y neutral trait for O. exasperata and its persistence in the species is simply due to a lack of natural selection against it. It is also known that individuals showing high leve ls of homozygosity due to inbreeding can be prone to higher levels of fluctuating asymme try (Mller and Eriksson 1995). Higher levels of homozygosity are found in organisms that are capable of self-pollination, and evidence of selfing ability in O. exasperata would be an important step toward confirming homoz ygosity as a cause of its persistent asymmetry. In addition resource constraints have been found to affect flower production in the neotropical orchid Brassavola nodosa (Murren and Ellison 1996), so it may be possible that O. exasperata also expresses asymmetry as a response to environme ntal conditions. The purpose of this study was to shed light on the cause or causes of the high asymmetry seen in O. exasperata by determining whether the orchid is capable of th e self-pollination that would imply inbreeding and homozygosity, and whethe r floral asymmetry is correlated with different floral characteristics, plant size or sun exposure. I hypothesized that, like many montane species, O. exasperata would show self-compatibility because of low pollin ator ability due to harsh environmental conditions. I predicted that fl oral asymmetry would not be correlated with other floral traits, as implied by Snayds previous study. I also hypothesized that asymmetry would be correlated with plant size and sun exposur e, because of past studies that suggest that environmental factors affect floral development. METHODS Study Organism Oerstedella exasperata is a terrestrial or epiphytic orchid distributed t hroughout the mountains of Costa Rica and Panama between 1000 and 2500 m, and is common along roadsides. It is one of the largest orchids in Costa Rica, reaching up to t hree m in size (Zuchowski 2005). It has adventitious roots and tolerates various conditions of elevation and rainfall, which helps account for its ubiquity. Its stems are highly branched, an d can reach over one cm in diameter at the base, with leaves ranging from five to fifteen cm long. I ts petals are usually greenish at the base fading to yellow toward the terminal end, and are more obl ique in shape than the oblanceolate sepals (Hgsater 1992). The column and labellum are flanke d by prominent, lobate fringes, which have a variable number of lobes. The white fringes, labe llum and callus are usually highlighted with purple markings, and the area of this coloration is also highly variable.
Study Site This study was conducted between October 30th and November 16th, 2008 near the Monteverde Cloud Forest in Costa Rica. I examined nineteen O. exasperata plants growing along the TV Tower Road leading to Cerro de los Amigos. All indi viduals were found in the clay bank of the west side of the road. Plants were marked with flag ging tape and given a unique identification letter. Self-Pollination Experiment I selected 40 young flowers with intact pollinia on nine O. exaperata plants on which to conduct my self-compatibility experiment. I self-pollinated 20 flowers and cross-pollinated 20 flowers, using a pin to remove pollinia from the pollinaria and deposit them onto the adhesive surface within the column. I then put pollinator bags aroun d the pollinated flowers to prevent pollinator interference. Pollination was performed from Novemb er 2nd to November 4th. I removed the pollinator bags on November 13th and recorded the presence or absence of a swollen pedicel and column that would presumably indicate fruit formati on and therefore fertilization. I entered the data into a contingency table and analyzed my findi ngs with a chi-square test. Asymmetry Experiment I sampled 124 individual flowers from 19 plants at the study site, labeling each flower with its parent plant letter and a unique number. I evaluate d fringe asymmetry and bilateral asymmetry following Snayds methods of taking the absolute va lue of the difference between the number of fringes on each side and the lengths of each side o f the labellum. I also evaluated petal asymmetry by taking the absolute value of the diffe rence in the lengths of the petals. I measured the length of each side of the labellum and the len gths of each of the petals using calipers. In addition to the three asymmetry measurements, I cre ated a scale to measure the amount of relative coverage of purple on the labellum, callus and fringes, and rated each flower from one (minimal purple) to five (saturation). I also weigh ed each flower to determine its mass. In addition to the floral characteristics, I determine d the size of each plant by counting the number of leaves and measuring the length of the longest l eaf (Murren and Ellison 1996). To evaluate the effects of sun exposure on plant development, I cat egorized each plant as being in the shade or sun, shade being defined as having other plants g rowing above it and preventing it from absorbing as much light as an uncovered plant. Biva riate regressions were performed to evaluate the relationship between the three different degree s of floral asymmetry, floral mass and color; between the average floral asymmetry, size and numb er of flowers on each plant; and between the number of flowers and plant size. RESULTS None of the pollinated flowers showed swollen pedic els, but as of the eleventh day of the experiment, 20 flowers did show swelling in the col umn, of which 35% were flowers that had been selfed. This implies that outcrossing may be a more common occurrence. The numbers of outcrossed flowers showing swelling and selfed flow ers lacking swelling both exceeded the expected value of ten, with thirteen flowers each. Seven selfed flowers had swelling and seven outcrossed flowers lacked swelling, both of which a re lower numbers than the expected ten
flowers. However, these differences are not statist ically significant (Figure 1). There was no significant difference between the average floral a symmetry of plants growing in different sunlight conditions, though fringe asymmetry was co nsiderably higher than lip or petal asymmetry (Figure 2). Based on statistical tests, a symmetry is not related to any of the plant characteristics tested (Table 1), nor are the three degrees of asymmetry correlated with each other (Figure 3). In addition, no correlations were found between the three degrees of floral asymmetry and flower mass (Figure 4) or flower colo r (Figure 5). However, larger plants were observed to produce significantly more flowers than smaller ones (Figure 6). n r r n r r n Figure 1: Difference between observed and expected values for presence or absence of swelling in the columns of selfed and outcrossed Oerstedella exasperata flowers in Monteverde, Costa Rica. Expected value for all = 10. N = 40 manually pollinated flowers. Chi-squared value = 3.6; df = 1; critical value = 3.84.
Figure 2: Mean value and standard error of three de grees of floral asymmetry with respect to sun conditions. Lip and petal values and corresponding error bars were multiplied by 10 to improve the resolution. N of sun plants = 11, n of shade pl ants = 8. Table 1. Statistical results of bivariate regressions compar ing fringe, lip and petal asymmetry to flower mass, flower color, size of plant and number of flowers. Values are F (p value) and R2. Fringe Asymmetry Lip Asymmetry Petal Asymmetry Flower Mass 2.259 (0.135) 0.018 0.752 (0.388) 0.006 1.559 (0.214) 0.013 Flower Color 0.281 (0.597) 0.002 0.202 (0.654) 0.002 1.134 (0.289) 0.009 # of Leaves 2.589 (0.126) 0.132 0.160 (0.695) 0.009 1.547 (0.230) 0.083 Longest Leaf 3.490 (0.079) 0.170 1.252 (0.279) 0.069 0.778 (0.390) 0.044 # of Flowers 0.0002 (0.988) 0.00001 1.758 (0.202) 0.094 0.126 (0.728) 0.007 0 0.2 0.4 0.6 0.8 1 1.2 1.4 SunShade Sun Conditions Fringe Lip Petal ! r
! # r # Figure 3: Three regressions showing: (a) relationsh ips between fringe asymmetry (absolute value of [# of fringes on left side number of fringes o n right side]) and lip asymmetry (absolute value of [length of left side of labellum length of rig ht side of labellum]) R2 = 0.010, p = 0.265; (b) relationships between fringe asymmetry and petal as ymmetry (absolute value of [length of left petal length of right petal]) R2 = 0.012, p = 0.220; (c) relationships between lip and petal asymmetry R2 = 0.003, p = 0.580. For all, n = 124 flowers.
! !! # !! $%&%' $%&' Figure 4: Mass of flower related to: a. fringe asym metry (R2 = 0.018, p = 0.135); b. lip (R2 = 0.0049, p = 0.338) and petal asymmetry (R2 = 0.0127, p = 0.214) related to floral mass. N = 1 24 flowers.
! ( # ! $%&$' $%&)%' Figure 5: Relationship between number of leaves and : a. average degree of fringe asymmetry (R2 = 0.132, p = 0.126; b. average degrees of lip (R2 = 0.009, p = 0.695) and petal asymmetry (R2 = 0.083, p = 0.230). N = 19 plants.
! ! Figure 6: Relationship between number of flowers an d number of leaves per plant. R2 = 0.509 p <0.05 N = 19 plants. DISCUSSION This study is the first to document manual pollinat ion attempts on the Monteverde population of O. exasperata as well as the first to examine the effects of su nlight on its floral morphology. Although the pollination data were not statisticall y significant, seven selfed individuals did show swelling, which suggests that O. exasperata is indeed self-compatible, though outcrossing seems to result in higher pollin ation success. Given that this species is usually found in montane conditions at midto high elevations, it could be that pollinators are often in short supply, and therefore self-pollinati on is utilized by this species when outcrossing is not possible. A repetition of the experiment with a larger sample size may provide significant support of this trend. It is of interest to note that the observed swellin g did not occur in the ovary area of the pedicel as expected, but rather in the column itsel f. This characteristic may be a precursor of fruit formation, and the time constraints of this study d id not allow for the observation of subsequent swelling in the pedicel. However, it may also be th at the swelling of the column is a response of the plant to successful pollination, but that no fr uit developed because fertilization is prevented by self-incompatibility, in which case O. exasperata would not be capable of self-pollination (Richards 1996). The pollination system of this spe cies is still poorly understood, and deserves more in-depth study. In order to avoid pollinating flowers that had alre ady received pollen, only flowers with intact pollinia were used in this experiment, as th e previous arrival of a pollinator would probably have dislodged the pollinia. However, they were not bagged as buds and then allowed to flower, which would have eliminated the chance o f previous pollination. Future studies should employ this method to obtain more accurate results.
If O. exasperata is found to be self-incompatible, it may still exp erience a high level of homzygosity due to its adventitious roots, which al low vegetative reproduction. This could result in patches of individuals who are essentially clone s of each other, which would reinforce homozygosity if its pollinators generally forage in a small area. In this case, outcrossing and selfing would both contribute to an increased level of homozygosity, as there would be no effective genetic variation between neighboring ind ividuals. It would be beneficial to perform genotyping on this population to determine whether it does indeed have the high level of homozygosity that can result in increased fluctuati ng asymmetry. The fact that floral asymmetry is not correlated wi th floral color or mass implies that it does not have a negative impact on the reproductive success of O. exasperata as more colorful and larger flowers would presumably be more attract ive to pollinators. This finding contradicts past studies on other species of flowering plants b ut agrees with Snayds previous study on the same population. It could be that this species of o rchid, or this population specifically, is an exception to the general rule that symmetrical flow ers confer a reproductive advantage. Perhaps its primary mode of reproduction is vegetative, and therefore it does not invest energy in producing symmetrical flowers to attract pollinator s. It could also be that its pollinator, as yet unknown, is a species or genus that is not attracte d by floral symmetry. Neal et al (1998) state that the environmental fact ors that trigger asymmetry may result in overall reduced fitness. If this condition were tru e for O. exasperata decreased sunlight would be expected to result in both smaller plant size an d increased asymmetry, as a plant starved of solar energy would not be able to invest extra reso urces in growth and the production of symmetrical flowers. This would lower this species fitness in terms of reproductive ability and overall health. The strong correlation between the number of leaves and flowers present on plants indicated that plant size was a good measure ment of both overall fitness and potential reproductive success. However, floral asymmetry was not correlated with sun exposure or plant size, which implies that higher asymmetry is not an indicator of poor plant quality, nor is it affected by environmental conditions. Additional st udies evaluating the effects of other environmental factors such as substrate, rainfall a nd elevation on asymmetry are needed to confirm this prediction. In addition, as Snayd (200 7) observed, one would expect a correlation between all measures of asymmetry if environmental factors do affect plant health. However, this study found no correlations between fringe, li p and petal asymmetry, which further suggests that increased asymmetry does not decrease this spe cies overall fitness. In conclusion, floral asymmetry in O. exasperata appears to be a neutral trait, showing no negative, predictable relationship with other flora l characteristics, plant size or levels of sunlight This conclusion contradicts previous studies that h ave found that symmetrical flowers have higher reproductive success, which implies that O. exasperata may be an exception to the general rule that floral asymmetry is a detrimental trait in plants. The high level of asymmetry in this population could be perpetuated due to a high level of homozygosity caused by either selffertilization or low levels of genetic variation as a result of vegetative reproduction. Future studies should focus on the pollination system of t his orchid, its genetic makeup and the effects of different environmental conditions on floral asy mmetry. ACKNOWLEDGMENTS I am eternally grateful to the Estacin Biolgica M onteverde for the opportunity to perform this study; Karen Masters for her orchid expertise and p atient help with statistics; Alan Masters for creating this truly unique study abroad experience; Chanchos de Monte for memorable evenings;
Pablo Allen and Moncho Caldern for supplies, snack s, humor and additional assistance with statistics; my classmates for companionship and com miseration; and whichever higher power, scientific or spiritual, that created the magical p lace that is the tropical rainforest. LITERATURE CITED Giurfa, M, A. Dafni and P. R. Neal. 1999. Floral Sy mmetry and Its Role in Plant-Pollinator Systems. International Journal of Plant Sciences 16 0: S41-S50. Hgsater. 1992. Oerstedella exasperata In: Orchids of Costa Rica, Part 2 D.E. Mora de Retana and J.T. Atwood eds. The Marie Selby Botanical Gard ens, Sarasota, FL, pp.1460-1461. Jenkins, J. B. 1990. Human Genetics Harper and Row, New York, NY. As cited on Webref.org.
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Asimetra de las estructuras florales y la posibilidad de auto-polinizacin en Oerstedella exasperata (Orchidaceae)
Asymmetry of floral structures and the possibility of self-pollination in Oerstedella exasperata (Orchidaceae)
The purpose of this study was to investigate the causes of the pervasive asymmetry found in the neotropical orchid Oerstedella exasperata. Previous studies have found that high levels of asymmetry decrease reproductive success, and can be caused by a high amount of homozygosity within a population. This study tested the ability of O. exasperata to self-pollinate, a condition that indicates the possibility of a high level of homozygosity. In addition, the relationships between three degrees of asymmetry and floral color, floral mass, plant size and sun exposure were investigated to determine whether higher asymmetry is correlated with lower reproductive success or reduced overall plant fitness. Results showed an overall trend suggesting that self-pollination is possible, which implies that O. exasperatas high level of asymmetry could be caused by homozygosity. Floral asymmetry was not related to any of the other examined factors, implying that it is a neutral trait that does not affect the overall fitness of this species of orchid. These results contradict other studies findings that asymmetry is detrimental to plant health, indicating that O. exasperata could be an exception to the general rule that symmetry indicates superior genetics in plants.
El propsito de este estudio fue investigar las causas de asimetra en la orqudea neotropical Oerstedella exasperata. Estudios anteriores han reportado que los niveles elevados de asimetra bajan el xito reproductivo, y pueden ser causados por homocigosis alta en una poblacin. Este estudio probo la capacidad de O. exasperata de auto-polinizarse, una condicin que indica la posibilidad de un nivel alto de homocigosis.
Text in English.
Orchids--Costa Rica--Puntarenas--Monteverde Zone
Orqudeas--Costa Rica--Puntarenas--Zona de Monteverde
Orqudeas polinizacin--Costa Rica
Tropical Ecology 2008
Oerstedella exasperata--Species fitness
Ecologa Tropical 2008
Oerstedella exasperata--Estado fsico de las especies
t Monteverde Institute : Tropical Ecology