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Directional and Fluctuating Asymmetry in the Black-Winged Damselfly Calopteryx Maculata (Beauvois) (Odonata: Calopterygidae)

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Directional and Fluctuating Asymmetry in the Black-Winged Damselfly Calopteryx Maculata (Beauvois) (Odonata: Calopterygidae) Directional and fluctuating asymmetry in the black-winged damselfly Calopteryx maculata (Beauvois) (Odonata: Calopterygidae). Jason Pither and Philip D. Taylor J. Pither (correspondence). Department of Biology, Queen’s University, Kingston, ON K7L 3N6. email: [email protected] P.D. Taylor. Department of Biology, Acadia University, Wolfville, NS B0P 1X0. Abstract Directional asymmetry (DA) has received considerably less attention than fluctuating asymmetry (FA) in the literature. Evidence for DA, however, is building am ong insect taxa. We examined asymmetries in two wing traits within both sexes of the damselfly Calopteryx maculata (Beauvois) (Odonata: Calopterygidae), sampled from three sites in southeastern Ontario. After accounting for measurement error, we show that proximal segments within right fore and hind wings are consistently longer than left in all but one sample group. Full wing lengths, however, exhibited fluctuating rather than directional asymmetry. Mean asymmetry values for both traits (segment and length) occurred in the direction of right-wingedness significantly more often than expected by chance. Patterns of asymmetry were generally consistent among the sexes and sites, although males tended to exhibit more pronounced DA. W e suggest that the wings of C. maculata may undergo compensatory development such that full lengths are more bilaterally symmetric than their component parts. Introduction flies, Klingenberg et al. 1998; wood butterfly; Windig and Nylin 1999; bumblebees; Klingenberg pers. comm.). Of the possible forms of subtle asymmetry in The degree to which methodological problems (Palmer bilateral characters, fluctuating asymmetry (FA) has and Strobeck 1997) have served to bias the literature received the most attention (e.g. Palmer 1994; Leung and away from reporting DA is unknown, but possibly Forbes 1996; Møller and Swaddle 1997). FA is defined significant (Rowe et al. 1997). T his bias may have been as subtle, random deviations from perfect symmetry (Van enhanced by the fact that DA is commonly deemed Valen 1962). FA arises when the stabilising processes uninterpretable in relation to stress or fitness relations inherent to organism development (i.e. developmental (the most common goal of FA papers; Graham et al. homeostasis) are unable to buffer against disruptive 1998). Indeed, whether in fact DA can be used in any factors during development (Palmer 1996). Increased way to infer developmental stability is still debated (e.g. levels of FA - beyond those which are expected solely Palmer 1994; M oller and T hornhill 199 7; Palmer and from the interplay of developmental "noise" and Strobeck 1997; Graham et al. 1998). stabilising processes - have been correlated with extreme Consider, for example, the case of FA in conditions of environment (e.g. high levels of pollutants; damselflies (Odonata: Zygoptera). Prior to 1997, several Graham et al. 1993), and restricted gene flow (e.g. papers purported to have found ideal FA in wing length, inbreeding or genetic bottlenecks; Leary et al. 1985; and reported that FA was correlated with measures of Wayne et al. 1986). Its potential as a tool for monitoring individual fitness and / or quality (Harvey and W alsh stress levels in natural populations, and as an indicator of 1993; Cordoba-Aguilar 1995; Bonn et al. 1996). individual quality or fitness (e.g. Leung and Forbes 1996; However, none of these papers properly evaluated the Moller 1997), has thus prompted this attention (Leary and contributions of both measurement error and other forms Allendorf 1989; Clarke 1993; Graham et al. 1993; Palmer of asymmetry (i.e. D A) to their quantification of FA. 1996). Subtle directional asymmetry (DA), the case Interestingly, subsequent experiments that used when one side’s character value is consistently greater appropriate methodology found inconsistent relationships than the other, has received considerably less attention between FA and measures of individual fitness and (e.g. Palmer 1994; Kraak 1997; Møller and Swaddle quality (Leung and Forb es 1997; Forbes et al. 1997), and 1997; Rowe et al. 1997). W e are aware of only a between FA and environmental stress (Hardersen and handful of studies that document the existence of Wratten 1998; Hardersen et al. 1999). Furthermore, significant DA within insects (crickets, Simmons and using these more appropriate methods, Leung and Forbes Ritchie 1996; honeybees, Smith et al. 1997; houseflies, (1997) and Hardersen et al. (1999) found some evidence Goulson et al. 1999; fruit flies, house flies, and tsetse for DA in the wings of their subject species. Thus, whether or not differences in damselfly wing lengths 0.05 mm x 0.05 mm. All measurements are presented in conform to a distribution consistent with ideal FA millimetres. (normal or leptokurtic distribution with mean zero; The scanned images of the wings were enlarged Palmer and Strobeck 1992) is debatable. Indeed, given on the computer screen to approximately 11 times their the recent findings of DA in several insect taxa (above), normal size and were measured using an on-screen it could be that shortcomings in methodology were pointer that is part of a freeware image analysis software masking a phenomenon that is widespread in the insect program (Image Tools; University of Texas). On all world (Klingengberg et al. 1998). individuals we measured a segment of the wing that runs As part of a previous study relating wing parallel to the R1 vein of the fore and hindwings - morphology to landscape structure (Pither 1997), we henceforth referred to as the “segment” (Fig. 1). On a found preliminary evidence suggesting the presence of subset of the individuals we measured a trait that better DA in the wings of the damselfly Calopteryx maculata represents the full length of the wing - henceforth referred (Beauvois) (Odonata: Calopterygidae) within several to as the “length” - (Fig. 1). Pigmentation of the distal populations sampled in Nova Scotia. Here we present the portions of male wings was too dark to measure wing results of a follow-up study, conducted in eastern Ontario, length on all individuals (some wings were also slightly that set out to more explicitly exam ine patterns of damaged distally). Thus, for 14 males and 14 females (to asymmetry in the wings of males and females of this maintain balanced sample sizes) from each site (total 42 species. We determine whether fluctuating or directional individuals per sex), we performed this second asymmetry is present within multiple traits, and compare measurement on the forewings and hindwings. All patterns of asymmetry among the sexes and three sample measurements were taken twice (using the same image, sites. see below) by the same person, each time in a haphazard order. Materials and Methods (c) Measurement error (a) Sample collections Two potential sources of measurement error Twenty five male and 25 female C. maculata exist: human measurement error and error due to the were collected from each of three streams in the vicinity scanning process. The former source of error was of the Queen’s University Biological Station (QUBS) in evaluated for both traits using mixed-model ANOVAs, eastern Ontario, Canada: Forsyth Creek (henceforth following the methods of Palmer and Strobeck (1986). called Forsyth); (391000 m E, 4933300 m N), Peterson To test for both potential sources of error simultaneously Creek (Peterson); (394200 m E, 4933000 m N), and (thus providing an additional test of the human Black Creek (B lack); (390200 m E , 4954500 m N). Only measurement error), we scanned a total of 18 individual reproductively mature individuals were collected (ad ult wings (9 left and 9 right of a pair) twice, at perpendicular and immature damselflies are easily distinguished visually orientations to each other, and repeated segment by eye), and all sam pling was conducted over a short measurements (in a haphazard order) for each scanned period of time within 14 days of the first emergence image twice. We then ran a mixed-model ANOVA with recorded at each of the three sites. Maturation period for “scan” and “individual” each as random effects this species is typically 7-11 days (Waage 1972). (individual nested within scan), and repeated Collection dates (all in 1998) were 28 May (Peterson), 31 measurements as the residual error. This enabled a May (Black), and between 29 May and 1 June (Forsyth). partitioning of variance components due to the scanning A standard insect flight net was used, and individuals effect (random effect 1, 1 df), the difference in segment were placed within mesh cages upon capture. Within two size among individuals (random effect 2, 16 df), and hours of capture all individuals were placed in vials and within-individual measurement errors (residual error, 18 stored in a freezer at -10°C until measured. All df) (cf. Yezerinac et al. 1992). This entire exercise was individuals received identical treatment throughout. conducted for each sex. We also scanned five individual (b) Measurements female wings from the front and back (i.e. flip sides), and The forewings and hindwings of each individual were conducted a similar procedure. We additionally tested clipped at their base, and fastened between two acetate for a difference in calculated left minus right values of the sheets (forming an envelope) using clear adhesive tape. segments (average of two left measurements minus The first millimetre of the proximal end of each wing was average of two right measurements) for the two “flip taped to one of the sheets. Each envelope of acetate side” scans. sheets (containing 15 - 18 individuals’ wings) was (d) Testing for normality scanned once at a resolution of 600 dpi using an Hewlett Asymmetry for each trait was taken as the Packard 4c Scanjet flatbed scanner with a background average of left minus the average of right repeated light source (i.e. a transparency adapter). Scanning at this measurements. The resulting asymmetry data for each resolution produces images whose pixel dimensions are site and sex were tested for normality using a Fig.
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