«Instituto de Genética e Bioquímica, Universidade Federal de Uberlândia, Uberlândia, MG, Brasil Instituto de Biologia, Universidade Federal de ...»
Climatic and anthropic influence on size and
fluctuating asymmetry of Euglossine bees
(Hymenoptera, Apidae) in a semideciduous
seasonal forest reserve
M.C. Silva1, C. Lomônaco2, S.C. Augusto2 and W.E. Kerr1
Programa de Pós-Gradução em Genética e Bioquímica,
Instituto de Genética e Bioquímica,
Universidade Federal de Uberlândia, Uberlândia, MG, Brasil
Instituto de Biologia, Universidade Federal de Uberlândia,
Uberlândia, MG, Brasil
Corresponding author: C. Lomônaco
Genet. Mol. Res. 8 (2): 730-737 (2009) Received December 12, 2008 Accepted January 26, 2009 Published June 23, 2009 ABSTRACT. We examined the influence of climate and man on size and fluctuating asymmetry in two species of Euglossine bees collected from a semideciduous forest reserve. Sixty males of each species were collected; four measurements were made of their wings to obtain a multivariable size index and a fluctuating asymmetry index.
No significant differences in the size of Eulaema nigrita Lepeletier were found between the areas and seasons. Larger males of Euglossa pleosticta Dressler were collected during the hot and wet season;
however, male size did not vary with location. Higher rainfall and a consequent increase in food availability could have influenced the increase in size of E. pleosticta. Bees collected during the hot and wet season at the forest border were more asymmetric than bees collected during the cold and dry season; the latter were found inside the forest.
This indicates that climate and anthropic interferences influence the stability of development of E. pleosticta. Consequently, this species Genetics and Molecular Research 8 (2): 730-737 (2009) ©FUNPEC-RP www.funpecrp.com.br Size and fluctuating asymmetry in Euglossine bees 731 could be used as a bioindicator of stress. Apparently, E. nigrita is more resistant to environmental interference.
Key words: Phenotypic plasticity; Development stability;
Stress bioindicators; Eulaema nigrita; Euglossa pleosticta InTRoduCTIon Environmental factors may cause phenotypic modifications by altering morphological, physiological and behavioral traits; most individual responses to environmental distress are due to phenotypic plasticity (Via and Lande, 1985). This latter term refers to the capability of an organism to alter its phenotypic characteristics without major genetic changes (Scheiner, 1993). Phenotypic plasticity can be considered an important adaptive tool for survival in unstable and heterogeneous environments (Parsons, 1990), including those that vary due to human interference.
Another source of phenotypic variation is developmental noise, which refers to modifications in the normal development program (Markow, 1995). The most common index utilized to describe phenotypic variations caused by developmental noise is fluctuating asymmetry (FA). This latter term is defined as small random deviations from perfect symmetry of a trait in organisms that have bilateral symmetry (Parsons, 1990). Given that expression of both sides of a bilateral trait is due to the same genes, then any asymmetry between the sides is a consequence of environmental disturbance. Therefore, FA could be a good indicator of stress levels or environmental impact (Markow, 1995).
On the other hand, organisms also have the ability to resist genetic and environmental disturbances during their development, through buffering mechanisms, allowing production of a pre-determined phenotype within normal limits of variation in expression. This ability is defined as developmental homeostasis, which is constituted of two components: canalization and developmental stability. Thus, an organism asymmetry may also serve as an indicator of how much of its development is being buffered under stress conditions; because it is inversely correlated with developmental homeostasis, it is frequently used to estimate developmental stability.
Euglossine bees (Hymenoptera, Apidae), found only in the Neotropics (Dressler, 1982), are good indicators of the environmental quality of natural and human-modified ecosystems (Morato, 1994; Peruquetti et al., 1999).
Eulaema nigrita Lepeletier and Euglossa pleosticta Dressler are widely distributed throughout Central America (Ackerman, 1983) and are also found as far as the extreme south of Brazil (Wittmann et al., 1988; Rebêlo and Garófalo, 1997; Nemésio, 2002; Neves and Viana, 2003; Sofia et al., 2004; Nemésio and Silveira, 2006). Contrary to E. pleosticta, E. nigrita is very common in habitats that have suffered human influence (Silveira and Cure, 1993). We examined climatic and anthropic influences on size and FA of these two species of Euglossine bees in a semideciduous forest reserve.
MATERIAL And METHodS
Specimens of E. nigrita and E. pleosticta were deposited in the entomological collection of the Laboratório de Ecologia de Insetos of the Universidade Federal de Uberlândia. They were collected monthly for 12 months (from December 1998 to November 1999), in a semideciduous Genetics and Molecular Research 8 (2): 730-737 (2009) ©FUNPEC-RP www.funpecrp.com.br M.C. Silva et al.
forest area (20°46’12’’S and 47°14’24’’W, altitude between 775 and 836 m). This area is a remnant forest of 49 ha surrounded by an area of 98 ha of unaltered Neotropical Savannah (cerrado).
Bees were collected from two sites: site A, located at the forest border close to the administration building, and site B, approximately 1000 m apart from site A, located in the interior of the forest fragment.
The climate of this area is between tropical and altitudinal tropical (Awb, accordingly to the Köeppen International System), characterized by the occurrence of two seasons: a humid and hot season from September to April (1400-1550 mm of rain and average temperature varying from 18.4° to 29°C), and a dry and cold season from May to August (35 mm of rain during the driest month and an average temperature of 23.5°C) (Teixeira et al., 2004).
Sixty males of each species were randomly selected, 30 from the forest border and 30 from the forest interior. Half were collected during the hot and wet season and the other half during the dry and cold season. All individuals had their wings (right and left) detached from the body; these were dry mounted on microscope slides, with a cover slip. Wing veins provide many well-defined morphological landmarks in an essentially plane structure and are commonly used in morphological analysis (Klingenberg and McIntyre, 1998). The slides were scanned and four measurements were made from the images using the computer program Adobe Photoshop, version 6.0. The following wing segments were measured (Figure 1). Each of these segments was measured three times.
Figure 1. Schematic representation of a bee wing showing the morphological characters of Eulaema nigrita and Euglossa pleosticta measured (adapted from Silveira et al.
, 2002). For abbreviations, see legend to Table 2.
Measurements were made of both sides in order to evaluate FA, calculated as the mean difference between the right and the left side, i.e., FA = [(Σ | (R - L) | / n] (Palmer and Strobeck, 1986). A two-factor analysis of variance (ANOVA) was used to determine whether the between-sides variation was significantly larger than the measurement error (Woods et al., 1998;
Perfectti and Camacho, 1999). According to Palmer and Strobeck (1986), it is necessary to distinguish FA from other kinds of asymmetry. A t-test was performed to determine whether the Genetics and Molecular Research 8 (2): 730-737 (2009) ©FUNPEC-RP www.funpecrp.com.br Size and fluctuating asymmetry in Euglossine bees 733 means of the signed right minus left distribution were significantly different from zero. Asymmetry was tested for by looking for significant departures of the right-left frequency distribution from normality using the Kolmogorov-Smirnov test. Size dependence of FA was tested by regressing the unsigned, absolute difference of the right minus left measurements on trait size.
The first of the three measurements was used in the principal component analysis (PCA) in order to obtain a general body size index, estimated by a correlation matrix of the original characters (Manly, 1994). Then, the transformed data (FA and size index) were analyzed with ANOVA to determine the effect of seasonal and anthropic influences (Zar, 1984).
All statistical procedures were performed using the computer software package Systat for Windows, version 9.0 (Willkinson, 1986).
Since the original morphological variables of E. nigrita and E. pleosticta were significantly correlated, the PCA was considered adequate to estimate the multivariate size index, the score distribution of which was normally adjusted (Dmax = 0.078, P = 0.861 and Dmax = 0.057, P = 0.991, respectively). The first component, which is the axis that varies most among individuals, explained 82.4% of the total variance for E. nigrita, and 78.6% for E. pleosticta morphological data.
Based on two-way ANOVA, there were no significant differences in E. nigrita sizes between areas (F = 0.251, P = 0.618) and seasons (F = 0.568, P = 0.454), nor were there significant interactions between these two factors (F = 0.413, P = 0.523; Table 1). There were significant differences in E. pleosticta sizes between seasons (F = 20.947, P = 0.001), but no differences in size were found between areas (F = 0.109, P = 0.743), and there was no significant interaction between areas and seasons (F = 2.405, P = 0.127; Table 1).
Table 1. Two-way ANOVA (considering climatic seasons and areas of human influence as factors) of a multivariate index of size of Eulaema nigrita and Euglossa pleosticta collected in a semideciduous seasonal forest reserve in the municipality of Patrocínio Paulista (SP, Brazil) from December 1998 to November 1999.
The measurement error of FA was negligible in the analysis of the two species, based on the highly significant side x individuals effect on two-way ANOVA (F = 2.470, P 0.001 for M1; F = 27.968, P 0.001 for M2; F = 88.040, P 0.001 for M3; F = 55.115, P 0.001 for M4 of E. nigrita, and F = 3.633, P 0.001 for M1; F = 4.226, P 0.001 for M2; F = 1.016, P
0.001 for M3; F = 1.000, P 0.001 for M4 of E. pleosticta). All FA distributions were nor
mal, and no evidence of antisymmetry was found. No correlations between size and FA were observed and the asymmetry in all traits fluctuated around a mean of zero (Table 2).
Table 2. Asymmetry, antisymmetry and fluctuating asymmetry correlation with size for four characters of two species of Euglossine bees.
M1 = distance between the distal extremity of the radial vein (Rs) and the interception of the second medial vein (2nd M-Cu) with cubital 1 (Cu 1); M2 = distance between the distal extremity of Rs to the interception of the medial vein (M) with the proximate portion of the Rs vein; M3 = distance between the interception of M with the proximal portion of Rs and the interception of the cubital (Cu) vein with the alar vein (1A), and M4 = distance between the interception of Cu and 1A and the 2nd M-Cu.
Based on ANOVA, there were no differences in E. nigrita FA indexes between areas and seasons, and there were no significant interactions between these two factors. For E. pleosticta the interaction of the factors used in two-way ANOVA was significant, but each factor independently did not affect FA (Table 3).
The environment of the forest border area did not affect E. nigrita or E. pleosticta body size, indicating that environmental degradation caused by anthropic influence was not intense. This could also be because the two sites were not distant enough to permit isolation of the feeding areas. According to Sofia and Suzuki (2004) Euglossine males have great vagility and are able to transpose forest fragments separated by distances greater than 1500 m (Janzen, 1971). For this reason, they are considered to be important Neotropical pollinators, especially for sparse vegetation species (Zucchi et al., 1969; Dressler, 1982), including some orchids that are exclusively visited by them (Williams and Whitten, 1983). Some researchers have investigated the impact of deforestation and forest fragmentation on bee populations; they found a strong decrease in the abundance of individuals (Morato, 1994; Tonhasca Jr. et al., 2003).
Body size of E. nigrita was not affected by season, different from what was found by Peruquetti (2003) in the Atlantic Forest, where variation in body biomass was found between males collected during distinct seasons. However, we did find seasonal variations in E. pleosticta size, which is similar to what was reported by Peruquetti (2003), who found that males collected during the hot and wet season were larger than males collected during the cold and dry season. The higher rainfall levels and the consequent increase in Genetics and Molecular Research 8 (2): 730-737 (2009) ©FUNPEC-RP www.funpecrp.com.br Size and fluctuating asymmetry in Euglossine bees 735 Table 3. Two-way ANOVA (considering climatic season and human influence as factors) of the fluctuating asymmetry (FA) of four characters for two species of Euglossine bees collected in a semideciduous seasonal forest reserve in the municipality of Patrocínio Paulista (SP, Brazil) from December 1998 to November 1999.
SS = sum of squares; d.f. = degrees of freedom; MS = mean square. For M1-M4 abbreviations, see legend to Table 2.
food availability could have an important effect on the size of E. pleosticta (Crozier and Pamilo, 1996; Peruquetti, 2003).
Body size influences several bee traits related to individual fitness. For females, larger bodies may give greater reproductive success due to better competitive ability and better capacity to do cell provisioning and greater egg production (Tengo and Baur, 1993). For males, greater body size may assure competitive reproductive success and better dispersion ability (Kukuk, 1996). Body size has low heritability; it is generally influenced by environmental and maternal factors, which may cause sufficient variability to affect reproductive capacity (Molumby, 1997).
Genetics and Molecular Research 8 (2): 730-737 (2009) ©FUNPEC-RP www.funpecrp.com.br M.C. Silva et al.