The Sex Life of Spiders (or Adventures in Spider Sexual Biology)

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Genetic Variability and Gene Flow in Metepeira Ventura (Araneae, Araneidae)

Levels of genetic variability and gene flow among three populations of Metepeira ventura o n Santa Catalina Island, California, were evaluated based on variation at 10 gene loci . Mean heterozygosity (observed) per population was 10 .4% and mean polymorphism was 36 .7%, consistent with levels of variabilit y in other arthropods. Values of FST for the five polymorphic loci (mean FST = 0 .009) suggest that gene flow prevents the genetic differentiation of these populations . The average number of migrants per generation (Nm) among these populations is estimated to be 28 .6 . The lack of inter-population genetic differentiation may resul t from aerial dispersal and/or crawling along vegetation by M. ventura . Such similarity may also be due to the more widespread vegetative cover of Santa Catalina prior to overgrazing, which may have physically unite d these populations in the recent past, allowing for gene flow among them

Evidence for Multiple Paternity in Broods of the Green Lynx Spider Peucetia viridans (Araneae: Oxyopidae)

In the green lynx spider Peucetia viridans (Hentz 1832), the two openings of a mated female’s epigynum are often sealed by copulatory plugs, sometimes with the two-pronged distal portion of the paracybium of a male palpus inserted in each opening and embedded in the plugs. The presence of copulatory plugs and paracymbia may prevent further mating by the female. However, not all mated females exhibit these structures, perhaps allowing some P. viridans females to mate with more than one male, despite the assertion of Whitcomb & Eason (1965) that females only mate once. We investigated this possibility by surveying the extent of multiple paternity in field-collected P. viridans broods from southern California. For adult females and their egg sacs, we determined the aspartate aminotransferase genotype for each mother and her spiderlings using allozyme electrophoresis in order to assess whether the progeny data best fit with a single male as the father. Two broods exhibited clear evidence of multiple paternity, verifying that multiple mating by females is possible in this species. Although most mothers of single paternity broods had one or both epigynal orifices blocked, some had no blockage at all, while the two mothers of multiple paternity broods had some kind of blockage to one or both orifices, suggesting that neither plugs nor inserted paracymbial processes are associated with a reduction in female remating

Phylogeny and Historical Biogeography of the Spider Genus Lutica (Araneae, Zodariidae)

Spiders of the genus Lutica from 19 populations in southern California and Baja California, including all the California Channel Islands except Anacapa, were compared electrophoretically on the basis of variability at 15 gene loci. Fixed allelic differences clearly define two species: new species A [Santa Barbara and Ventura Counties, northern Channel Islands (San Miguel, Santa Rosa, Santa Cruz), southern Channel Islands (San Nicolas, Santa Barbara, Santa Catalina)] and new species C [Guerrero Negro, central Baja California], while morphological features define two others: new species B [Los Angeles, Orange and San Diego Counties, northern Baja California] and clementea [San Clemente Island] . Phylogenetic analysis of the electrophoretic data using a variety of methods revealed that evolutionary rates among the populations sampled have been very unequal . The phylogenetic relationships among populations consistently supported by the electrophoretic cladograms generally correspond with the geological history of the Channel Islands and adjacent mainland and suggest certain likely colonization events involving some of the islands

Web Orientation of the Banded Garden Spider Argiope Trifasciata (Araneae, Araneidae) in a California Coastal Population

Magnetic heading (direction the spider’s ventrum faced) and web inclination (degree of slant from the vertical) in Argiope trifasciata Forska°l, a diurnal orb-weaving spider, were studied at a coastal site in southern California for nine weeks in fall 1999. Throughout the study, A. trifasciata largely occupied east-west oriented webs with their venters facing south and southwest. Mean magnetic heading was unaffected by ambient temperature extremes. Inclination levels varied from week to week over an approximately 58 range. However, these changes followed no discernible pattern and were independent both of the sun’s decrease in maximum altitude during the course of the study and of magnetic heading. Since the ventrum of A. trifasciata is dark, the consistent southern orientation exhibited by spiders at our study site suggests that they sought to maximize solar radiation in an attempt to gain heat. In addition, the east-west, facing-the-sun orientation of webs at this site places them parallel to the prevailing western breezes, minimizing their exposure to wind disturbance. As for inclination, it is unclear what factors may be influencing the degree of slant in A. trifasciata webs and further study will be needed to ascertain what they might be

Gene flow among habitat patches on a fragmented landscape in the spider Argiope trifasciata (Araneae: Araneidae)

The banded garden spider (Argiope trifasciata) is a common orb weaver of old field habitats in the United States. In this study, we determined levels of genetic variability and gene flow among seven populations in central Pennsylvania, based on variation at eight allozyme loci. Mean heterozygosity (observed) per population was 7.5% and mean polymorphism was 39.3%, consistent with levels of variability in other arthropods. Values of GST for the four polymorphic loci (mean GST=0.011) suggest that gene flow prevents the genetic differentiation of these populations. The average number of migrants per generation (Nm) among these populations is estimated to be 31.3. The lack of significant interpopulation genetic differentiation among these disjunct populations may result from spiderling aerial dispersal (ballooning), a more continuous distribution of suitable habitat in the past, and perhaps the use of roadside vegetation as gene flow corridors. On the other hand, the study populations did not exhibit isolation-by-distance, suggesting that suitable habitat in our study area is experienced as less than continuous by A. trifasciata. Thus, although A. trifasciata is an excellent ballooner, ballooning does not confer unlimited access among all populations, which suggests that ballooning may be a far less effective means of long-distance dispersal than previously thought

Heterozygosity and fitness in a California population of the labyrinth spider Metepeira ventura (Araneae, Araneidae).

The relationship between individual heterozygosity and characteristics likely to be associated with fitness was investigated in the labyrinth spider Metepeira ventura. Adult females and their egg sacs were collected at a coastal site in southern California, and three measures of bodily condition (carapace width, weight, residual index) and six measures of reproductive output (number of egg sacs, variation in egg number among sacs [coefficient of variation], total number of eggs, mean eggs/sac, mean eggs/sac divided by carapace width, mean eggs/sac divided by weight) were determined for each spider. The sample was polymorphic at three allozyme loci that were in Hardy-Weinberg equilibrium, and individual females were heterozygous at up to two of the three loci, forming three heterozygosity classes (0, 1, and 2). None of the bodily condition measures were significantly related to the number of heterozygous loci, while four of the reproductive output estimators (total number of eggs, mean eggs/sac, mean eggs/sac divided by carapace width, mean eggs/sac divided by weight) were significantly influenced by heterozygosity. In each significant case, values for class 2 females were less than those for class 0 and 1 females, whose values were usually more similar. Thus, while female bodily condition was comparable among classes, the most heterozygous females produced fewer total eggs and eggs per sac than their less heterozygous peers. The fact that females of M. ventura engage in a reproductive investment-number trade-off suggests that high-variability and low-variability females may be pursuing distinct reproductive strategies in the wild, with more heterozygous females being K-selected (smaller clutches, heavier eggs) and more homozygous females being r-selected (larger clutches, lighter eggs). Further investigation will be needed to assess more fully the fitness value of heterozygosity in M. ventura

Male Remating Success and the Frequency of Copulatory Plugs in the Green Lynx Spider Peucetia viridans (Araneae, Oxyopidae)

Peucetia viridans males were allowed to mate with three virgin females and most matings resulted in live spiderlings, even when males lacked palpal paracymbial processes. Among females, the presence of copulatory plugs was inconsistent, and when present, their condition was not uniform; broken-off male paracymbia were often found in epigynal orifices. There was no size effect in a male’s ability to completely plug a female’s genital orifices, as well as no significant change in the pattern of plug production over consecutive mating trials. Among mated, field-collected females, the presence of plugs and paracymbia was variable, with females from some sites possessing neither structure. Field-collected females with no plugs were in significantly better condition than those with two plugs and in nearly significantly better condition than those with two paracymbia. Females in the best condition may excel at resisting the emplacement of genital obstructions and/or voiding such structures, potentially enabling them to mate with multiple males. Enhanced prey access mediated by increased water availability may be why females at two sites were in relatively better condition. If plugs help prevent sperm desiccation in inseminated females, this may have contributed to the absence of plugs from females at these two moister sites

Relationships between physiological characteristics and trace metal body burdens of banded garden spiders Argiope trifasciata (Araneae, Araneidae)

Banded garden spiders (Argiope trifasciata) were collected at the Ballona Wetlands, a metal contaminated salt marsh. The relationship between spider body size and individual metal loads was investigated. Biochemical markers were identified in spider fecal material and found to correlate to body metal levels. Body metal dry weight concentrations of Cd, Cr, Cu, Zn and total metals in female A. trifasciata exhibited distinct patterns of spatial and annual variation during 2006 and 2007. Spider body size was homogeneous across sites in both years, while increased Cd and Cr concentrations were sometimes associated with a reduction in spider size, though the influence of Cr was quite minor. Spiders with higher body Cu levels showed a reduction in peak area for hypoxanthine and an un-identified component in fecal material chromatograms. Spatial and annual differences in metal bioaccumulation are likely mediated by variation in site-specific environmental parameters and rainfall, while the negative relationships between body size and metal levels are presumably a consequence of a spider\u27s expenditure of energy for metal tolerance mechanisms vs. foraging and growth. Finally, correlating body metal levels with excreta products constitutes a novel method to non-invasively predict metal levels in spiders

Evaluation of static network equivalent models for N-1 line contingency analysis

Although large scale simulation models may better represent the behavior of practical power systems, they are time-consuming and turn out to be impractical for some desired applications, particularly when the focus of the study is on only a small portion of the entire system and the use of a complete model would dramatically increase the computational effort and time. Therefore, network equivalent models can be used in this case to facilitate and accelerate the completion of the required analysis related to the specific subsystem. In this sense, and considering here small and large sample networks, the evaluation of static equivalents derived from popular Ward and REI reduction methods is presented in this paper. The performance of the reduced networks is assessed under the N-1 contingency criteria, and takes into account not only the base case condition but also post contingency response with the whole set of lines in the particular area of study. Obtained equivalents are compared in simulation in terms of deviations from the original system and average computational time needed to complete the involved power flow calculations