Pollinator community structure and sources of spatial variation in plant–pollinator interactions in <Emphasis Type="Italic">Clarkia xantiana</Emphasis> ssp. <Emphasis Type="Italic">xantiana</Emphasis>

The structure of diverse floral visitor assemblages and the nature of spatial variation in plant–pollinator interactions have important consequences for floral evolution and reproductive interactions among pollinator-sharing plant species. In this study, I use surveys of floral visitor communities across the geographic range of Clarkia xantiana ssp. xantiana (hereafter C. x. xantiana) (Onagraceae) to examine the structure of visitor communities, the specificity of the pollination system, and the role of variation in the abiotic vs. biotic environment in contributing to spatial variation in pollinator abundance and community composition. Although the assemblage of bee visitors to C. x. xantiana is very diverse (49 species), few were regular visitors and likely to act as pollinators. Seventy-four percent of visitor species accounted for only 11% of total visitor abundance and 69% were collected in three or fewer plant populations (of ten). Of the few reliable visitors, Clarkia pollen specialist bees were the most frequent visitors, carried more Clarkia pollen compared to generalist foragers, and were less likely to harbor foreign pollen. Overall, the core group of pollinators was obscured by high numbers of incidental visitors that are unlikely to contribute to pollination. In a geographic context, the composition of specialist pollinator assemblages varied considerably along the abiotic gradient spanning the subspecies range. However, the overall abundance of specialist pollinators in plant populations was not influenced by the broad-scale abiotic gradient but strongly affected by local plant community associations. C. x. xantiana populations sympatric with pollinator-sharing congeners were visited twice as often by specialists compared to populations occurring alone. These positive indirect interactions among plant species may promote population persistence and species coexistence by enhancing individual reproductive success.Electronic Supplementary Material Supplementary material is available for this article at     

The evolution of male genitalia: patterns of genetic variation and covariation in the genital sclerites of the dung beetle Onthophagus taurus

Three main hypotheses, have been invoked to explain divergent genital evolution, the lock and key, pleiotropy, and sexual selection hypotheses, each of which make different predictions about how genital traits are inherited. Here we used a half-sib breeding design to examine the patterns of genetic variation and covariation between male genital sclerites, and their covariance with general body morphology in the dung beetle Onthophagus taurus. We found CV(A)'s and CV(P)'s were similar for both genital and general morphological traits and that CV(R)'s were large for both trait types. We found that male genital sclerites were negatively genetically correlated with general morphological traits. Variation in male genital morphology has direct implications for a male's fertilization success and the resulting sexual selection acting on male genitalia is predicted to maintain high levels of additive genetic variance. Contrary to this prediction, we found that individual genital sclerites all had low levels of additive genetic variance and large maternal and environmental sources of variation. Our data suggest that the genital sclerites in O. taurus are not inherited independently but as a genetically integrated unit. More importantly, the way the different sclerites function to influence male fertilization success reflects this genetic integration. Even though levels of V(A) in individual genital sclerites may be low, there may still be sufficient V(A) in multivariate trait space for selection to generate evolutionary change in the overall morphology of male genitalia.     

Temporal variation of the population structure and genetic diversity of Farfantepenaeus notialis assessed by allozyme loci

Population genetic studies carried out on penaeid shrimps have disclosed different patterns of population subdivision, revealing new aspects of shrimp biology as well as the effects of historical contingency molding those patterns. However, the stability of observed allele frequencies over time still remains untested. The objective of this article is to show the analysis of the temporal variation of allozymes in a shrimp species inhabiting Cuba which proves that the genetic structure of this species could significantly change in time. The study involves four populations of Farfantepenaeus notialis sampled in a period of 8 years. The significant statistics obtained from partitions observed in 1995 were not detected in 2003 (as suggested by AMOVA and F(ST)), whereas temporal genetic differentiation and heterozygosity became highly significant. The results strongly suggest that the effect of migrations could be the cause for the loss of F. notialis genetic structure in 2003. It is therefore imperative to call attention on the vulnerability of these populations when facing unstable environmental and habitat conditions.     

Differences in growth,total lipid content and fatty acid composition among 60 clones of <Emphasis Type="Italic">Cylindrotheca fusiformis</Emphasis>

Differences between clones of the diatom Cylindrotheca fusiformi were studied with respect to growth rate, total lipid content and fatty acid composition. Sixty clones were isolated and cultivated under batch conditions. All clones were grown under identical conditions (temperature 22±1°C, light intensity 100 μmol photon m⁻² s⁻¹, salinity 28, F/2 medium) and were harvested in the late exponential growth phase for lipid and fatty acid analysis. The results show a wide variation in growth, total lipid content and fatty acid profiles among clones (p<0.05). The major fatty acids in the 60 clones were 14:0 (4.6–9.1%), 16:0 (18.2–32.0%), 16:1n-7 (21.6–33.1%), 20:4n-6 (4.1–13.5%) and 20:5n-3 (6.2–17.2%), with the highest proportion of 20:4n-6 in clone CF13 (13.5%), and the highest proportion of 20:5n-3 in clone CF5 (17.2%). The results support the view that some microalgal fatty acid variation is not restricted to interspecific variation and external factors, but also varies from clone to clone within the same species.     

Response of wetland plant species to hydrologic conditions

Understanding hydrologic requirements of native and introduced species is critical to sustaining native plant communities in wetlands of disturbed landscapes. We examined plant assemblages, and 31 of the most common species comprising them, from emergent wetlands in an urbanizing area of the Pacific Northwest, USA, in relation to in situ, fine-scale hydrology. Percent cover by plant species was estimated in 2208 1-m² plots across 43 sites, with water depth at time of vegetation sampling measured in 432 plots. Three years of bi-weekly hydrologic data from each of the 43 sites were used to estimate mean surface water level and mean absolute difference (MAD) in surface water level for every plot. Nine assemblages of plant species that co-occur in the field were identified using TWINSPAN. The assemblage richest in native species occurred under intermediate hydrologic conditions and was bracketed by pasture grass dominated assemblages at drier conditions with low water level variability, and Phalaris arundinacea L. assemblages with higher mean water levels and variability. Results suggest minor changes in average water levels (10 cm) or in variability (±2 cm in MAD) could promote a shift from assemblages dominated by natives to those dominated by invasive or alien taxa. Canonical correspondence analysis segregated the species into four groups related to hydrologic gradients. Each species response group was typified by taxa with similar optima for a given environmental variable, with each group related to a characteristic suite of hydrologic conditions. The most common species (P. arundinacea, Juncus effusus L., and Typha latifolia L.), each representing a different response group, exhibited unique responses in occurrence/abundance in relation to water level variability, but were abundant over a wide range of water depth. The realized niches of other species in each response group were more restricted, with peaks in cover confined to narrower ranges of water depth and variability.     

Androgynous rex - the utility of chevrons for determining the sex of crocodilians and non-avian dinosaurs

The sex of non-avian dinosaurs has been inferred on numerous occasions using a variety of anatomical criteria, but the efficacy of none has been proven. Nearly 50 years ago Romer suggested that the cranial-most or first chevron in the tails of some reptiles, including crocodilians, is sexually dimorphic. Recent work on this subject purportedly substantiated that the female first chevron articulates in a more caudal position than in males. Furthermore, it was concluded that this element is shorter in females. These phenotypic attributes theoretically provide a broader cloacal passageway for eggs by ovipositing females and a greater attachment area for male “penile retractor muscles”. Because theropod dinosaurs such as Tyrannosaurus rex presumably show similar variation in chevron anatomy, the same criteria has been advocated for sexing dinosaurs. We tested the neontological model for the chevron sexual dimorphism hypothesis using a skeletonized growth series of American alligators (Alligator mississippiensis) of known sex. No statistical support for the hypothesis was found. Furthermore, analysis of a diversity of crocodilian taxa from museum collections revealed similar findings suggesting the alligator results are not taxon specific. Study of well-preserved tyrannosaurid dinosaurs in museum collections showed nearly invariant chevron positioning like that seen in crocodilians. This suggests the usefulness of chevron anatomy for sexing dinosaurs is tenuous.     

The contribution of spontaneous mutation to variation in environmental responses of Arabidopsis thaliana: responses to light

It has been hypothesized that new, spontaneous mutations tend to reduce fitness more severely in more stressful environments. To address this hypothesis, we grew plants representing 20 Arabidopsis thaliana mutation-accumulation (M-A) lines, advanced to generation 17, and their progenitor, in differing light conditions. The experiment was conducted in a greenhouse, and two treatments were used: full sun and shade, in which influx of red light was reduced relative to far-red. The shade treatment was considered the more stressful because mean absolute fitness was lower in that treatment, though not significantly so. Plants from generation 17 of M-A developed significantly faster than those from generation 0 in both treatments. A significant interaction between generation and treatment revealed that, counter to the hypothesis, M-A lines tended to have higher fitness on average relative to the progenitor in the shaded conditions, whereas, in full sun, the two generations were similar in fitness. A secondary objective of this experiment was to characterize the contribution of new mutations to genotype x environment interaction. We did not, however, detect a significant interaction between M-A line and treatment. Plots of the line-specific environmental responses indicate no tendency of new mutations to contribute to fitness trade-offs, between environments. They also do not support a model of conditionally deleterious mutation, in which a mutant reduces fitness only in a particular environment. These results suggest that interactions between genotype and light environment previously documented for A. thaliana are not explicable primarily as a consequence of steady input of spontaneous mutations having environment-specific effects.     

Quantitative genetics of natural variation of behavior in Drosophila melanogaster: the possible role of the social environment on creating persistent patterns of group activity

Using a set of nine effectively isogenic lines collected from nature in 1998, we observed unperturbed behaviors of mixed-sex groups of Drosophila melanogaster. We repeatedly scanned replicated groups of genetically identical individuals, five females and five males, and recorded the behavior of each individual (i.e., walking, feeding, grooming, flying, courting, mating, fighting, or resting). From these behaviors, we made a composite variable of activity for our quantitative genetic analysis. Genotypes differed in activity, explaining 14.41% of the variation in activity; 8.60% of the variation was explained by a significant genotype x sex interaction, which signifies genetic variation for sexual dimorphism in behavior. Phenotypic plasticity explained 11.13% of the variation in activity. Different genotypes and sexes within genotypes had different rank orders of the component behaviors that contribute to activity. We found no effect of common rearing environment. Instead, differences between replicate groups within genotype accounted for 19.47% variation in activity, and activity was significantly repeatable across scans. This emergent group behavior is likely caused by differences between groups of interacting individuals, even though individuals were genetically identical across groups. Thus, emergent group behavior explained almost as much variation in activity as the combined sources of genetic variation (23.01%), and this is an additional level on which selection could operate: individuals and groups. We discuss how differences among groups could change patterns of additive genetic variation available for evolution. Furthermore, because the behavior of an individual is influenced by conspecifics, genotype interactions between individuals could contribute to indirect selection. Finally, if we consider activity as a syndrome governing all component behaviors with strong genetic correlations among behaviors within an individual, then these component behaviors cannot evolve independently. These results suggest that reductionist approaches of molecular behavior genetics may be incomplete and/or misleading when considering similar phenotypes at the population level or when trying to understand how behaviors evolve.     

The effect of intraspecific sample size on type I and type II error rates in comparative studies

Comparative studies have increased greatly in number in recent years due to advances in statistical and phylogenetic methodologies. For these studies, a trade-off often exists between the number of species that can be included in any given study and the number of individuals examined per species. Here, we describe a simple simulation study examining the effect of intraspecific sample size on statistical error in comparative studies. We find that ignoring measurement error has no effect on type I error of nonphylogenetic analyses, but can lead to increased type I error under some circumstances when using independent contrasts. We suggest using ANOVA to evaluate the relative amounts of within- and between-species variation when considering a phylogenetic comparative study. If within-species variance is particularly large and intraspecific sample sizes small, then either larger sample sizes or comparative methods that account for measurement error are necessary.     

DOES SELECTION ON FLORAL ODOR PROMOTE DIFFERENTIATION AMONG POPULATIONS AND SPECIES OF THE SEXUALLY DECEPTIVE ORCHID GENUS OPHRYS?

Sexually deceptive orchids from the genus Ophrys attract their pollinators primarily through the chemical mimicry of female hymenopteran sex pheromones, thereby deceiving males into attempted matings with the orchid labellum. Floral odor traits are crucial for the reproductive success of these pollinator-limited orchids, as well as for maintaining reproductive isolation through the attraction of specific pollinators. We tested for the signature of pollinator-mediated selection on floral odor by comparing intra and interspecific differentiation in odor compounds with that found at microsatellite markers among natural populations. Three regions from southern Italy were sampled. We found strong floral odor differentiation among allopatric populations within species, among allopatric species and among sympatric species. Population differences in odor were also reflected in significant variation in the attractivity of floral extracts to the pollinator, Colletes cunicularius. Odor compounds that are electrophysiologically active in C. cunicularius males, especially alkenes, were more strongly differentiated among conspecific populations than nonactive compounds in the floral odor. In marked contrast to these odor patterns, there was limited population or species level differentiation in microsatellites (FST range 0.005 to 0.127, mean FST 0.075). We propose that the strong odor differentiation and lack of genetic differentiation among sympatric taxa indicates selection imposed by the distinct odor preferences of different pollinating species. Within species, low FST values are suggestive of large effective population sizes and indicate that divergent selection rather than genetic drift accounts for the strong population differentiation in odor. The higher differentiation in active versus non-active odor compounds suggests that divergent selection among orchid populations may be driven by local pollinator preferences for those particular compounds critical for pollinator attraction.