Geographic Intraspecific Variation Research Articles

Geographic intra-specific variation in social organization is driven by population density

Social flexibility enables individuals to switch between group and solitary living and is suggested to be an adaptation to varying environments. Several previous studies on different species compared two populations and hypothesized that observed differences in the social organization were due to differences in population density but lacked the necessary sample size to test this prediction. In a previous 8-year long-term study, we showed that one population of African striped mice (Rhabdomys pumilio) displayed temporal social flexibility, living solitarily in years when population density was low but in groups in years when population density was high. Building on this temporal variation, we now tested whether geographic variation reveals the same pattern. We studied 6 populations in discrete geographical locations simultaneously, predicting more solitary living in populations with lower population density. Population density correlated significantly with the percentage of striped mice living in groups whereas other (environmental) factors were not significant. Moreover, some individuals dispersed over unoccupied habitats between these populations, switching from group to solitary living. Geographic variation in population density could make social flexibility adaptive because it allows individuals to respond quickly to the prevailing conditions they experience post dispersal. Our results suggest that geographic variation drives the evolution of social flexibility in our metapopulation of striped mice, causing intra-specific variation in its social organization, which might also be important in other species, especially in species with a fast life history. Populations of the same species can differ in their social organization. It has often been assumed that this is due to differences in population density. We studied 6 populations of the African striped mice, showing that more mice were solitary living when population density was low. Thus, we demonstrated that population differences in social organization were due to differences in population density.

Geographical origin determines responses to salinity of Mediterranean caddisflies.

Many freshwater ecosystems worldwide, and particularly Mediterranean ones, show increasing levels of salinity. These changes in water conditions could affect abundance and distribution of inhabiting species as well as the provision of ecosystem services. In this study we conduct laboratory experiments using the macroinvertebrate Smicridea annulicornis as a model organism. Our factorial experiments were designed to evaluate the effects of geographical origin of organisms and salinity levels on survival and behavioral responses of caddisflies. The experimental organisms were captured from rivers belonging to three hydrological basins along a 450 Km latitudinal gradient in the Mediterranean region of Chile. Animals were exposed to three conductivity levels, from 180 to 1400 μS/cm, close to the historical averages of the source rivers. We measured the behavioral responses to experimental stimuli and the survival time. Our results showed that geographical origin shaped the behavioral and survival responses to salinity. In particular, survival and activity decreased more strongly with increasing salinity in organisms coming from more dilute waters. This suggests local adaptation to be determinant for salinity responses in this benthic invertebrate species. In the current scenario of fast temporal and spatial changes in water levels and salt concentration, the conservation of geographic intra-specific variation of aquatic species is crucial for lowering the risk of salinity-driven biodiversity loss.

Population variation affects interactions between two California salt marsh plant species more than precipitation.

Species that occur along broad environmental gradients often vary in phenotypic traits that make them better adapted to local conditions. Variation in species interactions across gradients could therefore be due to either phenotypic differences among populations or environmental conditions that shift the balance between competition and facilitation. To understand how the environment (precipitation) and variation among populations affect species interactions, we conducted a common garden experiment using two common salt marsh plant species, Salicornia pacifica and Jaumea carnosa, from six salt marshes along the California coast encompassing a large precipitation gradient. Plants were grown alone or with an individual of the opposite species from the same site and exposed to one of three precipitation regimes. J. carnosa was negatively affected in the presence of S. pacifica, while S. pacifica was facilitated by J. carnosa. The strength of these interactions varied by site of origin but not by precipitation treatment. These results suggest that phenotypic variation among populations can affect interaction strength more than environment, despite a threefold difference in precipitation. Geographic intraspecific variation may therefore play an important role in determining the strength of interactions in communities.

Morphological variability of the cranium of Lontra longicaudis (Carnivora: Mustelidae): a morphometric and geographic analysis.

Studies of morphometric variation make it possible to delimit species and geographic intraspecific variation, mainly in species with wide distribution ranges. In the Neotropical otter, Lontra longicaudis, variation in the shape of the rhinarium of three potential subspecies has been described but it is not known whether there is a pattern to the morphometric variation in the skull throughout the distribution of this species. We analyzed morphological variation in the cranium (ventral view) and the mandible (lateral view) of the Neotropical otter, comparing male and female specimens and evaluating the differences between specified geographic units utilizing methods from geometric morphometrics. Specimens from the entire distribution of the species were analyzed. Between sexes, variability in the shape was determined by calculating the Procrustes distances and using Goodall's F test. Geographic variationwas analyzed using a discriminant analysis, an analysis of variance (ANOVA) on a matrix of partial warp scores, and a cluster analysis with Mahalanobis distances, allowing for similarities in shape to be identified between different geographic units. Variation in the size of the two structures was calculated based on the values for centroid size using a one-way ANOVA with a Bonferroni correction and a 95 % confidence interval. There was sexual dimorphism in shape for both views, with males the largest. In general, there was geographic variation in the shape and size of both the cranium and the mandible in the Neotropical otter, exhibiting a pattern that resembled Bergmann's rule. Variation in shape between geographic units could result from the presence of geographic barriers, the spatial configuration of hydrological regions, and/or the large distances between populations throughout this species' distribution. The Neotropical otter exhibits dimorphism in the size, but not in the shape of the skull. There is geographic variation between geographic units, and our results suggest that L.longicaudis could bea group of species. An integrative study using molecular and morphological data could elucidate its taxonomy.

Sperm polymorphism and genetic divergence in the mussel Perumytilus purpuratus

For the first time in a marine bivalve species, geographic intraspecific variation in sperm morphology in the Chilean mussel, Perumytilus purpuratus, is reported. Samples were obtained in December 2010 from the rocky intertidal at three locations: Antofagasta (23o44′01.08′′S, 70o26′26.11′′W), Valparaiso (32o57′23.07′′S, 71o33′01.86′′W), and Valdivia (39°50′44.26′′S, 73°25′51.99′′W). Scanning electron microscopy was used to describe sperm traits, and genetic data were obtained by sequencing a 16S mtDNA 427-bp fragment. The results showed geographic intraspecific variation with two statistically significant patterns of genetically differentiated sperm morphologies. One genetic clade was found for Antofagasta and Valparaiso, displaying a unique sperm morphotype with a short acrosome accounting for 26 % of the head length, and the other from Valdivia, with a larger acrosome occupying 47 % of the head length. These results will lead to new research to distinguish among alternative hypotheses: the presence of cryptic species, incipient speciation, or a polymorphism in sperm traits of P. purpuratus.

Geographic intraspecific variation in buoyancy within Antarctic notothenioid fishes

AbstractWe investigated intraspecific geographic variation in buoyancy by obtaining percentage buoyancy (%B) measurements for the Antarctic notothenioid speciesPleuragramma antarcticum,Trematomus hansoni,T. bernacchiiandGymnodraco acuticepsfrom both McMurdo Sound in East Antarctica and the South Shetland Islands in West Antarctica. Mean percentage buoyancies in these species ranged from 0.22–0.52% in the neutrally buoyantP. antarcticumto 3.34–3.67% in the benthicT. bernacchii. Dispersion (1 standard deviation) of percentage buoyancy (%B) values around the mean was ± 0.2–0.5 %B units for the entire sample. Although intraspecific differences in mean percentage buoyancy were statistically significant (P < 0.05) inP. antarcticumandT. hansoni, we consider these differences as normal variation without substantive biological significance. The dispersion in buoyancy measurements during adult life reflects the density of the fish and this may be influenced, in both the short- and long-term, by gut contents, nutritional condition, and reproductive state. Mitigation of the effects of these variables is not biologically realistic because they constitute normal aspects of the daily and yearly life cycles. The results of our measurements of buoyancy are consistent with what is known about the ecology of these four species and this is considered in the discussion.

Reappraisal of Vipera aspis Venom Neurotoxicity

BackgroundThe variation of venom composition with geography is an important aspect of intraspecific variability in the Vipera genus, although causes of this variability remain unclear. The diversity of snake venom is important both for our understanding of venomous snake evolution and for the preparation of relevant antivenoms to treat envenomations. A geographic intraspecific variation in snake venom composition was recently reported for Vipera aspis aspis venom in France. Since 1992, cases of human envenomation after Vipera aspis aspis bites in south-east France involving unexpected neurological signs were regularly reported. The presence of genes encoding PLA2 neurotoxins in the Vaa snake genome led us to investigate any neurological symptom associated with snake bites in other regions of France and in neighboring countries. In parallel, we used several approaches to characterize the venom PLA2 composition of the snakes captured in the same areas.Methodology/Principal FindingsWe conducted an epidemiological survey of snake bites in various regions of France. In parallel, we carried out the analysis of the genes and the transcripts encoding venom PLA2s. We used SELDI technology to study the diversity of PLA2 in various venom samples. Neurological signs (mainly cranial nerve disturbances) were reported after snake bites in three regions of France: Languedoc-Roussillon, Midi-Pyrénées and Provence-Alpes-Côte d'Azur. Genomes of Vipera aspis snakes from south-east France were shown to contain ammodytoxin isoforms never described in the genome of Vipera aspis from other French regions. Surprisingly, transcripts encoding venom neurotoxic PLA2s were found in snakes of Massif Central region. Accordingly, SELDI analysis of PLA2 venom composition confirmed the existence of population of neurotoxic Vipera aspis snakes in the west part of the Massif Central mountains.Conclusions/SignificanceThe association of epidemiological studies to genetic, biochemical and immunochemical analyses of snake venoms allowed a good evaluation of the potential neurotoxicity of snake bites. A correlation was found between the expression of neurological symptoms in humans and the intensity of the cross-reaction of venoms with anti-ammodytoxin antibodies, which is correlated with the level of neurotoxin (vaspin and/or ammodytoxin) expression in the venom. The origin of the two recently identified neurotoxic snake populations is discussed according to venom PLA2 genome and transcriptome data.