Genetic diversity and spatial genetic structure support the specialist-generalist variation hypothesis in two sympatric woodpecker species

Species are often arranged along a continuum from “specialists” to “generalists”. Specialists typically use fewer resources, occur in more patchily distributed habitats and have overall smaller population sizes than generalists. Accordingly, the specialist-generalist variation hypothesis (SGVH) proposes that populations of habitat specialists have lower genetic diversity and are genetically more differentiated due to reduced gene flow compared to populations of generalists. Here, expectations of the SGVH were tested by examining genetic diversity, spatial genetic structure and contemporary gene flow in two sympatric woodpecker species differing in habitat specialization. Compared to the generalist great spotted woodpecker (Dendrocopos major), lower genetic diversity was found in the specialist middle spotted woodpecker (Dendrocoptes medius). Evidence for recent bottlenecks was revealed in some populations of the middle spotted woodpecker, but in none of the great spotted woodpecker. Substantial spatial genetic structure and a significant correlation between genetic and geographic distances were found in the middle spotted woodpecker, but only weak spatial genetic structure and no significant correlation between genetic and geographic distances in the great spotted woodpecker. Finally, estimated levels of contemporary gene flow did not differ between the two species. Results are consistent with all but one expectations of the SGVH. This study adds to the relatively few investigations addressing the SGVH in terrestrial vertebrates.

     

Genetic diversity of coastal Northwest Atlantic herring populations: implications for management

Analysis of nine tetranucleotide microsatellite loci for Atlantic herring at five locations in the Northwest Atlantic including the Bras d'Or Lakes shows considerable genetic variation and significant population structure within the Coastal Nova Scotia management component, and among coastal populations and herring collected from Georges Bank. However, results are also consistent with gene flow across the Gulf of Maine. The magnitude of differentiation between the Bras d'Or Lakes sample and all others considered was sufficient to warrant further investigation. These data support the precautionary spawning-ground based management approach implemented in this area.     

Phenology and floral visitors of two sympatric Heliconia species in the Brazilian Atlantic forest

The main goal of the present study was to test if two sympatric species of Heliconia, Heliconia spathocircinata and Heliconia laneana var. flava, differ in their reproductive seasons and guilds of flower visitors. Twenty-four sampling sessions were conducted monthly between February 2001 and January 2003. Individuals of H. spathocircinata and of H. laneana var. flava were tagged within a transect of 4000×4 m. Reproductive phenophases (flowering, unripe, and ripe fruits) were recorded, and circular statistics were used to test for the regularity and frequency of phenophases. Flower visitors were observed during two flowering periods for each species. Both species had well-defined and synchronous reproductive periods, with a small overlap, suggesting a sequential flowering model. Biotic and abiotic factors seem to affect phenophases of both species. However, herbarium data suggest that the flowering period in H. spathocircinata varies less than in H. laneana, despite its wider geographic distribution. The hummingbird Phaethornis idaliae was the main floral visitor of both Heliconia species studied. H. laneana was also legitimately visited by another hummingbird species, Glaucis hirsuta, and by Lepidoptera. Some Lepidoptera, particularly the bee Trigona spinipes, were illegitimate visitors (‘nectar robbers’). We suggest that the flower of H. spathocircinata allowed a wider array of visitors, because it is bigger.     

Genetic variation inSolanum pennellii: Comparisons with two other sympatric tomato species

Genetic variation—primarily in 19 genetic loci of seven enzyme systems—was analyzed in accessions from various parts of the geographic range ofSolanum pennellii, which according to all tested biosystematic criteria behaves like a species ofLycopersicon. In comparison with the largely sympatricL. hirsutum andL. pimpinellifolium, this species exhibits the same trends of reduced allogamy and decreased genetic variation toward the north and south margins of its distribution, though to a much lesser degree; it does not exhibit their trends toward smaller flower size in the same peripheral regions. All three species agree to a considerable extent in the ranking of their tested loci in respect to degree of variablility; however, overall polymorphy is highest inS. pennellii. Except for the appearance of self-compatibility at its southernmost margin,S. pennellii is exclusively and rigidly self-incompatible. Alleles are distributed much more uniformly over the range than in the previously mentioned species, marginal and internal endemic mutants being much less abundant. A marked geographic disagreement is evident in regions of high and low variation. These differences in patterns of genetic variability are reconciled in terms of observed differences in mating systems, probable age of distributions, and adaptive strategies.     

Genetic diversity and parasite prevalence in two species of bumblebee

Many bumblebee species have been suffering from significant declines across their ranges in the Northern Hemisphere over the last few decades. The remaining populations of the rare species are now often isolated due to habitat fragmentation and have reduced levels of genetic diversity. The persistence of these populations may be threatened by inbreeding depression, which may result in a higher susceptibility to parasites. Here we investigate the relationship between genetic diversity and prevalence of the parasitic mite Locustacarus buchneri in bumblebees, using the previously-studied system of Bombus muscorum and Bombus jonellus in the Western Isles of Scotland. We recorded L. buchneri prevalence in 17 populations of B. muscorum and 13 populations of B. jonellus and related the results to levels of heterozygosity. For B. muscorum, we found that prevalence of the mite was higher in populations with lower genetic diversity but there was no such relationship in the more genetically diverse B. jonellus. In contrast to population-level measures of genetic diversity, the heterozygosity of individual bees was not correlated with infection status. We suggest population-level genetic homogeneity may facilitate parasite transmission and elevate prevalence, with potential consequences for population persistence.     

Ecophysiological responses to warming events by two sympatric zooplankton species

Under current climate change scenarios, numerous lakes are predictedto experience increases in maximum temperatures and in the timing,frequency and duration of heat waves. To investigate the effectsof the magnitude and timing of thermal stress on zooplanktoncommunities we compared the thermal responses and seasonal phenologiesof two sympatric species, Epischura lacustris and Daphnia catawba.Thermal responses were measured in the laboratory as differencesin demography of field animals at temperatures characteristicof temperate lakes in spring, summer and autumn (15–30°C).Epischura lacustris had a significantly lower temperature tolerancethan D. catawba, which appears to explain differences in theirseasonal phenologies. Divergence in thermal tolerances and seasonalphenologies of these sympatric species strongly suggests thatthe response of the zooplankton community in this lake to warmingevents will differ greatly with the timing and severity of thethermal stress. Similar effects are to be expected in otherlakes containing these species and other zooplankton communities.     

Cross-incompatibility between two sympatric polyploid Solanum species

Summary Preliminary results from a large number of reciprocal crosses between the closely related sympatric species S. gourlayi Hawkes (2n=4x=48) and S. oplocense Hawkes (2n=6x=72) indicated that they are difficult to hybridize. Pollen-pistil incompatibility barriers were detected via fluorescent microscopy. The cross incompatibility reaction occurred at three sites in 6x×4x crosses; on the stigma, in the first one-third of the style, and in the first two-thirds of the style. In the reciprocal 4x×6x crosses the incompatibility reaction invariably occurred in the ovary. Backcrosses of interspecific pentaploid hybrids (that were occasionally formed) to both parental populations were fully compatible, partially compatible, and fully incompatible with three sites of cross-incompatibility reaction similar to those observed in 6x×4x crosses, respectively. Both polyploid species were found to be selfcompatible, whereas their F1 hybrids were found to be self-incompatible. An hypothesis based on interactions of dominant cross-incompatibility (CI) genes in pistils and dominant specific complementary genes in pollen grains is postulated to explain these observations. The cross-incompatibility system that appears to be operating in nature between 4x S. gourlayi and 6x S. oplocense provides a way for gene exchange between sympatric populations without threatening the identity of either species.     

Genetic diversity of Nostoc symbionts endophytically associated with two bryophyte species

The diversity of the endophytic Nostoc symbionts of two thalloid bryophytes, the hornwort Anthoceros fusiformis and the liverwort Blasia pusilla, was examined using the tRNA(Leu) (UAA) intron sequence as a marker. The results confirmed that many different Nostoc strains are involved in both associations under natural conditions in the field. The level of Nostoc diversity within individual bryophyte thalli varied, but single DNA fragments were consistently amplified from individual symbiotic colonies. Some Nostoc strains were widespread and were detected from thalli collected from different field sites and different years. These findings indicate a moderate level of spatial and temporal continuity in bryophyte-Nostoc symbioses.     

Genetic divergence in adaptive characters between sympatric species of stickleback

This study explored the genetic basis of phenotypic differences between two sympatric species of ecologically and morphologically divergent sticklebacks (Gasterosteus aculeatus complex). The aim was to understand how many loci determine the differences and to what extent the differences are due to additive or nonadditive gene action. I reared the two parental species, F1 and F2 hybrids, and both backcrosses in the laboratory and measured the following quantitative characters: gill raker number and length (both involved in feeding), lateral plate number and pelvic spine length (both involved in predator defense), and growth (a fitness component). I then applied joint-scaling regression models to estimate composite additive, dominance and epistatic effects, and their contribution to divergence of parental lines. A simple additive model was sufficient for gill raker number and growth; additive and dominance effects contributed significantly to divergence in plate number and pelvic spine length; and additive, dominance, and epistatic effects contributed significantly to divergence in gill raker length. Wright's estimator for the number of loci for the four morphological characters ranged from 1 to 50. My results suggest that adaptive divergence between limnetic and benthic sticklebacks has taken place through a variety of genetic mechanisms specific to different traits. Though interspecific hybrids are completely fertile and viable in the laboratory, they are selected against in the wild. The pattern of inheritance for the traits examined here directly influences how well hybrids can exploit the two major resource environments in the wild.     

The effect of hummingbird flower mites on nectar availability of two sympatric Heliconia species in a Brazilian Atlantic forest

BACKGROUND AND AIMS: Hummingbird flower mites feed and reproduce in flowers of host plants pollinated by hummingbirds, and use the nostrils and bill of the hummingbird to move from plant to plant. These mites compete with the pollinator for the nectar produced by flowers. An investigation was made of the relationship between the pattern of nectar production and the effects of hummingbird flower mites in the flowers of two sympatric species of Heliconia (Heliconiaceae). METHODS: Nectar production was sampled by carrying out two experiments: 2-hour intervals and accumulated nectar. Flowers with and without mites were used in both experiments. KEY RESULTS: Exclusion of mites increased nectar production, especially in accumulated daily production (a maximum of 49 % more nectar). Both Heliconia species had the same pattern of nectar production: a high concentration in the morning, which was progressively reduced as the day passed. This pattern of nectar production coincides with the behaviour of the pollinator, which makes more frequent visits in the morning, as observed in a previous study. CONCLUSIONS: The results suggest that the impact of mites on nectar availability of Heliconia is more important with regard to total volume of nectar produced irrespective of flower longevity. A high variation among individuals in nectar produced in the populations was also observed. Hummingbird flower mites strongly affect availability of nectar for hummingbirds.     

Genetic diversity and karyotype of Pitcairnia azouryi: an endangered species of Bromeliaceae endemic to Atlantic Forest inselbergs

Plant species of various families, such as those of Bromeliaceae, occur on inselbergs where they are subject to geographic isolation and environmental conditions that can lead to genetic erosion. This, in turn, can result in the loss of natural populations due to homozygosis, or changes in ploidy that may lead to reproductive isolation. The genetic diversity of five natural populations of Pitcairnia azouryi was measured using nine microsatellite markers transferred from P. albiflos and P. geyskesii. Chromosome numbers and nuclear DNA content were also evaluated. The results indicated moderate genetic differentiation among populations (F_ST?=?0.188), and significant gene flow (N_m?=?1.073) in four of the five populations. P. azouryi has, predominantly, 2n?=?50 chromosomes and DNA content of 2C?=?1.16 pg, but the tetraploid condition was found (2n?=?100 and 2C?=?2.32 pg) in seedlings of an individual of the most geographically isolated population. The moderate level of genetic structuring observed for P. azouryi seems to be related to its disjoint geographical distribution and the locally aggregated spatial structure of the populations, which are isolated from each other, hindering the inter and intrapopulational gene flow. This interpretation was also evidenced by the mantel test (r?=?0.777, P?<?0.05). The occurrence of diploid individuals with tetraploid seedlings is indicative of events of eupolyploidization, possibly due to the environmental conditions of this geographically isolated population.

     

Genetic diversity and the evolutionary history of plant immunity genes in two species of Zea

Plant pathogenesis-related genes (PR genes) code for enzymes, enzyme inhibitors, and other peptides that confer resistance to pathogens and herbivores. Although several PR genes have been the subject of molecular population genetic analyses, a general understanding of their long-term evolutionary dynamics remains incomplete. Here we analyze sequence data from 17 PR genes from two closely related teosinte species of central Mexico. In addition to testing whether patterns of diversity at individual loci depart from expectations under a neutral model, we compared patterns of diversity at defense genes, as a class, to nondefense genes. In Zea diploperennis, the majority of defense genes have patterns of diversity consistent with neutral expectations while at least two genes showed evidence of recent positive selection consistent with arms-race models of antagonistic coevolution. In Zea mays ssp. parviglumis, by contrast, analyses of both defense and nondefense genes revealed strong and consistent departures from the neutral model, suggestive of nonequilibrium population dynamics or population structure. Nevertheless, we found a significant excess of replacement polymorphism in defense genes compared to nondefense genes. Although we cannot exclude relaxed selective constraint as an explanation, our results are consistent with temporally variable (transient or episodic) selection or geographically variable selection acting on parviglumis defense genes. The different patterns of diversity found in the two Zea species may be explained by parviglumis' greater distribution and population structure together with geographic variation in selection.     

Exposure to light enhances pre-adult fitness in two dark-dwelling sympatric species of ants

Background  

In insects, circadian clocks play a key role in enhancing fitness by regulating life history traits such as developmental time and adult lifespan. These clocks use environmental light/dark (LD) cycles to fine-tune a wide range of behavioral and physiological processes. To study the effect of environmental LD conditions on pre-adult fitness components, we used two dark-dwelling sympatric species of ants (the night active Camponotus compressus and the day active Camponotus paria), which normally develop underground and have fairly long pre-adult developmental time.     

Genetic Structure of Capelin (Mallotus villosus) in the Northwest Atlantic Ocean

Capelin (Mallotus villosus) is a commercially exploited, key forage-fish species found in the boreal waters of the North Pacific and North Atlantic Oceans. We examined the population structure of capelin throughout their range in the Canadian northwest Atlantic Ocean using genetic-based methods. Capelin collected at ten beach and five demersal spawning locations over the period 2002 through 2008 (N = 3,433 fish) were genotyped using six polymorphic microsatellite loci. Temporally distinct samples were identified at three beach spawning locations: Chance Cove, Little Lawn and Straitsview, Newfoundland. Four capelin stocks are assumed for fisheries management in the northwest Atlantic Ocean based on meristics, morphometrics, tag returns, and seasonal distribution patterns. Our results suggested groupings that were somewhat different than the assumed structure, and indicate at least seven genetically defined populations arising from two ancestral populations. The spatial mosaic of capelin from each of the two basal cluster groups explains much of the observed geographic variability amongst neighbouring samples. The genetic-defined populations were resolved at Jost’s D _est ≥ 0.01 and were composed of fish collected 1) in the Gulf of St. Lawrence, 2) along the south and east coasts of Newfoundland, 3) along coastal northern Newfoundland and southern Labrador, 4) along coastal northern Labrador, 5) near the Saguenay River, and at two nearshore demersal spawning sites, 6) one at Grebes Nest off Bellevue Beach on the east coast of Newfoundland, and 7) one off the coast of Labrador at Domino Run. Moreover, the offshore demersal spawners on the Scotian Shelf and Southeast Shoal appeared to be related to the inshore demersal spawners at Grebes Nest and in Domino Run and to beach spawners from the Gulf of St. Lawrence.     

Biosystematics of two sympatric species ofEucharis (Amaryllidaceae)

Eucharis candida andE. formosa are two often sympatric species of bulbous geophytes restricted to neotropical rain forest understory. The species are most common in eastern Ecuador, and are the only two east Ecuadorean species of the genus found north of the Pastaza valley. Data from phenetic, karyotypic, and preliminary isozyme electrophoretic analyses of both species are represented. The species are distinguishable phenetically and karyologically, but isozyme-based relationships are more complex. Phenetic resolution of the isozyme phenotypes supports recognition of two species in Ecuador. A Peruvian isolate ofE. formosa, though not morphologically distinct, shows both allozyme and chromosomal divergence from Ecuadorean populations. Cladistic relationships based on overall allozyme data do not support species distinction, but a novel electrophoretic phenotype for glutathione reductase is shared only by individuals ofE. candida. An apparent geographic component within the monophyletic groups resolved in the cladogram suggests that some degree of gene flow between these two species has been maintained without the complete loss of morpholgoical species identity. This may have been mediated either by artificial population structures due to a probable long history of cultivation, or via Pleistocene refugia effects. Both species may have originated in eastern Ecuador from a common ancestral population which has since radiated outward, perhaps several times.     

Interspecific competition during transmission of two sympatric malaria parasite species to the mosquito vector

The role of species interactions in structuring parasite communities remains controversial. Here, we show that interspecific competition between two avian malaria parasite species, Plasmodium gallinaceum and P. juxtanucleare, occurs as a result of interference during parasite fertilization within the bloodmeal of the mosquito. The significant reduction in the transmission success of P. gallinaceum to mosquitoes, due to the co-infecting P. juxtanucleare, is predicted to have compromised its colonization of regions occupied by P. juxtanucleare and, thus, may have contributed to the restricted global distribution of P. gallinaceum. Such interspecies interactions may occur between human malaria parasites and, thus, impact upon parasite species epidemiology, especially in regions of seasonal transmission.     

Genetic diversity and structure in two species of Leavenworthia with self-incompatible and self-compatible populations

Self-fertilization is a common mating system in plants and is known to reduce genetic diversity, increase genetic structure and potentially put populations at greater risk of extinction. In this study, we measured the genetic diversity and structure of two cedar glade endemic species, Leavenworthia alabamica and L. crassa. These species have self-incompatible (SI) and self-compatible (SC) populations and are therefore ideal for understanding how the mating system affects genetic diversity and structure. We found that L. alabamica and L. crassa had high species-level genetic diversity (H_e=0.229 and 0.183, respectively) and high genetic structure among their populations (F_ST=0.45 and 0.36, respectively), but that mean genetic diversity was significantly lower in SC compared with SI populations (SC vs SI, H_e for L. alabamica was 0.065 vs 0.206 and for L. crassa was 0.084 vs 0.189). We also found significant genetic structure using maximum-likelihood clustering methods. These data indicate that the loss of SI leads to the loss of genetic diversity within populations. In addition, we examined genetic distance relationships between SI and SC populations to analyze possible population history and origins of self-compatibility. We find there may have been multiple origins of self-compatibility in L. alabamica and L. crassa. However, further work is required to test this hypothesis. Finally, given their high genetic structure and that individual populations harbor unique alleles, conservation strategies seeking to maximize species-level genetic diversity for these or similar species should protect multiple populations.     

Differences in species diversity of Monogenea between the Pacific and Atlantic Oceans

Data from five extensive surveys each in the Pacific and Atlantic Oceans show that relative species diversity (number of parasite species per host species) of gill Monogenea of coastal marine fishes is greater in the northern and southwestern Pacific than in the northeastern and central- and southwestern Atlantic. Relative species diversity is markedly lower in the cold northeastern Atlantic than in the warmer parts of the Atlantic examined, and in the northern Pacific than in the warm southwestern Pacific. The difference between the northern Pacific and Atlantic is entirely or almost entirely due to a much greater number of species of Gyrodactylidae in the northern Pacific. A species-area relationship cannot explain the difference, because the area of the northern Pacific is not larger than that of the northern Atlantic and because Gyrodactylidae are cold-water forms which cannot have immigrated from warmer seas. The difference is tentatively explained by an evolutionary time hypothesis: more species of Gyrodactylidae have accumulated in the much older Pacific than in the Atlantic Ocean. Alternatively, an ecological time hypothesis may explain the difference: ice sheets during the last glaciation covered much more of the continental shelf in the northern Atlantic than in the northern Pacific, possibly extinguishing more Monogenea in the former than in the latter Ocean.