Fine-scale spatial genetic structure and gene flow in a small, fragmented population of Sinojackia rehderiana (Styracaceae), an endangered tree species endemic to China

Populations of Sinojackia rehderiana are highly threatened and have small and scattered distribution due to habitat fragmentation and human activities. Understanding changes in genetic diversity, the fine-scale spatial genetic structure (SGS) at different life stages and gene flow of S. rehderiana is critical for developing successful conservation strategies for fragmented populations of this endangered species. In this study, 208 adults, 114 juveniles and 136 seedlings in a 50 × 100-m transect within an old-growth forest were mapped and genotyped using eight microsatellite makers to investigate the genetic diversity and SGS of this species. No significant differences in genetic diversity among different life-history stages were found. However, a significant heterozygote deficiency in adults and seedlings may result from substantial biparental inbreeding. Significant fine-scale spatial structure was found in different life-history stages within 19 m, suggesting that seed dispersal mainly occurred near a mother tree. Both historical and contemporary estimates of gene flow (13.06 and 16.77 m) indicated short-distance gene dispersal in isolated populations of S. rehderiana. The consistent spatial structure revealed in different life stages is most likely the result of limited gene flow. Our results have important implications for conservation of extant populations of S. rehderiana. Measures for promoting pollen flow should be taken for in situ conservation. The presence of a SGS in fragmented populations implies that seeds for ex situ conservation should be collected from trees at least 19-m apart to reduce genetic similarity between neighbouring individuals.     

Fragmentation can increase spatial genetic structure without decreasing pollen-mediated gene flow in a wind-pollinated tree

Fragmentation reduces population sizes, increases isolation between habitats and can result in restricted dispersal of pollen and seeds. Given that diploid seed dispersal contributes more to shaping fine-scale spatial genetic structure (SGS) than haploid pollen flow, we tested whether fine-scale SGS can be sensitive to fragmentation even if extensive pollen dispersal is maintained. Castanopsis sclerophylla (Lindley & Paxton) Schottky (Fagaceae), a wind-pollinated and gravity seed-dispersed tree, was studied in an area of southeast China where its populations have been fragmented to varying extents by human activity. Using different age classes of trees in areas subject to varying extents of fragmentation, we found no significant difference in genetic diversity between prefragmentation vs. postfragmentation C. sclerophylla subpopulations. Genetic differentiation among postfragmentation subpopulations was also only slightly lower than among prefragmentation subpopulations. In the most fragmented habitat, selfing rates were significantly higher than zero in prefragmentation, but not postfragmentation, cohorts. These results suggest that fragmentation had not decreased gene flow among these populations and that pollen flow remains extensive. However, significantly greater fine-scale SGS was found in postfragmentation subpopulations in the most fragmented habitat, but not in less fragmented habitats. This alteration in SGS reflected more restricted seed dispersal, induced by changes in the physical environments and the prevention of secondary seed dispersal by rodents. An increase in SGS can therefore result from more restricted seed dispersal, even in the face of extensive pollen flow, making it a sensitive indicator of the negative consequences of population fragmentation.     

Ecology and biology of the lake sturgeon: a synthesis of current knowledge of a threatened North American <Emphasis Type="Italic">Acipenseridae</Emphasis>

The lake sturgeon is one of the largest North American freshwater fish and was once common in most inland rivers and lakes of the US and Canadian Midwest. World demand for caviar and sturgeon meat led to a dramatic decline of lake sturgeon populations throughout much of its range. Along with overfishing, lake sturgeon populations have been negatively affected by habitat degradation. Recruitment factors and early life history are poorly understood. Today, renewed interest in lake sturgeon restoration has led to numerous state and federally-funded research activities. Research has focused on identifying and assessing the size structure of remnant stocks, the availability of spawning habitat, and factors affecting reproductive success. Additional studies are needed to improve hatchery techniques, to better understand recruitment mechanisms, and how genetic diversity among and within meta-populations may affect long-term recovery of depleted populations.     

Fine-scale spatial genetic structure within continuous and fragmented populations of Trillium camschatcense

Spatial genetic structure (SGS) within populations was analyzed for the ling-lived understory perennial herb Trillium camschatcense using allozyme loci. We used Sp statistics to compare SGS between 2 life-history stages, juveniles (J) and reproductives (R), as well as between 2 populations, continuous and fragmented, with different habitat conditions. In the continuous population, significant SGS was detected in both stages but the extent was greatly reduced with the progress of the stage (J, Sp = 0.0475; R, Sp = 0.0053). We inferred that limited seed dispersal and subsequent random loss of individuals from the family patches are responsible for the J and R stage structures, respectively. The fragmented population differed in the patterns of SGS; significant structure was detected in the R stage, but not in the J stage (J, Sp = 0.0021; R, Sp = 0.0165) despite significant positive inbreeding coefficients (J, F(IS) = 0.251). The observed differences in the J-stage structures between populations may be explained by habitat fragmentation effects because reduced recruitment in the fragmented population prevents the development of maternal sibling cohort. Such comparative analysis between populations and life-history stages can be useful to understand the different underlying causes of SGS.     

Life-history variation in agricultural and wild populations of Erucastrum nasturtiifolium (Brassicaceae)

In the present study we relate the variability in life-history traits (such as flowering time and lifespan) of the herbaceous biennial–perennial Erucastrum nasturtiifolium (Brassicaceae) to habitat type. We studied plant populations from arable fields and from eroded mountain habitats, such as badlands and rocky slopes. Collection sites ranged from low basin to sub-alpine regions in the NE Iberian Peninsula. Plants were grown under common garden conditions to evaluate genetic variation among and within populations. Plants were also subjected to a resource gradient to detect genetic variation in phenotypic plasticity. The populations exhibited differentiation across a number of life-history traits (mainly flowering time and lifespan) and morphological traits related to growth (basal stem diameter, plant height and number of branches). This suggests that life-history differences among populations are genetically based. Moreover, our results show that variation in flowering time and lifespan are affected by habitat type independent of other abiotic factors such as altitude or continentality. Thus, populations from arable fields started flowering in their first year and displayed annual cycles, whereas those from wild habitats generally flowered in their second year and showed biennial or even perennial cycles. Intra-population differences in flowering time were observed in only one population, and were related to nutrient availability. We suggest that early-flowering and shorter lifespan populations of E. nasturtiifolium may have been selected from late-flowering and longer lifespan populations as part of a selective process ensuring survival and future offspring amidst unpredictable and frequently disturbed environments such as exist in many agricultural habitats.     

Adaptation of timing of life history traits and population dynamic responses to climate change in spatially structured populations

Changes in the seasonal timing of life history events are documented effects of climate change. We used a general model to study how dispersal and competitive interactions affect eco-evolutionary responses to changes in the temporal distribution of resources over the season. Specifically, we modeled adaptation of the timing of reproduction and population dynamic responses in two competing populations that disperse between two habitats characterized by an early and late resource peak. We investigated three scenarios of environmental change: (1) food peaks advance in both habitats, (2) in the late habitat only and (3) in the early habitat only. At low dispersal rates the evolutionarily stable timing of reproduction closely matched the local resource peak and the environmental change typically caused population decline. Larger dispersal rates rendered less intuitive eco-evolutionary population responses. First, dispersal caused mismatch between evolutionarily stable timing of reproduction and local resource peaks and as a result, reproductive output for subpopulations could increase as well as decrease when resource availability underwent temporal shifts. Second, population responses were contingent on competition between populations. This could accelerate population declines and cause extinctions or even reverse population trends from negative to positive compared to the low dispersal case. When dispersal rate was large and the early resource peak was advanced available niche space was reduced. Hence, even when a population survived the environmental change and obtained positive equilibrium population density, subsequent adaptation of competing populations could drive it to extinction due to convergent evolution and competitive exclusion. These results shed new light on the role of competition and dispersal for the evolution of timing of life history events and provide guidelines for understanding short and long-term population response to climate change.     

鹅掌楸贵州烂木山居群的微卫星遗传多样性及空间遗传结构

濒危植物鹅掌楸(Liriodendron chinense)目前仅零散分布于我国亚热带及越南北部地区, 残存居群生境片断化较为严重。研究濒危植物片断化居群的遗传多样性及小尺度空间遗传结构(spatial genetic structure)有助于了解物种的生态进化过程以及制定相关的保育策略。本研究采用13对微卫星引物, 对鹅掌楸的1个片断化居群进行了遗传多样性及空间遗传结构的研究, 旨在揭示生境片断化条件下鹅掌楸的遗传多样性及基因流状况。研究结果表明: 鹅掌楸烂木山居群内不同生境斑块及不同年龄阶段植株的遗传多样性水平差异不显著(P>0.05), 居群内存在寨内和山林2个遗传分化明显的亚居群。烂木山居群个体在200 m以内呈现显著的空间遗传结构, 而2个亚居群内的个体仅在20 m的距离范围内存在微弱或不显著的空间遗传结构。鹅掌楸的空间遗传结构强度较低(Sp = 0.0090), 且寨内亚居群的空间遗传结构强度(Sp = 0.0067)要高于山林亚居群(Sp = 0.0053)。鹅掌楸以异交为主, 种子较轻且具翅, 借助风力传播, 在一定程度上降低了空间遗传结构的强度。此外, 居群内个体密度及生境特征也对鹅掌楸的空间遗传结构产生了一定影响。该居群出现显著的杂合子缺失, 近交系数(F_IS)为0.099 (P < 0.01), 表明生境片断化的遗传效应正逐渐显现。因此, 对鹅掌楸的就地保护应注意维护与强化生境的连续性, 促进基因交流。迁地保护时, 取样距离应不小于20 m, 以涵盖足够多的遗传变异。     

Adaptive inter-population differences in blue tit life-history traits on Corsica

Few studies of natural populations have investigated how phenotypic variation across populations relates to key factors in the environment and landscape structure. In the blue tits of southern France, inter-population differences in reproductive life-history traits (e.g. laying date and clutch size) are small, whatever the timing of maximum caterpillar availability, a key factor for offspring survival in tits. These small differences are attributed to gene flow between local populations occupying different habitat types. In contrast, in blue tits on the island of Corsica, we noted large differences in reproductive life-history traits between two populations, where each population is synchronized with the peak-date of caterpillar abundance. These occur over a short geographical distance (25km). Considering our study within a framework of long-term population studies in tits, our results support the hypothesis that different blue tit populations on Corsica show adaptive differences in life-history traits, and suggest that landscape structure at a small spatial scale can have profound effects on adaptive between-population differentiation in life-history traits that are closely linked with fitness.     

Low but significant genetic differentiation underlies biologically meaningful phenotypic divergence in a large Atlantic salmon population

Despite decades of research assessing the genetic structure of natural populations, the biological meaning of low yet significant genetic divergence often remains unclear due to a lack of associated phenotypic and ecological information. At the same time, structured populations with low genetic divergence and overlapping boundaries can potentially provide excellent models to study adaptation and reproductive isolation in cases where high‐resolution genetic markers and relevant phenotypic and life history information are available. Here, we combined single nucleotide polymorphism (SNP)‐based population inference with extensive phenotypic and life history data to identify potential biological mechanisms driving fine‐scale subpopulation differentiation in Atlantic salmon (Salmo salar) from the Teno River, a major salmon river in Europe. Two sympatrically occurring subpopulations had low but significant genetic differentiation (F_ST = 0.018) and displayed marked differences in the distribution of life history strategies, including variation in juvenile growth rate, age at maturity and size within age classes. Large, late‐maturing individuals were virtually absent from one of the two subpopulations, and there were significant differences in juvenile growth rates and size at age after oceanic migration between individuals in the respective subpopulations. Our findings suggest that different evolutionary processes affect each subpopulation and that hybridization and subsequent selection may maintain low genetic differentiation without hindering adaptive divergence.     

Reproductive allocation between the sexes,across natural and novel habitats,and its impact on genetic diversity

Allocation to reproductive mode (sexual and asexual) can vary depending on environmental conditions but is often examined at the population level, whereas selection acts upon the individual. We examined individual variation in reproductive mode to identify how the interaction of sex and the environment affect population genetic diversity. Using the plant Marchantia inflexa, we tested whether reproductive allocation pattern varies consistently between males and females and among plants collected from different environments, and determined if morphological responses were the result of individual plasticity or genetic differences. We then quantified genetic variability between the different environments and between the sexes. Male and female plants were collected from two strikingly different habitats within the same region: along natural sites (rivers) and along novel human-modified sites (roadsides). Using a common garden approach, we found a strong sex by habitat interaction: male and female responses differed significantly by their source habitat. For females, relative to river-collected, road-collected plants had higher growth and asexual reproduction, while the pattern was reversed, although not significant, for males. Genetic differentiation was significant between the two habitats with no evidence of individual differences in plasticity for growth, but there was a genotype effect for asexual propagule production. Males and females did not differ genetically; but river-collected plants with lower sexual potential were more diverse than roadside-collected plants, possibly the result of founder events. These results show that individual variation in reproduction is controlled by the interaction of both the environment and genetics. Due to different selection pressures between natural and novel habitats, there are observable differences in life history traits with an associated evolutionary response to the novel habitat.     

Small-scale spatial genetic structure in the Central African rainforest tree species Aucoumea klaineana: a stepwise approach to infer the impact of limited gene dispersal, population history and habitat fragmentation

Under the isolation-by-distance model, the strength of spatial genetic structure (SGS) depends on seed and pollen dispersal and genetic drift, which in turn depends on local demographic structure. SGS can also be influenced by historical events such as admixture of differentiated gene pools. We analysed the fine-scale SGS in six populations of a pioneer tree species endemic to Central Africa, Aucoumea klaineana. To infer the impacts of limited gene dispersal, population history and habitat fragmentation on isolation by distance, we followed a stepwise approach consisting of a Bayesian clustering method to detect differentiated gene pools followed by the analysis of kinship-distance curves. Interestingly, despite considerable variation in density, the five populations situated under continuous forest cover displayed very similar extent of SGS. This is likely due to an increase in dispersal distance with decreased tree density. Admixture between two gene pools was detected in one of these five populations creating a distinctive pattern of SGS. In the last population sampled in open habitat, the genetic diversity was in the same range as in the other populations despite a recent habitat fragmentation. This result may due to the increase of gene dispersal compensating the effect of the disturbance as suggested by the reduced extent of SGS estimated in this population. Thus, in A. klaineana, the balance between drift and dispersal may facilitate the maintenance of genetic diversity. Finally, from the strength of the SGS and population density, an indirect estimate of gene dispersal distances was obtained for one site: the quadratic mean parent-offspring distance, sigma(g), ranged between 210 m and 570 m.     

Ancestral populations perform better in a novel environment: domestication of medfly populations from five global regions

Geographically isolated populations of a species may differ in several aspects of life-history, morphology, behavior, and genetic structure as a result of adaptation in ecologically diverse habitats. We used a global invasive species, the Mediterranean fruit fly to investigate, whether adaptation to a novel environment differs among geographically isolated populations that vary in major life history components such as life span and reproduction. We used wild populations from five global regions (Kenya, Hawaii, Guatemala, Portugal, and Greece). Adult demographic traits were monitored in F(2), F(5), F(7) and F(9) generations in captivity. Although domestication in constant laboratory conditions had a different effect on the mortality and reproductive rates of the different populations, a general trend of decreasing life span and age of first reproduction was observed for most medfly populations tested. However, taking into account longevity of both sexes, age-specific reproductive schedules, and average reproductive rates we found that the ancestral Kenyan population kept the above life history traits stable during domestication compared to the other populations tested. These findings provide important insights in the life-history evolution of this model species, and suggest that ancestral medfly populations perform better than the derived - invasive ones in a novel environment.     

Spatial genetic structure in Milicia excelsa (Moraceae) indicates extensive gene dispersal in a low-density wind-pollinated tropical tree

In this study, we analysed spatial genetic structure (SGS) patterns and estimated dispersal distances in Milicia excelsa (Welw.) C.C. Berg (Moraceae), a threatened wind-pollinated dioecious African tree, with typically low density (∼10 adults/km²). Eight microsatellite markers were used to type 287 individuals in four Cameroonian populations characterized by different habitats and tree densities. Differentiation among populations was very low. Two populations in more open habitat did not display any correlation between genetic relatedness and spatial distance between individuals, whereas significant SGS was detected in two populations situated under continuous forest cover. SGS was weak with a maximum S _p-statistic of 0.006, a value in the lower quartile of SGS estimates for trees in the literature. Using a stepwise approach with Bayesian clustering methods, we demonstrated that SGS resulted from isolation by distance and not colonization by different gene pools. Indirect estimates of gene dispersal distances ranged from σ _g = 1 to 7.1 km, one order of magnitude higher than most estimates found in the literature for tropical tree species. This result can largely be explained by life-history traits of the species. Milicia excelsa exhibits a potentially wide-ranging wind-mediated pollen dispersal mechanism as well as very efficient seed dispersal mediated by large frugivorous bats. Estimations of gene flow suggested no major risk of inbreeding because of reduction in population density by exploitation. Different strategy of seed collection may be required for reforestation programmes among populations with different extent of SGS.     

Interpopulation variation in life-history traits of Poeciliopsis prolifica: implications for the study of placental evolution

Placental reproduction is widespread across vertebrate taxa, but little is known about its life-history correlates and putative adaptive value. We studied variation in life-history traits in two populations of the placental poeciliid fish Poeciliopsis prolifica to determine whether differences in post-fertilization maternal provisioning to embryos have a genetic basis and how food availability affects reproduction. Life histories were characterized for wild-caught females and for second-generation lab-born females raised under two levels of food availability. We found that the two populations did not differ significantly in the wild for any life-history traits except for the lipid dry weight in females and in embryos at an advanced stage of development. When environmental effects were experimentally controlled, however, populations exhibited significant differences in several traits, including the degree of maternal provisioning to embryos. Food availability significantly affected female size at first parturition, brood size and offspring dry weight at birth. Altogether, these results demonstrate that population differences in maternal provisioning and other life-history traits have a genetic basis and show a plastic response to food availability. We infer that phenotypic plasticity may mask population differences in the field. In addition, when comparing life-history patterns in these two populations with known patterns in placental and non-placental poeciliids, our results support the hypotheses that placentation is an adaptive reproductive strategy under high-resource conditions but that it may represent a cost under low-food conditions. Finally, our results highlight that age at maturity and reproductive allotment may be key life-history traits accompanying placental evolution.     

Effect of habitat permanence on life-history: extending the Daphnia model into new climate spaces

We use an Australian freshwater invertebrate species, Daphnia carinata, to assess whether variation in habitat permanence influences life-history traits in subpopulations. Using a life table experiment, we measure the life-history traits of populations from both permanent and temporary pools. We show that these habitat classes are associated with clear differences in important life-history traits and evidence of trade-offs in important traits influencing reproduction, diapause, and growth rate and suggest this is evidence for local adaptation. Here we use Daphnia from Australian populations spanning semi-arid and temperate climates generating results that are in broad agreement with similar studies in the northern hemisphere, and so extend these results to a new continent and its particular climate. Variation in habitat permanence, it appears, is a very general driver of life-history divergence.

     

Local specialization and maladaptation in the Mediterranean blue tit (Parus caeruleus)

Although the evolutionary interplay between gene flow and local adaptation of organisms in heterogeneous environments has been widely discussed from a theoretical point of view, few empirical studies have been designed to test predictions on the consequences of habitat patchiness on the evolution of life history traits. Using blue tits in Mediterranean habitat mosaics as a model, we defined two nested levels of habitat heterogeneity: an inter-regional level which compares two isolated landscapes (mainland, southern France vs the island of Corsica), and an intra-regional level which compares two habitat types within each landscape (deciduous vs evergreen trees). Deciduous habitats are more common than evergreen habitats on the mainland whereas the opposite is true on the island. Results suggest that: (1) on a regional scale, each population is specialized to the more common habitat, i.e. life history traits have evolved in such a way that breeding success is high; (2) in the less common habitats within each landscape, birds are clearly mistimed because they mismatch the best period of food availability, and hence their breeding success is lower; and (3) the density of the populations and the morphometry of the birds support the model of ideal despotic distribution. These results, which are supported by preliminary data on the genetic variation and gene flow of populations in the mainland landscape, are consistent with a source-sink model of population structure within each landscape. They are discussed in the framework of metapopulation theory and habitat selection models.     

Habitat persistence,habitat availability and the evolution of dispersal

Many organisms live in ephemeral habitats, making dispersal a vital element of life history. Here, we investigate how dispersal rate evolves in response to habitat persistence, mean habitat availability and landscape pattern. We show that dispersal rate is generally lowered by reduced habitat availability and by longer habitat persistence. However, for habitats that persist for an average of ten times the length of a generation, we show a clear non-monotonic relationship between habitat availability and dispersal rate. Some patterns of available habitat result in populations with dispersal polymorphisms. We explain these observations as a metapopulation effect, with the rate of evolution a function of both within-population and between-population selection pressures. Individuals in corridors evolve much lower dispersal rates than those in the mainland populations, especially within long, narrow corridors. We consider the implications of the results for conservation.     

Life History Variation in Female Gambusia hubbsi

Gambusia hubbsi populations occur in a variety of fresh and brackish-water habitats on Andros, Bahamas. These include shallow water sites (tidal creeks, lakes, roadside ditches), and blueholes (vertical solution caves). In some blueholes G. hubbsi is the only species present, in others it co-occurs with other species, principal among these is a predator, Eleotris pisonis. By contrast to blueholes, shallow water sites have highly variable temperature and depth. In addition, they are frequented by avian piscivores and may be occasionally occupied by piscivores such as Eleotris. We sampled 10 shallow water sites, 14 blueholes where Eleotris is absent and 12 blueholes where Eleotris co-occurs with G. hubbsi. We measured and compared variation in female body size, fecundity, and reproductive investment among these three habitats. The observed patterns of life history variation are only partially in accord with expectations from theory regarding the effects of predation and seasonality on life history variation. Samples from populations that colonized a series of man-made trenches (Well Fields), a set of introductions into that habitat, and changes in life history traits of lab-raised females from three blueholes, suggest that the observed pattern of life history variation in other habitats also reflects differences in food availability among habitats, and imperfectly reflects the potential phenotypic variability of this species.     

Reproductive strategies and body shape in the European pond turtle (Emys orbicularis) from contrasting habitats in Italy

Comparisons within and among populations offer important insights into variation in life-history traits and possible adaptive patterns to environmental conditions. We present the results of observed differences in body size, body shape and patterns of reproduction in four separate populations of the European pond turtle Emys orbicularis in central and southern Italy – coastal ( n =3) and mountainous ( n =1) sites and pond ( n =2) and canal ( n= 2) habitats – to determine whether phenotypic plasticity affects reproductive output. Although we did not find any significant latitudinal variation in body size, we observed significant differences in body shape between canal (rounded body shape) and pond (elongated body shape) systems and smaller size with rounded shape in the mountainous population. Reproductive output is similar among populations (median=5 eggs per clutch), whereas reproductive investment (relative clutch mass to maternal body mass) is higher in the mountain population (one clutch per year) than in coastal populations (two clutches per year), suggesting differential trade-offs between geographic locality, elevation and habitat type. Turtle shell shape and geographic location together affect reproductive output in E. orbicularis in Italy.     

Stable isotope analysis of vertebrae reveals ontogenetic changes in habitat in an endothermic pelagic shark

Ontogenetic changes in habitat are driven by shifting life-history requirements and play an important role in population dynamics. However, large portions of the life history of many pelagic species are still poorly understood or unknown. We used a novel combination of stable isotope analysis of vertebral annuli, Bayesian mixing models, isoscapes and electronic tag data to reconstruct ontogenetic patterns of habitat and resource use in a pelagic apex predator, the salmon shark (Lamna ditropis). Results identified the North Pacific Transition Zone as the major nursery area for salmon sharks and revealed an ontogenetic shift around the age of maturity from oceanic to increased use of neritic habitats. The nursery habitat may reflect trade-offs between prey availability, predation pressure and thermal constraints on juvenile endothermic sharks. The ontogenetic shift in habitat coincided with a reduction of isotopic niche, possibly reflecting specialization upon particular prey or habitats. Using tagging data to inform Bayesian isotopic mixing models revealed that adult sharks primarily use neritic habitats of Alaska yet receive a trophic subsidy from oceanic habitats. Integrating the multiple methods used here provides a powerful approach to retrospectively study the ecology and life history of migratory species throughout their ontogeny.