Combined effects of drought stress and psyllid herbivory on the invasive weed Scotch broom,Cytisus scoparius

Increases in the frequency and duration of droughts under global climate change could have implications for plant–insect interactions, and could either increase or decrease the effects of weed biological control agents. In this study, we used greenhouse and field experiments to examine the impacts of drought stress on the abundance and impacts of the adventive psyllid Arytainilla spartiophila Forster (Hemiptera: Psyllidae, Arytaininae) on its target weed Scotch broom, Cytisus scoparius (L.) Link (Fabaceae), in California, USA. The psyllid impacted plant growth in both field and greenhouse experiments, whereas drought stress affected plant growth in the greenhouse only, suggesting that other factors besides water availability may be more limiting for plants in the field. Effects on psyllid survival were consistent with the plant vigor hypothesis, which predicts that herbivores will perform better on vigorously growing plants; psyllid numbers were lower on drought-stressed plants in the greenhouse and were correlated with plant growth in both the greenhouse and the field. In the greenhouse, the combined effects of the psyllid and drought stress were additive, indicating that the effects of the psyllid were consistent across unstressed and drought-stressed plants. Although the psyllid is unlikely to control Scotch broom on its own, results suggest that it may work in conjunction with drought stress to suppress Scotch broom.     

Demography of the invasive shrub Scotch broom (Cytisus scoparius) at Barrington Tops,New South Wales: insights for management.

The exotic shrub Scotch Broom (Cytisus scoparius) has invaded large areas of eucalypt woodland at Barrington Tops, New South Wales, where it forms dense stands that have significant impacts on vegetation structure, flora and fauna. Data are presented from four 25 m² plots, which have been studied since 1985. Two plots were located in uniform broom thickets of different ages, and two were located across the margins of broom stands, which have since expanded to cover the entire plots. All broom plants in the plots (other than young seedlings, which were counted) were mapped, tagged and monitored annually. New seedlings appeared annually, but there was no relationship between their numbers (varying between years) and subsequent recruitment of older plants. The probability of seedlings reaching first flowering was less than 2%, and of surviving to mature size (>10 cm² basal area) was negligible. Seedlings mainly died through suppression (shade). Individuals less than 50 cm high were also browsed. Recruitment occurred only where light levels were high, either before closure of the broom canopy or after senescence had led to canopy opening. From approximately 12–30 years after initial invasion, broom stands underwent self‐thinning of mature plants, accelerated by collapse of plants on to each other. Recruitment of new, maturing plants, after this period produced a stand that was less dense than that found after initial invasion. Broom is creating more disturbance‐prone environments due to its impacts on other biota, likely alterations to the fire regime, and by harbouring feral pigs. Further disturbance favours broom, and elsewhere it has resulted in massive seedling regeneration. While fire or other disturbance can be used to stimulate germination, and thereby reduce a large part of the soil seed bank, denser broom infestations are likely to result unless follow‐up treatments can be applied over long time periods. A wiser management option, at least in the short term, may be avoidance of all disturbance, especially for stands of mature broom.     

Introduced Scotch broom (Cytisus scoparius) invades the genome of native populations in vulnerable heathland habitats

Cytisus scoparius is a global invasive species that affects local flora and fauna at the intercontinental level. Its natural distribution spans across Europe, but seeds have also been moved among countries, mixing plants of native and non‐native genetic origins. Hybridization between the introduced and native gene pool is likely to threaten both the native gene pool and the local flora. In this study, we address the potential threat of invasive C. scoparius to local gene pools in vulnerable heathlands. We used nuclear single nucleotide polymorphic (SNP) and simple sequence repeat (SSR) markers together with plastid SSR and indel markers to investigate the level and direction of gene flow between invasive and native heathland C. scoparius. Analyses of population structures confirmed the presence of two gene pools: one native and the other invasive. The nuclear genome of the native types was highly introgressed with the invasive genome, and we observed advanced‐generation hybrids, suggesting that hybridization has been occurring for several generations. There is asymmetrical gene flow from the invasive to the native gene pool, which can be attributed to higher fecundity in the invasive individuals, measured by the number of flowers and seed pods. Strong spatial genetic structure in plastid markers and weaker structure in nuclear markers suggest that seeds spread over relatively short distances and that gene flow over longer distances is mainly facilitated by pollen dispersal. We further show that the growth habits of heathland plants become more vigorous with increased introgression from the invaders. Implications of the findings are discussed in relation to future management of invading C. scoparius.     

Pollinators exert positive selection on flower size on urban,but not on rural Scotch broom (Cytisus scoparius L. Link)

Aims Adaptive evolution of invasive species is both particularly exciting for the evolutionary biologist and worrisome for those interested in controlling or halting spread. Invasive species often have a distinct timeline and well-recorded population expansion. As invaders encounter new environments, they undergo rapid adaptive evolution. Our aim in this study was to measure variation of floral size in the invasive shrub Cytisus scoparius (Scotch broom) and measure natural selection by pollinators on that trait. Past research has found that this invasive plant is pollinator limited in Washington State and that declines in pollinator populations can contribute to local extinction in another invaded range (New Zealand). This plant is pollinated by both native and introduced species of bees, representing a broad range of pollinator sizes. Cytisus scoparius has a flower structure that is highly conducive to studies on pollinator choice, even in the absence of direct pollinator observations.Methods We surveyed urban and rural sites in and around the city of Olympia in Washington State. Measuring banner width, we were able to show that flower size varies substantially between plants but minimally within plants. By measuring the proportion of flowers that were 'tripped', we could determine pollinator visitation rates and thus determine the level of selection due to pollinator choice alone.Important findings We found that C. scoparius is under natural selection by pollinators for increased flower size. However, such positive natural selection was only seen in urban populations although it was consistent across two flowering seasons. Rural populations of Scotch broom do not appear to be under selection on flower size. The natural selection by pollinators on broom flowers could result in adaptive evolution into a new pollination niche by an invading species. A higher level of variation in broom flowers seen here than seen in previous works in native regions suggests that C. scoparius may be highly diverse and primed for adaptive evolution.     

Post-establishment assessment of host plant specificity of Arytainilla spartiophila (Hemiptera: Psyllidae), an adventive biological control agent of Scotch broom,Cytisus scoparius

Scotch broom, Cytisus scoparius (Fabaceae), is a shrub native to Europe that is invasive in the USA, New Zealand and Australia. The psyllid Arytainilla spartiophila has been purposely introduced to Australia and New Zealand as a biological control agent of C. scoparius, but is an accidental introduction to California. Lupines (Lupinus spp.) are the closest native taxon to Cytisus in North America, and are therefore considered to be at the highest risk for non-target damage. However, because no lupines are native to Australia or New Zealand, only one imported forage species was evaluated during prior host specificity testing. We conducted a laboratory nymphal transfer experiment, a field choice experiment and a field survey to assess risk to three lupine species (Lupinus albifrons, Lupinus bicolor and Lupinus formosus). In the laboratory, 20% of third-instar nymphs were able to develop to adulthood on L. formosus but not on the other lupine species, while 40% completed development on C. scoparius. In the field experiment, potted lupine and C. scoparius plants were placed beside large infested C. scoparius plants; oviposition occurred on all the potted C. scoparius plants, but on none of the lupines. In the field survey, no A. spartiophila eggs or nymphs were found on naturally occurring lupines growing adjacent to infested C. scoparius. The results indicate that A. spartiophila is not likely to damage or reproduce on lupines in the field. This study provides an example of how field studies can help clarify the host specificity of biological control agents.     

Chloroplast microsatellites: measures of genetic diversity and the effect of homoplasy

Chloroplast microsatellites have been widely used in population genetic studies of conifers in recent years. However, their haplotype configurations suggest that they could have high levels of homoplasy, thus limiting the power of these molecular markers. A coalescent-based computer simulation was used to explore the influence of homoplasy on measures of genetic diversity based on chloroplast microsatellites. The conditions of the simulation were defined to fit isolated populations originating from the colonization of one single haplotype into an area left available after a glacial retreat. Simulated data were compared with empirical data available from the literature for a species of Pinus that has expanded north after the Last Glacial Maximum. In the evaluation of genetic diversity, homoplasy was found to have little influence on Nei's unbiased haplotype diversity (H(E)) while Goldstein's genetic distance estimates (D2sh) were much more affected. The effect of the number of chloroplast microsatellite loci for evaluation of genetic diversity is also discussed.     

Microsatellite loci isolated from the tropical tree Hymenaea courbaril L. (Fabaceae)

We developed 11 microsatellite markers for Hymenaea courbaril for the purpose of studying spatial genetic structure and gene flow. The microsatellite loci were screened in 44 trees from two populations. All loci were polymorphic, exhibiting between two and 16 alleles, and levels of expected heterozygosity from 0.174 to 0.909. Departures from Hardy-Weinberg equilibrium were detected for all loci in one population. The estimated null allele frequency is low or moderate. No locus combinations exhibited linkage disequilibrium.     

Population differentiation of sessile oak at the altitudinal front of migration in the French Pyrenees

To assess the effects of altitude on the level and structure of genetic diversity, a genetic survey was conducted in 12 populations of sessile oak (Quercus petraea) located between 130 and 1660 m in two parallel valleys on the northern side of the Pyrenees Mountains. Genetic diversity was monitored at 16 nuclear microsatellite loci and 5 chloroplast DNA (cpDNA) markers. The cpDNA survey suggested that extant populations in both valleys shared the same source populations from the plain. There was no visible trend of nuclear genetic diversity along altitude, even if indirect estimates of effective population sizes revealed a consistent reduction at higher altitudes. Population differentiation, although low, was mostly present among populations of the same valleys and reached similar levels than differentiation across the range of distribution of sessile oak. Contribution to the overall differentiation in the valleys was mostly due to the genetic divergence of the highest populations and the altitudinal variation of allelic frequencies at a few loci. Bayesian inference of migration between groups of populations showed that gene flow is preferentially unidirectional from lower altitudes in one valley to other groups of populations. Finally, we found evidence of clonal reproduction in high altitude populations. The introgression of Quercus robur and Quercus pubescens was also more frequent at the altitudinal margin suggesting that this mechanism may have contributed to the present migration and adaptation of Q. petraea and may also facilitate its future upslope shift in the context of climate change.     

Genetic structure of Araucaria angustifolia (Araucariaceae) populations in Brazil: implications for the in situ conservation of genetic resources

The distribution of the genetic variation within and among natural populations of A. ANGUSTIFOLIA growing in different regions in Brazil was assessed at microsatellite and AFLP markers. Both markers revealed high gene diversity ( H = 0.65; AR = 9.1 for microsatellites and H = 0.27; P = 77.8 % for AFLPs), moderate overall differentiation ( RST = 0.13 for microsatellites and FST = 0.10 for AFLPs), but high divergence of the northernmost, geographically isolated population. In a Bayesian analysis, microsatellite data suggested population structure at two levels: at K = 2 and at K = 3 in agreement to the geographical distribution of populations. This result was confirmed by the UPGMA dendrogram based on microsatellite data (bootstrap support > 95 %). Non-hierarchical AMOVA revealed high variation among populations from different A POSTERIORI defined geographical groups. The genetic distance between sample locations increased with geographical distance for microsatellites ( R = 0.62; P = 0.003) and AFLPs ( R = 0.32; P = 0.09). This pattern of population differentiation may be correlated with population history such as geographical isolation and postglacial colonization of highlands. Implications of the population genetic structure for the conservation of genetic resources are discussed.     

Founding events in species invasions: genetic variation, adaptive evolution, and the role of multiple introductions

Invasive species are predicted to suffer from reductions in genetic diversity during founding events, reducing adaptive potential. Integrating evidence from two literature reviews and two case studies, we address the following questions: How much genetic diversity is lost in invasions? Do multiple introductions ameliorate this loss? Is there evidence for loss of diversity in quantitative traits? Do invaders that have experienced strong bottlenecks show adaptive evolution? How do multiple introductions influence adaptation on a landscape scale? We reviewed studies of 80 species of animals, plants, and fungi that quantified nuclear molecular diversity within introduced and source populations. Overall, there were significant losses of both allelic richness and heterozygosity in introduced populations, and large gains in diversity were rare. Evidence for multiple introductions was associated with increased diversity, and allelic variation appeared to increase over long timescales (~100 years), suggesting a role for gene flow in augmenting diversity over the long‐term. We then reviewed the literature on quantitative trait diversity and found that broad‐sense variation rarely declines in introductions, but direct comparisons of additive variance were lacking. Our studies of Hypericum canariense invasions illustrate how populations with diminished diversity may still evolve rapidly. Given the prevalence of genetic bottlenecks in successful invading populations and the potential for adaptive evolution in quantitative traits, we suggest that the disadvantages associated with founding events may have been overstated. However, our work on the successful invader Verbascum thapsus illustrates how multiple introductions may take time to commingle, instead persisting as a ‘mosaic of maladaptation’ where traits are not distributed in a pattern consistent with adaptation. We conclude that management limiting gene flow among introduced populations may reduce adaptive potential but is unlikely to prevent expansion or the evolution of novel invasive behaviour.     

Contrasting patterns in genetic diversity following multiple invasions of fresh and brackish waters

Biological invasions may combine the genetic effects of population bottlenecks and selection and thus provide valuable insight into the role of such processes during novel environmental colonizations. However, these processes are also influenced by multiple invasions, the number of individuals introduced and the degree of similarity between source and receiving habitats. The amphipod Gammarus tigrinus provides a useful model to assess these factors, as its invasion history has involved major environmental transitions. This species is native to the northwest Atlantic Ocean, although it invaded both brackish and freshwater habitats in the British Isles after introduction more than 65 years ago. It has also spread to similar habitats in Western Europe and, most recently, to Eastern Europe, the Baltic Sea, and the Laurentian Great Lakes. To examine sources of invasion and patterns of genetic change, we sampled populations from 13 native estuaries and 19 invaded sites and sequenced 542 bp of the mitochondrial COI gene. Strong native phylogeographical structure allowed us to unambiguously identify three allopatrically evolved clades (2.3-3.1% divergent) in invading populations, indicative of multiple introductions. The most divergent clades occurred in the British Isles and mainland Europe and were sourced from the St Lawrence and Chesapeake/Delaware Bay estuaries. A third clade was found in the Great Lakes and sourced to the Hudson River estuary. Despite extensive sampling, G. tigrinus did not occur in freshwater at putative source sites. Some European populations showed reduced genetic diversity consistent with bottlenecks, although selection effects cannot be excluded. The habitat distribution of clades in Europe was congruent with the known invasion history of secondary spread from the British Isles. Differences in salinity tolerance among lineages were suggested by patterns of habitat colonization by different native COI clades. Populations consisting of admixtures of the two invading clades were found principally at recently invaded fresh and brackish water sites in Eastern Europe, and were characterized by higher genetic diversity than putative source populations. Further studies are required to determine if these represent novel genotypes. Our results confirm that biological invasions need not result in diminished genetic diversity, particularly if multiple source populations, each with distinctive genetic composition, contribute to the founding populations.     

Fire increases variance in genetic characteristics of Florida Sand Skink (Plestiodon reynoldsi) local populations

Fire is a complex event that maintains many ecological systems. The Florida Sand Skink (Plestiodon reynoldsi) is precinctive to Florida Scrub, a habitat that is maintained by infrequent fire. We characterize the effect of fire on genetic diversity and genetic differentiation at eight microsatellite loci in the Florida Sand Skink (n=470) collected from 30 replicate sites over three 'time since last fire' categories at the Archbold Biological Station. Long unburned sites had greater allelic richness and expected heterozygosity than either recently or intermediately burned sites. More recently, burned sites had greater standard deviations of allelic richness and private allelic richness. Expected heterozygosity positively correlated with 'time since fire' (r=0.36, P=0.05) and abundance (r=0.53, P=0.002). There was a significant spatial component to genetic differentiation, and results indicate individuals rarely disperse >1 km. Genetic differentiation was positively correlated with geographic distance in long unburned units (r=0.59, P=0.04), yet this relationship was disrupted by fire in recently (r=0.00, 1) and intermediately (r= -0.81, 0.05) burned areas. Simulations indicate that demographic changes to a local population could have generated the observed differences among 'time since fire' categories. Our findings indicate that infrequent fire may be beneficial to the Florida Sand Skink and that local populations begin to recover from changes attributable to the fire after 10 years. Too frequent fires may reduce genetic diversity because it may take multiple generations for local populations to recover.     

Isolation and characterization of 15 nuclear microsatellite markers for Baillonella toxisperma Pierre (Sapotaceae), a low-density tree species of Central Africa

Baillonella toxisperma is a scattered endemic species of the Guineo-Congolian forest. We developed 15 nuclear microsatellite markers specifically for B. toxisperma to analyse the genetic diversity of 188 individuals distributed in two natural populations in Gabon. On average, 8.3 alleles per locus were identified; observed heterozygosity ranged from 0.056 to 0.872 and expected heterozygosity from 0.282 to 0.823 among populations. Only three loci showed a significant departure to Hardy-Weinberg equilibrium, but no linkage disequilibrium was observed for any pair of loci.     

Phylogeography and genetic diversity of the widespread katydid Ducetia japonica (Thunberg, 1815) across China

Habitat fragmentation can lower migration rates and genetic connectivity among remaining populations of native species. Ducetia japonica is one of the most widespread katydids in China, but little is known about its genetic structure and phylogeographic distribution. We combined the five‐prime region of cytochrome c oxidase subunit I (COI‐5P), 11 newly developed microsatellite loci coupled with an ecological niche model (ENM) to examine the genetic diversity and population structure of D. japonica in China and beyond to Laos and Singapore. Both Bayesian inference (BI) and haplotype network methods revealed six mitochondrial COI‐5P lineages. The distribution of COI‐5P haplotypes may not demonstrate significant phylogeographic structure (N _ST > G _ST, p > .05). The STRUCTURE analysis based on microsatellite data also revealed six genetic clusters, but discordant with those obtained from COI‐5P haplotypes. For both COI‐5P and microsatellite data, Mantel tests revealed a significant positive correlation between geographic and genetic distances in mainland China. Bayesian skyline plot (BSP) analyses indicated that the population size of D. japonica''s three major mitochondrial COI‐5P lineages were seemingly not affected by last glacial maximum (LGM, 0.015–0.025 Mya). The ecological niche models showed that the current distribution of D. japonica was similar to the species’ distribution during the LGM period and only slightly extended in northern China. Further phylogeographic studies based on more extensive sampling are needed to identify specific locations of glacial refugia in northern China.     

A multilocus population genetic survey of the greater sage-grouse across their range

The distribution and abundance of the greater sage-grouse (Centrocercus urophasianus) have declined dramatically, and as a result the species has become the focus of conservation efforts. We conducted a range-wide genetic survey of the species which included 46 populations and over 1000 individuals using both mitochondrial sequence data and data from seven nuclear microsatellites. Nested clade and structure analyses revealed that, in general, the greater sage-grouse populations follow an isolation-by-distance model of restricted gene flow. This suggests that movements of the greater sage-grouse are typically among neighbouring populations and not across the species, range. This may have important implications if management is considering translocations as they should involve neighbouring rather than distant populations to preserve any effects of local adaptation. We identified two populations in Washington with low levels of genetic variation that reflect severe habitat loss and dramatic population decline. Managers of these populations may consider augmentation from geographically close populations. One population (Lyon/Mono) on the southwestern edge of the species' range appears to have been isolated from all other greater sage-grouse populations. This population is sufficiently genetically distinct that it warrants protection and management as a separate unit. The genetic data presented here, in conjunction with large-scale demographic and habitat data, will provide an integrated approach to conservation efforts for the greater sage-grouse.     

Range-wide genetic structure in a north-east Asian spruce (Picea jezoensis) determined using nuclear microsatellite markers

Aim We used microsatellite markers to determine the range‐wide genetic structure of Picea jezoensis and to test the hypothesis that the past population history of this widespread cold‐temperate spruce has resulted in a low level of genetic variation and in imprints of inbreeding and bottlenecks in isolated marginal populations. Location The natural range of the three infraspecific taxa of P. jezoensis throughout north‐east Asia, including isolated marginal populations. Methods We analysed a total of 990 individuals across 33 natural populations using four nuclear microsatellite loci. Population genetic structure was assessed by analysing genetic diversity indices for each population, examining clustering (model‐based and distance‐based) among populations, evaluating signals of recent bottlenecks, and testing for isolation by distance (IBD). Results The 33 populations were clustered into five groups. The isolated marginal groups of populations (in Kamchatka, Kii in Japan and South Korea) exhibited low levels of allelic richness and gene diversity and a complete or almost complete loss of rare alleles. A recent bottleneck was detected in the populations in Hokkaido across to mid‐Sakhalin. The IBD analysis revealed that genetic divergence between populations was higher for populations separated by straits. Main conclusions Picea jezoensis showed a higher level of genetic differentiation among populations (F_ST = 0.101) than that observed in the genus Picea in general. This might be attributable to the fact that historically the straits around Japan acted as barriers to the movement of seeds and pollen. The low levels of genetic diversity in the isolated marginal population groups may reflect genetic drift that has occurred after isolation. Evidence of a significant bottleneck between the Hokkaido and mid‐Sakhalin populations implies that the cold, dry climate in the late Pleistocene resulted in the decline and contraction of populations, and that there was a subsequent expansion followed by a founder effect when conditions improved. The high polymorphism observed in P. jezoensis nuclear microsatellites revealed cryptic genetic structure that organellar DNA markers failed to identify in a previous study.     

中国西南地区重要木本油料植物扁核木的遗传结构及成因

扁核木(Prinsepia utilis)为中国西南地区温带森林重要的木本油料植物, 但对其野生资源种群遗传结构及成因的了解严重不足。我们采用核微卫星分子标记, 对32个扁核木自然种群共377个个体的群体演化历史进行了探讨, 并评估其遗传资源。研究发现扁核木种群自西向东可划分4个遗传群组, 即喜马拉雅、横断山以及云贵高原西部和东部群组。其中, 最大的遗传分化存在于喜马拉雅和其他区域种群间。与喜马拉雅和云贵高原东部群组相比, 横断山和云贵高原西部群组混合了其他群组的遗传成分。种群动态历史分析显示中部2个群组在喜马拉雅和云贵高原东部群组形成后形成, 不同群组间的分化均发生在更新世晚期。地理隔离和环境隔离分析表明扁核木种群间的遗传分化主要由环境差异导致。环境差异分析显示不同群组间的气候存在不同程度的差异, 其中喜马拉雅和云贵高原东部群组与中部2个群组间的差异显著。此外, 结合该物种不同时期的生态位模拟数据, 我们认为喜马拉雅和云贵高原地区的遗传资源在未来需要优先保护。