Parallel evolution of dwarf ecotypes in the forest tree Eucalyptus globulus

Three small populations of a dwarf ecotype of the forest tree Eucalyptus globulus are found on exposed granite headlands in south-eastern Australia. These populations are separated by at least 100 km. Here, we used 12 nuclear microsatellites and a chloroplast DNA marker to investigate the genetic affinities of the dwarf populations to one another and to their nearest populations of tall E. globulus. Cape Tourville was studied in greater detail to assess the processes enabling the maintenance of distinct ecotypes in close geographical proximity. The three dwarf populations were not related to one another and were more closely related to adjacent tall trees than to one another. At Cape Tourville the dwarf and tall ecotypes were significantly differentiated in microsatellites and in chloroplast DNA. The dwarf and tall populations differed in flowering time and no evidence of pollen dispersal from the more extensive tall to the dwarf population was found. The three dwarf populations have evolved in parallel from the local tall ecotypes. This study shows that small marginal populations of eucalypts are capable of developing reproductive isolation from nearby larger populations through differences in flowering time and/or minor spatial separation, making parapatric speciation possible.     

Complete Nucleotide Sequence of the Chloroplast Genome from the Tasmanian Blue Gum, Eucalyptus globulus (Myrtaceae)

The complete nucleotide sequence of the chloroplast genome ofthe hardwood species Eucalyptus globulus is presented and comparedwith chloroplast genomes of tree and non-tree angiosperms andtwo softwood tree species. The 160 286 bp genome is similarin gene order to that of Nicotiana, with an inverted repeat(IR) (26 393 bp) separated by a large single copy (LSC) regionof 89 012 bp and a small single copy region of 18 488 bp. Thereare 128 genes (112 individual gene species and 16 genes duplicatedin the inverted repeat) coding for 30 transfer RNAs, 4 ribosomalRNAs and 78 proteins. One pseudogene (-infA) and one pseudo-ycf(-ycf15) were identified. The chloroplast genome of E. globulusis essentially co-linear with that of another hardwood treespecies, Populus trichocarpa, except that the latter lacks rps16and rpl32, and the IR has expanded in Populus to include rps19(part of the LSC in E. globulus). Since the chloroplast genomeof E. globulus is not significantly different from other treeand non-tree angiosperm taxa, a comparison of hardwood and softwoodchloroplasts becomes, in essence, a comparison of angiospermand gymnosperm chloroplasts. When compared with E. globulus,Pinus chloroplasts have a very small IR, two extra tRNAs andfour additional photosynthetic genes, lack any functional ndhgenes and have a significantly different genome arrangement.There does not appear to be any correlation between plant habitand chloroplast genome composition and arrangement.     

Phenology of Eucalyptus weevil, Gonipterus scutellatus Gyllenhal (Coleoptera: Curculionidae), and chrysomelid beetles in Eucalyptus globulus plantations in south-western Australia

1 In south‐western Australia, Eucalyptus globulus plantations are defoliated by a complex of beetle species, yet only scant information exists on these species under such climatic conditions. To improve management of these defoliating beetles in the region, canopy fogging and shoot clipping were conducted in plantations between 1999 and 2002 to identify and document the phenology of the beetle species present. 2 Eucalyptus weevil, Gonipterus scutellatus, was the most common and destructive defoliating beetle. Gonipterus scutellatus undergoes one principal generation each year with a lesser second generation or cohort in some seasons, which contrasts greatly with reports of two to four annual generations for the species in other regions. This limited reproduction by G. scutellatus may be due to the limited availability from summer onwards of new flushing foliage, which is essential for feeding and oviposition. 3 Several species of chrysomelid beetles were collected in plantations, but these were present in much lower numbers than G. scutellatus and were only a minor concern. However, some species, such as Chrysophtharta variicollis, appear to be capable of developing short‐lived outbreaks. 4 A diverse suite of natural enemies was fogged from plantations but they were significantly less abundant than defoliating beetles and are not likely to provide significant control of beetles. 5 In terms of managing these defoliating beetles, monitoring and control should focus on G. scutellatus, and be conducted during spring when most damage occurs.     

Gene flow of the canker pathogen Botryosphaeria australis between Eucalyptus globulus plantations and native eucalypt forests in Western Australia

Abstract The eucalypt plantation industry in Western Australia provides a unique opportunity to study the movement of pathogens between closely related host taxa. Eucalyptus globulus, a native to Tasmania and south‐eastern Australia, is the predominant species in Western Australian plantations, often being planted adjacent to native forest containing Eucalyptus marginata and Eucalyptus diversicolor. Since the commencement of the plantation industry 20 years ago, several fungal species, previously known only to eastern Australia or overseas, have been reported on E. globulus in Western Australia. Botryosphaeria australis is a newly described species, recently found causing cankers on Acacia spp. in eastern Australia. However, during a routine survey, B. australis was found to be the predominant species associated with E. globulus plantations and native Eucalyptus spp. in Western Australia. In this study, six short simple repeat markers were used to evaluate genetic diversity and gene flow between collections of B. australis from native eucalypt forest and E. globulus plantations at two locations in south‐western Australia. In both cases, there was no restriction to gene flow between the plantations and the adjacent native forest. Botryosphaeria australis has now been isolated from a wide range of hosts across south‐western Australia and was not isolated from E. globulus in Tasmania or South Australia. This extensive distribution and host range suggests B. australis is native to Western Australia. This study demonstrates the ability of a pathogen to move between plantation and forests.     

Fruiting of putative ectomycorrhizal fungi under blue gum (Eucalyptus globulus) plantations of different ages in Western Australia

 The species richness of putative ectomycorrhizal (EM) fungi fruiting in blue gum (Eucalyptus globulus Labill.) plantations in Western Australia was investigated in relation to plantation age. Eleven plantations, 1–8 years old, were selected for study and two native Eucalyptus forest sites in the same region were chosen for comparison. Sporocarps of 44 species of putative EM fungi were collected from the 13 sites. Of these, 30 species were found in blue gum plantations. The number of fungal species was highly positively correlated with plantation age and inversely correlated with soil pH. Young plantations (1–5 years) had 2–9 fungal species and were overwhelmingly dominated by species of Laccaria and Scleroderma. In older plantations (6–8 years), the relative abundance of sporocarps of each species within the fungal community decreased, accompanied by an increase in the number of fungal species (12–17 per site). A brief survey of the two native eucalypt forests in this region revealed a much higher number of fungal species than that observed in plantations. In plantations, species of Descolea, Laccaria, Pisolithus and Scleroderma typically fruited in young plantations. Species of epigeous fungi of the genera Boletus, Cortinarius, Hydnum, Inocybe, Lactarius, Paxillus, Russula and hypogeous fungi, including species of Descomyces, Hysterangium and Mesophellia, were found only in older plantations, or in native forests. Some of the fungi that fruit in young plantations are now being evaluated for use in commercial spore inoculation programs to increase the species diversity of EM fungi in exotic eucalypt plantations. Accepted: 8 October 1998     

Patterns of hybridization and asymmetrical gene flow in hybrid zones of the rare Eucalyptus aggregata and common E. rubida

The patterns of hybridization and asymmetrical gene flow among species are important for understanding the processes that maintain distinct species. We examined the potential for asymmetrical gene flow in sympatric populations of Eucalyptus aggregata and Eucalyptus rubida, both long-lived trees of southern Australia. A total of 421 adults from three hybrid zones were genotyped with six microsatellite markers. We used genealogical assignments, admixture analysis and analyses of spatial genetic structure and spatial distribution of individuals, to assess patterns of interspecific gene flow within populations. A high number of admixed individuals were detected (13.9–40% of individuals), with hybrid populations consisting of F₁ and F₂ hybrids and backcrosses in both parental directions. Across the three sites, admixture proportions were skewed towards the E. aggregata genetic cluster (x=0.56–0.65), indicating that backcrossing towards E. aggregata is more frequent. Estimates of long-term migration rates also indicate asymmetric gene flow, with higher migration rates from E. aggregata to hybrids compared with E. rubida. Taken together, these results indicate a greater genetic input from E. aggregata into the hybrid populations. This asymmetry probably reflects differences in style lengths (E. rubida: ∼7 mm, E. aggregata: ∼4 mm), which can prevent pollen tubes of smaller-flowered species from fertilizing larger-flowered species. However, analyses of fine-scale genetic structure suggest that localized seed dispersal (<40 m) and greater clustering between hybrid and E. aggregata individuals may also contribute to directional gene flow. Our study highlights that floral traits and the spatial distributions of individuals can be useful predictors of the directionality of interspecific gene flow in plant populations.     

Expansion of the rare Eucalyptus risdonii under climate change through hybridization with a closely related species despite hybrid inferiority

Background and AimsHybridization is increasingly recognized as an integral part of the dynamics of species range expansion and contraction. Thus, it is important to understand the reproductive barriers between co-occurring species. Extending previous studies that argued that the rare Eucalyptus risdonii was expanding into the range of the surrounding E. amygdalina by both seed and pollen dispersal, we here investigate the long-term fitness of both species and their hybrids and whether expansion is continuing.MethodsWe assessed the survival of phenotypes representing a continuum between the two pure species in a natural hybrid swarm after 29 years, along with seedling recruitment. The performance of pure species as well as of artificial and natural hybrids was also assessed over 28 years in a common garden trial.Key ResultsIn the hybrid zone, E. amygdalina adults showed greater mortality than E. risdonii, and the current seedling cohort is still dominated by E. risdonii phenotypes. Morphologically intermediate individuals appeared to be the least fit. Similar results were observed after growing artificial first-generation and natural hybrids alongside pure species families in a common garden trial. Here, the survival, reproduction, health and growth of the intermediate hybrids were significantly less than those of either pure species, consistent with hybrid inferiority, although this did not manifest until later reproductive ages. Among the variable progeny of natural intermediate hybrids, the most E. risdonii-like phenotypes were the most fit.ConclusionsThis study contributes to the increasing number of reports of hybrid inferiority in Eucalyptus, suggesting that post-zygotic barriers contribute to the maintenance of species integrity even between closely related species. However, with fitness rapidly recovered following backcrossing, it is argued that hybridization can still be an important evolutionary process, in the present case appearing to contribute to the range expansion of the rare E. risdonii in response to climate change.     

The influence of drought on the relationship between leaf and conducting sapwood area in Eucalyptus globulus and Eucalyptus nitens

 Development of the relationship between leaf area (A _l ) and sapwood area (A _s ) was investigated in two important hardwoods, Eucalyptus globulus (Labill) and E. nitens (Deane and Maiden) Maiden, growing in an experimental plantation established in a low rainfall zone (approx. 515 mm year^–1) of Tasmania. The experiment compared irrigated controls and a rainfed treatment which was subjected to cyclical summer droughts from age 1 to 6 years old. Leaf area and sapwood area were determined by destructive sampling at ages 2, 3 and 6 years old. There was no effect of stand age on A _l :A _s when sapwood area was measured at crown break. At age 3 years old A _l :A _s was significantly greater in the rainfed than the irrigated trees. It was concluded that this difference was due to earlier canopy closure in the irrigated trees. When the plantation was 6 years old A _l :A _s was significantly greater in the irrigated than the rainfed treatment. An analysis based on an equation which links A _l :A _s with transpiration and volumetric flow rate (Whitehead et al. 1984) was used to infer a positive correlation between stem hydraulic conductivity (k _h ) and water availability. Independent of water availability E. globulus maintained a higher A _l :A _s than E. nitens at all ages. Received: 20 March 1997 / Accepted: 30 December 1997     

Differential gene flow of mitochondrial and nuclear DNA markers among chromosomal races of Australian morabine grasshoppers (Vandiemenella, viatica species group)

Recent theoretical developments have led to a renewed interest in the potential role of chromosomal rearrangements in speciation. Australian morabine grasshoppers (genus Vandiemenella, viatica species group) provide an excellent study system to test this potential role of chromosomal rearrangements because they show extensive chromosomal variation and formed the basis of a classic chromosomal speciation model. There are three chromosomal races, viatica19, viatica17, and P24(XY), on Kangaroo Island, South Australia, forming five parapatric populations with four putative contact zones among them. We investigate the extent to which chromosomal variation among these populations may be associated with barriers to gene flow. Population genetic and phylogeographical analyses using 15 variable allozyme loci and the elongation factor-1alpha (EF-1alpha) gene indicate that the three races represent genetically distinct taxa. In contrast, analyses of the mitochondrial cytochrome c oxidase subunit I (COI) gene show the presence of three distinctive and geographically localized groups that do not correspond with the distribution of the chromosomal races. These discordant population genetic patterns are likely to result from introgressive hybridization between the chromosomal races and range expansions/contractions. Overall, these results suggest that reduction of nuclear gene flow may be associated with chromosomal variation, or underlying genetic variation linked with chromosomal variation, whereas mitochondrial gene flow appears to be independent of this variation in these morabine grasshoppers. The identification of an intact contact zone between P24(XY) and viatica17 offers considerable potential for further investigation of molecular mechanisms that maintain distinct nuclear genomes among the chromosomal races.     

Nutrient dynamics throughout the rotation of Eucalyptus clonal stands in Congo

The dynamics of the main nutrient fluxes of the biological cycle were quantified in a clonal Eucalyptus plantation throughout the whole planted crop rotation: current annual requirements of nutrients, uptake from the soil, internal translocations within trees, return to soil (litterfall and crown leaching) and decomposition in the forest floor. As reported for other species, two growth periods were identified in these short-rotation plantations: (1) a juvenile phase up to canopy closure, during which the uptake of nutrients from the soil reserves supplied most of the current requirements; and (2) a second phase up to harvest, characterized by intense nutrient recycling processes. Internal translocation within trees supplied about 30 % of the annual requirements of N and P from 2 years of age onwards, and about 50 % of the K requirement. The mineralization of large amounts of organic matter returned to the soil with litterfall during stand development represented a key process providing nutrients to the stand at the end of the rotation. The importance of the recycling processes was clearly shown by the small amounts of nutrients permanently immobilized in the ligneous components of trees, compared with the total requirements accumulated over the stand rotation which were two to four times higher. Small pools of nutrients circulating quickly in the ecosystem made it possible to produce high amounts of biomass in poor soils. The sustainability of these plantations will require fertilizer inputs that match the changes in soil fertility over successive rotations, mainly linked to the dynamics of organic matter in this tropical soil.     

Chloroplast DNA diversity associated with protected slopes and valleys for hybridizing Eucalyptus species on isolated ranges in south‐eastern Australia

Aim To relate genetic diversity to topographic features and to investigate genetic interactions between Eucalyptus species in a local centre of endemism and diversity in south‐eastern Australia. Location Grampian Ranges, Victoria, Australia. Methods We documented chloroplast DNA (cpDNA) variation for a group of endemic Eucalyptus species (E. serraensis, E. verrucata and E. victoriana) that dominate rocky, high‐elevation ridgelines of the Grampian Ranges and for one closely‐related, widespread species (E. baxteri) occupying flanking slopes and valleys. We documented genetic patterns across the landscape using cpDNA microsatellites, and related them to topographic features (exposed west‐facing versus protected east‐facing slopes and valleys). We also determined the extent of local haplotype sharing between populations of endemic species and neighbouring E. baxteri downslope with cpDNA microsatellites, and haplotype sharing between the endemic group and more distantly related species (E. obliqua, E. pauciflora and E. willisii) with sequences of the J_LA+ chloroplast region. Results We detected 26 cpDNA microsatellite haplotypes in a relatively small area of c. 20 km × 50 km. Populations of E. baxteri on east‐facing slopes and valleys had greater cpDNA microsatellite diversity than E. baxteri and endemic species on exposed west‐facing slopes. Endemic species frequently shared chloroplast haplotypes with E. baxteri downslope. Sharing of J_LA+ haplotypes with species outside the endemic group was mostly restricted to E. victoriana, which had cpDNA more similar to the species from other sections of Eucalyptus (E. obliqua, E. willisii and E. pauciflora). Main conclusions Intensive sampling of related species on small isolated mountain ranges allowed us to relate genetic diversity to fine‐scale habitats and to document extensive local haplotype sharing between species. This study contributes to a general understanding of the environmental conditions that enable plant population persistence by linking concentrations of genetic diversity to particular habitats.     

The relationship of the female reproductive success of Eucalyptus globulus to the endogenous properties of the flower

Low capsule and seed set is a major factor limiting seed production in Eucalyptus globulus seed orchards. Controlled pollination studies showed that the reproductive success (number of seeds produced per flower pollinated) was primarily determined by the female. We aimed to identify the factors contributing to the differences in reproductive success between female genotypes in terms of the physical and anatomical properties of the flower. We studied pairs of genotypes of high and low reproductive success from each of three races (Furneaux Group, Strzelecki Ranges and Western Otways) growing in a seed orchard. Controlled pollinations were performed on six females and along with flower physical measurements, pollen tube growth and seed set were assessed. Overall tree reproductive success was positively correlated with flower size, ovule numbers, style size, cross-sectional area of conductive tissue within the style (all of which were inter-correlated) and the proportion of pollen tubes reaching the bottom of the style. Significant positive correlations of reproductive success and flower physical properties between different ramets of the same genotypes across seasons suggests a genetic basis to the variation observed. The majority of pollen tube attrition occurred within the first millimetre of the cut style and appeared to be associated with differences in style physiology. When examined as pairs within races the difference in reproductive success for the Western Otways pair was simply explained by differences in flower size and the number of ovules per flower. Physical features did not differ significantly for the Strzelecki Ranges pair, but the proportion of pollen tubes reaching the bottom of the style was lower in the less reproductively successful genotype, suggesting an endogenous physiological constraint to pollen tube growth. The difference in reproductive success between the females from the Furneaux Group was associated with a combination of these factors.     

Insect pests of Tasmanian blue gum,Eucalyptus globulus globulus,in south‐western Australia: History,current perspectives and future prospects

Trends in, and potential causes of, insect pest problems of the Tasmanian blue gum, Eucalyptus globulus globulus, in south‐western Australia are reviewed. Historical evidence suggests that insect pest problems of E. g. globulus in south‐western Australia have greatly increased in the last 10 years, which corresponds to a time of rapid expansion of the blue gum industry in the region. Current major establishment pests include the African black beetle, Heteronychus arator, spring beetles, Liparetrus spp. and Heteronyx spp., and the wingless grasshopper, Phaulacridium vittatum. Current major pests of established trees are the Eucalyptus weevil, Gonipterus scutellatus, and chrysomelid beetles, Chrysophtharta spp. and Cadmus excrementarius. The occurrence of these insects on an introduced eucalypt is not unexpected because insect‐rich native eucalypt forests dominate the landscape where E. g. globulus plantations are grown. Insect damage may also be exacerbated because E. g. globulus is grown as a monoculture.     

Introgression and phylogeography of Betula nana (diploid), B. pubescens (tetraploid) and their triploid hybrids in Iceland inferred from cpDNA haplotype variation

Aim The objective was to find direct genetic evidence supporting introgressive hybridization between tetraploid tree birch (Betula pubescens) and diploid dwarf birch (B. nana), via triploid hybrids, and to investigate an association between the introgression and phylogeographical distribution of Icelandic birch. Location Samples were collected from 463 trees in 12 woodlands in Iceland and eight locations in Norway, Sweden, Scotland and Greenland. Methods Ploidy status of individual trees was determined by chromosome counting. Variation in the chloroplast genome was assessed using polymerase chain reaction‐restriction fragment length polymorphism. The geographical distribution of the haplotypes was mapped. The haplotype variation and introgression ratios (IG) were analysed statistically. Results Thirteen haplotypes were identified among Icelandic samples. The most common haplotype (T, 49% occurrence) was present in all ploidy groups and in all woodlands. All common haplotypes were shared between the triploid group and the parental species, indicating introgressive hybridization. This was supported by the statistical analysis of IG indices and the variation components. Considerable differences existed among samples, shaped by isolation by distance and local introgression. An east–west phylogeographical distribution in Iceland was observed. Main conclusions Despite extensive introgression across species and ploidy levels, a biogeographical pattern has been observed, and this may indicate different population histories or multiple origins of Icelandic birch. The chloroplast haplotype diversity found in Iceland resembles that found in birch populations from northern Scandinavia.     

Soil–atmosphere exchange of greenhouse gases in a Eucalyptus marginata woodland, a clover-grass pasture, and Pinus radiata and Eucalyptus globulus plantations

Soils provide the largest terrestrial carbon store, the largest atmospheric CO₂ source, the largest terrestrial N₂O source and the largest terrestrial CH₄ sink, as mediated through root and soil microbial processes. A change in land use or management can alter these soil processes such that net greenhouse gas exchange may increase or decrease. We measured soil–atmosphere exchange of CO₂, N₂O and CH₄ in four adjacent land‐use systems (native eucalypt woodland, clover‐grass pasture, Pinus radiata and Eucalyptus globulus plantation) for short, but continuous, periods between October 2005 and June 2006 using an automated trace gas measurement system near Albany in southwest Western Australia. Mean N₂O emission in the pasture was 26.6 μg N m⁻² h⁻¹, significantly greater than in the natural and managed forests (< 2.0 μg N m⁻² h⁻¹). N₂O emission from pasture soil increased after rainfall events (up to 100 μg N m⁻² h⁻¹) and as soil water content increased into winter, whereas no soil water response was detected in the forest systems. Gross nitrification through ¹⁵N isotope dilution in all land‐use systems was small at water holding capacity < 30%, and under optimum soil water conditions gross nitrification ranged between < 0.1 and 1.0 mg N kg⁻¹ h⁻¹, being least in the native woodland/eucalypt plantation < pine plantation < pasture. Forest soils were a constant CH₄ sink, up to −20 μg C m⁻² h⁻¹ in the native woodland. Pasture soil was an occasional CH₄ source, but weak CH₄ sink overall (−3 μg C m⁻² h⁻¹). There were no strong correlations (R < 0.4) between CH₄ flux and soil moisture or temperature. Soil CO₂ emissions (35–55 mg C m⁻² h⁻¹) correlated with soil water content (R < 0.5) in all but the E. globulus plantation. Soil N₂O emissions from improved pastures can be considerable and comparable with intensively managed, irrigated and fertilised dairy pastures. In all land uses, soil N₂O emissions exceeded soil CH₄ uptake on a carbon dioxide equivalent basis. Overall, afforestation of improved pastures (i) decreases soil N₂O emissions and (ii) increases soil CH₄ uptake.     

DNA fingerprinting supports notions of clonality in a rare mallee, Eucalyptus argutifolia

DNA fingerprinting was used to test for genetic variation within putative clones of Eucalyptus argutifolia, a rare multistemmed (mallee) species endemic to Western Australia. Hybridization of the M13 repeat sequence to HaeIII digested DNA samples was able to discriminate between seedlings from the same maternal plant, demonstrating the capability of this probe in detecting different individuals of this species. Each of the putative clones yielded a unique banding pattern, but no variation was found within clones. These results are consistent with the notion of clonality in E. argutifolia and reaffirm that populations are likely to contain considerably fewer individuals than originally anticipated.     

Phylogeography of the world's tallest angiosperm, Eucalyptus regnans: evidence for multiple isolated Quaternary refugia

Aim There is a need for more Southern Hemisphere phylogeography studies, particularly in Australia, where, unlike much of Europe and North America, ice sheet cover was not extensive during the Last Glacial Maximum (LGM). This study examines the phylogeography of the south‐east Australian montane tree species Eucalyptus regnans. The work aimed to identify any major evolutionary divergences or disjunctions across the species’ range and to examine genetic signatures of past range contraction and expansion events. Location South‐eastern mainland Australia and the large island of Tasmania. Methods We determined the chloroplast DNA haplotypes of 410 E. regnans individuals (41 locations) based on five chloroplast microsatellites. Genetic structure was examined using analysis of molecular variance (AMOVA), and a statistical parsimony tree was constructed showing the number of nucleotide differences between haplotypes. Geographic structure in population genetic diversity was examined with the calculation of diversity parameters for the mainland and Tasmania, and for 10 regions. Regional analysis was conducted to test hypotheses that some areas within the species’ current distribution were refugia during the LGM and that other areas have been recolonized by E. regnans since the LGM. Results Among the 410 E. regnans individuals analysed, 31 haplotypes were identified. The statistical parsimony tree shows that haplotypes divided into two distinct groups corresponding to mainland Australia and Tasmania. The distribution of haplotypes across the range of E. regnans shows strong geographic patterns, with many populations and even certain regions in which a particular haplotype is fixed. Many locations had unique haplotypes, particularly those in East Gippsland in south‐eastern mainland Australia, north‐eastern Tasmania and south‐eastern Tasmania. Higher haplotype diversity was found in putative refugia, and lower haplotype diversity in areas likely to have been recolonized since the LGM. Main conclusions The data are consistent with the long‐term persistence of E. regnans in many regions and the recent recolonization of other regions, such as the Central Highlands of south‐eastern mainland Australia. This suggests that, in spite of the narrow ecological tolerances of the species and the harsh environmental conditions during the LGM, E. regnans was able to persist locally or contracted to many near‐coastal refugia, maintaining a diverse genetic structure.     

Divergence,gene flow,and the origin of leapfrog geographic distributions: The history of colour pattern variation in Phyllobates poison‐dart frogs

The geographic distribution of phenotypic variation among closely related populations is a valuable source of information about the evolutionary processes that generate and maintain biodiversity. Leapfrog distributions, in which phenotypically similar populations are disjunctly distributed and separated by one or more phenotypically distinct populations, represent geographic replicates for the existence of a phenotype, and are therefore especially informative. These geographic patterns have mostly been studied from phylogenetic perspectives to understand how common ancestry and divergent evolution drive their formation. Other processes, such as gene flow between populations, have not received as much attention. Here, we investigate the roles of divergence and gene flow between populations in the origin and maintenance of a leapfrog distribution in Phyllobates poison frogs. We found evidence for high levels of gene flow between neighbouring populations but not over long distances, indicating that gene flow between populations exhibiting the central phenotype may have a homogenizing effect that maintains their similarity, and that introgression between ‘leapfroging’ taxa has not played a prominent role as a driver of phenotypic diversity in Phyllobates. Although phylogenetic analyses suggest that the leapfrog distribution was formed through independent evolution of the peripheral (i.e. leapfrogging) populations, the elevated levels of gene flow between geographically close populations poise alternative scenarios, such as the history of phenotypic change becoming decoupled from genome‐averaged patterns of divergence, which we cannot rule out. These results highlight the importance of incorporating gene flow between populations into the study of geographic variation in phenotypes, both as a driver of phenotypic diversity and as a confounding factor of phylogeographic inferences.     

More introgression with less gene flow: chloroplast vs. mitochondrial DNA in the Picea asperata complex in China, and comparison with other Conifers

Recent work has suggested that rates of introgression should be inversely related to levels of gene flow because introgressed populations cannot be 'rescued' by intraspecific gene flow if it is too low. Mitochondrial and chloroplast DNA (mtDNA and cpDNA) experience very different levels of gene flow in conifers due to their contrasted maternal and paternal modes of transmission, hence the prediction that mtDNA should introgress more readily than cpDNA in this group. Here, we use sequence data from both mtDNA and cpDNA to test this hypothesis in a group of closely related spruces species, the Picea asperata complex from China. Nine mitochondrial and nine chloroplast haplotypes were recovered from 459 individuals in 46 natural populations belonging to five species of the Picea asperata complex. Low variation was found in the two mtDNA introns along with a high level of differentiation among populations ( G _ST = 0.90). In contrast, we detected higher variation and lower differentiation among populations at cpDNA markers ( G _ST = 0.56), a trend shared by most conifer species studied so far. We found that cpDNA variation, although far from being fully diagnostic, is more species-specific than mtDNA variation: four groups of populations were identified using cpDNA markers, all of them related to species or groups of species, whereas for mtDNA, geographical variation prevails over species differentiation. The literature suggests that mtDNA haplotypes are often shared among related conifer species, whereas cpDNA haplotypes are more species-specific. Hence, increased intraspecific gene flow appears to decrease differentiation within species but not among species.