European ash ( Fraxinus excelsior L.) trees with low susceptibility to ash dieback have higher reproductive fitness compared to highly susceptible trees, although most pronounced for female success. Selection at generation turnover therefore supports the future recovery of ash forests.
The introduced invasive pathogen Hymenoscyphus fraxineus (T. Kowalski) Baral, Queloz, and Hosoya cause extensive damage on European ash (Fraxinus excelsior L.). Heritable variation in susceptibility to ash dieback has been observed among ash trees in natural and planted populations, but it is not clear how variation in susceptibility influences reproductive fitness.
We hypothesize that healthier male and female trees contribute more gametes to the following generation compared to unhealthy ones.
We tested the hypothesis by studying gender, seed production, and paternal success in a clonal field trial with 39 replicated clones. In the trial, the susceptibility level of each clone has been recorded in terms of percent crown damage since 2007. We used a linear regression model to explore the relationship between susceptibility and reproductive success (female and male).
The clones revealed a clear gender dimorphism with an approximate 2:2:1 male/female/hermaphrodite ratio. Females with low levels of crown damage produced substantially more seeds compared to highly damaged females. The male clone with the lowest level of susceptibility was the most effective pollen donor, but highly susceptible males also sired some offspring.
The results overall represent good news for the potential recovery of ash forests: selection against most susceptible genotypes at generation turnover is expected to facilitate building up disease resistance in ash populations.
Forest tree species have to cope with various threats, such as emerging infectious diseases, climate change, habitat loss, and fragmentation, which in combination can put them under severe pressure. In forest trees, the primary causes of emerging infectious diseases are fungal and fungal-like pathogens, whose numbers increased 13-fold only from 1995 to 2010 due to expanding networks of international trade and travel (Santini et al. 2013). They present a real challenge for conservation by causing rapid and extensive reduction in numerical abundance and by changing the genetic composition of host populations (Altizer et al. 2003). The outcome of an epidemic in terms of host mortality depends not only on pathogen establishment, development, and virulence but also on how a disease outbreak influences the reproductive success and ultimately the fitness of the host species.
Fraxinus excelsior L. or common ash (Oleaceae, hereinafter ash) is an important component of European forest ecosystems and a valuable timber species (Pliûra and Heuertz 2003). Ash has been decimated in continental Europe by an invasive fungal pathogen, Hymenoscyphus fraxineus (T. Kowalski) Baral, Queloz, Hosoya (Baral et al. 2014), since the early 1990s (McKinney et al. 2014), when it was first observed in Poland (Przybył 2002). Genome-wide population diversity in H. fraxineus indicates an eastern Russian origin of European ash dieback (Sønstebø et al. 2017), where it is asymptomatically associated with Asian Fraxinus species (Cleary et al. 2016; Nielsen et al. 2017). High mortality due to the pathogen in the European ash stands of Fraxinus excelsior regardless of age (Bakys et al. 2009; Lenz et al. 2016; Skovsgaard et al. 2010) raises concern about the future existence of the species.
Previous research based on clonal ash stands, such as in Denmark (McKinney et al. 2011), Sweden (Cleary et al. 2014; Stener 2013), and Germany (Enderle et al. 2014), has documented a high level of genetic variation in ash dieback susceptibility among ash individuals, giving rise to high broad-sense heritability estimates (e.g. H2 = 0.42–0.54 in Tapsøre, Denmark; McKinney et al. 2011). Analyses based on variance among young half-sib families have further revealed high levels of narrow-sense heritability in dieback susceptibility (e.g. h2 = 0.40–0.49 in Pliūra et al. 2011; h2 = 0.37–0.52 in Kjaer et al. 2012; h2 = 0.42–0.53 in Lobo et al. 2014; h2 = 0.42 in Muñoz et al. 2016), suggesting that disease tolerance is to a large extent inherited from parents to offspring. This is further supported by the results of Lobo et al. (2015), who compared disease symptoms in parents with necrosis development in their offspring. The mechanisms behind the observed ash dieback resistance are not yet well understood. However, since parts of the mechanisms differ from classical induced resistance (Harper et al. 2016) and the variation is of quantitative nature, we use the term tolerance rather than resistance in the present work. While genetic variation in disease tolerance is known to affect individual survival (Lobo et al. 2014), much less is known on how the level of susceptibility of a given tree influences its reproductive success. However, a recent study found a clear overrepresentation of healthy trees among parents of seedlings on the forest floor of a Danish mixed forest (Semizer-Cuming et al. submitted). Sexual selection may therefore provide an important contribution to natural selection against disease susceptibility. This issue is particularly important to clarify, as the frequency of tolerant ash trees in nature is low (McKinney et al. 2011).
Fitness is a measure of the ability to survive and produce successful offspring. Plants that produce more seeds and fertilize more ovules than the population average therefore have higher fitness (Primack and Kang 1989). Female fitness can be estimated by counting the relative number of mature seeds assuming equal germination percentage while the estimation of male fitness requires assignments based on paternity analysis using highly polymorphic codominant markers (Marshall et al. 1998). The aim of the present study is to test the hypothesis that healthier male and female trees have higher gamete contribution to the subsequent generation compared to the unhealthy ones. We study the variation in gender and fertility in a Danish trial with the clonal replication of 39 genotypes that have been characterized in terms of ash dieback susceptibility based on 10 years of measurements. The implications of the observed relationship between disease tolerance and reproductive success are discussed in relation to the conservation and management of the species.
Materials and methods
Study site and sampling
The study site is located in Tuse Nӕs (55° 45′ 57.99″ N 11° 42′ 47.48″ E) in Northern Zealand, Denmark. It is a clonal field trial, established in 1998 based on a randomized complete block design with 39 individual F. excelsior genotypes (39 clones). Each clone was grafted onto rootstocks with approximately 26 replications (ramets) and planted in a single-tree, random block design. The initial objective of the trial was to serve as seed orchard, and the spacing was therefore 3 × 6 m. Details of the trial and the origin of the 39 clones can be found in McKinney et al. (2011). In the trial, seeds were collected in October 2012 from the ramets of three open pollinated female clones, representing relatively healthy genotypes (clone nos. 30, 33, 35) and located in the centre of the trial. Seeds were germinated and grown for 2 years in a greenhouse. At the time of seed collection in 2012, the number of ramets per clone had already been reduced to 4–21 due to high mortality caused by ash dieback.
All ramets of the 39 clones have been monitored in terms of crown damage since 2007. Crown damage was evaluated according to five-scale damage classes: class 0 represented no crown damage, class 1–3 indicated an increasing damage (class 1 < 10%, class 2 10–50%, class 3 > 50%), and class 4 meant 100% crown damage (dead). For statistical analyses, crown damage scores were converted into percent damage scores (PDS) based on the median values of the class percent ranges (cf. McKinney et al. 2011). Inflorescences of all flowering trees were observed in May 2015, and genders were scored according to a 1–9 scale, corresponding to the proportion of female flowers: (1) 0%, (2) 12.5%, (3) 25%, (4) 37.5%, (5) 50%, (6) 62.5%, (7) 75%, (8) 87.5%, and (9) 100%. Gender of one genotype (clone no. 28) could not be determined due to lack of flowers, and this clone was therefore excluded from further analyses concerning gender. Female reproductive success of all living trees were scored based on the visual observation of relative seed productivity in July 2015 (a year with abundant flowering) using a relative scoring scale from 0 to 9, where 0 corresponded to no seeds and 9 to very heavy seed production.
DNA genotyping was performed in order to assign fathers to offspring by paternity analysis and thereby estimate male reproductive success of the 39 clones. Leaves were collected from the 39 clones and from 285 2-year-old seedlings, and stored at − 20 °C until DNA extraction. DNA extractions were carried out with ~ 40 mg leaf tissues using the DNeasy® 96 Plant Kit (Cat No. 69181) according to the manufacturer’s protocol (Qiagen, Hilden, Germany). Microsatellite analyses were performed with nine selected primer pairs: FEMSATL11 and FEMSATL19 (Lefort et al. 1999); FEMSATL12 (Gerard et al. 2006); ASH2429 (Bai et al. 2011); FRESTSSR308, FRESTSSR427, and FRESTSSR528 (Aggarwal et al. 2011); and Fp18437 and Fp21064 (Noakes et al. 2014). The primers were labelled with four different fluorescent dyes (6-FAM: FEMSATL12, Fp18437, and Fp21064; VIC: FEMSATL11, FRESTSSR308, and FRESTSSR528; NED: FEMSATL19 and ASH2429; PET: FRESTSSR427) and combined in three multiplexes (Multiplex-1: FEMSATL11, FEMSATL12 and FEMSATL19; Multiplex-2: Fp18437, Fp21064, and FRESTSSR528; Multiplex-3: FRESTSSR308, FRESTSSR427, and ASH2429). PCR amplifications were carried out in 15 μl reactions using the QIAGEN Multiplex PCR Kit (Cat No. 206143) according to the manufacturer’s instructions. PCR amplifications were performed under the following conditions: initial denaturation at 95 °C for 15 min, 30 cycles of denaturation at 94 °C for 30 s, annealing at 57 °C, 62 °C, and 59 °C (for Multiplex-1, − 2, and − 3, respectively), extension at 72 °C for 60 s, and final extension at 60 °C for 30 min. PCR products were analysed on the ABI 3130xl Genetic Analyser (Applied Biosystems, Foster City, CA, USA). Each individual genotype and each mother-offspring pair were checked for errors, and the analyses were repeated in case of uncertain or non-amplified peaks. Alleles at each locus were binned into their size classes using a combination of manual and automated allele binning (Matschiner and Salzburger 2009) in order to detect potential binning errors.
Paternity analysis was carried out using CERVUS 3.0.7 (Kalinowski et al. 2007). The assignments were made based on Delta (Δ), the difference in LOD scores between the two most likely parents, to increase the certainty to identify the true parent when multiple parents had positive LOD scores (Marshall et al. 1998). The critical values of Δ were calculated at strict (95%) and relaxed (80%) confidence levels during the simulations. The assigned father was considered true when the likelihood of the paternity was 80% or more. The minimum number of loci was set to 8, and error rate was kept at 0.01. Selfing was allowed in the model as no clones were found to be completely female. To increase the reliability of the critical LOD values, 100,000 offspring were simulated. The percentage of sampled potential fathers was set to 70%. Confident paternity assignments were further checked and confirmed with the gender score of each genotype.
Analysis of gender and the effect of ash dieback susceptibility on reproductive fitness
We applied a general linear analysis of variance model to test for significant differences in gender scores among the 38 clones (excluding clone no. 28 with unassessed gender) using the ramets as replications. For the further analysis, the clones with a gender score (proportion of female flowers) of 0–20% were considered predominantly male (M), whereas the clones with gender score more than 80% were regarded as predominantly female (F). The remaining clones were considered hermaphrodites (H), and these clones were not included in the analysis of either male or female fitness. In this regard, the numbers of predominantly male, predominantly female, and hermaphroditic clones were M = 15, F = 16, and H = 7, respectively. The ratio does not necessarily reflect the situation in native populations, because the clones were specifically selected for the trial in 1998 with the objective of containing equal numbers of males and females.
Female reproductive success was assessed based on seed score (see as explained in “Materials and methods”) while male reproductive success was assessed based on realised paternities. The effect of ash dieback susceptibility (observed as percent crown damage; PDS) on reproductive success (Y) was analysed based on a linear regression model. The relationship between male fertility and disease susceptibility was analysed based on the data from the 15 predominantly male clones, while the relationship between female fertility and disease susceptibility was analysed based on the data from the 16 predominantly female clones. We applied the linear regression model E (Yi) = β0+ β PDSi, where PDSi was the susceptibility of the clones to ash dieback. The analyses for males and females were performed independently. In order to isolate the effect of survival in the clonal trial (differences in number of living ramets per clone), we analysed the relationship between reproductive fitness and ash dieback susceptibility at four levels: (i) total female reproductive success per clone (i.e. summed relative seed score across all ramets), (ii) average female reproductive success per living ramet of each clone, (iii) total male reproductive success per clone (i.e. number of realised paternities), and (iv) average male reproductive success per living ramet of each clone. Statistical analyses were conducted using the ‘tidyverse’ package (version 1.1.1; Wickham 2017) implemented in R (R Core Team 2017).
The assessment of gender showed significant differences among clones (F = 48.4, p < 0.001). The average gender scores ranged from purely male (gender score = 0% for clone nos. 11 and 20) to mostly female (gender score = 92% for clone no. 22) (Table 1). The 16 predominantly female clones formed a distinct group with gender scores of > 80% whereas the border between male and hermaphroditic clones was less clear (Fig. 1). The predominantly males did not carry any seed, but several of the predominantly females had sired offspring (Table 1). The level of disease susceptibility was not correlated with the gender score (t = − 0.25, p > 0.05; Fig. 1).
Paternity was assigned with high exclusion probability (0.9999), and no mother-offspring mismatches were detected. Paternity analysis successfully assigned candidate fathers to 144 (51%) of the analysed seedlings; hereof 102 (36%) at strict confident level (95%), and 1 was self-fertilized. The number of successful pollinations varied among males. Eleven clones were not assigned as candidate fathers whereas a single clone (clone no. 18), the healthiest male clone in the trial, was assigned as a pollen donor for 46 offsprings (Table 1), which corresponds to 32% of the total assignments. When correcting for the number of living ramets per clone, the variation in number of sired offspring varied among clones from 0 to 2.9 per ramet (Table 1).
The effect of ash dieback susceptibility on reproductive fitness
The negative effect of ash dieback susceptibility on female reproductive success was highly significant, and the crown damage score (PDS) alone explained a high proportion of the observed variation in total female reproductive success and average female reproductive success per living ramet (R2 = 0.873 and 0.518) among the females (Table 2; Fig. 2a, b). PDS, in general, explained less variation (R2 = 0.504–0.349) on the male side (Table 2; Fig. 2c, d). The relationships between reproductive fitness and ash dieback susceptibility among the tested clones were significant at all four levels (Table 2).
In the present study, we used percent damage score based on the level of crown damage to account for susceptibility to ash dieback. Crown damage is caused by the occlusion of branches leading to necrotic tissue and defoliation. The symptoms are clearly recognisable, and thus, it has been successfully used to evaluate, monitor, and quantify the disease level. Significant variation among clones in crown damage has previously been documented in the present trial with an estimated broad-sense heritability of around 0.4 (McKinney et al. 2011), and the same approach to assess disease intensity has been applied in many other studies (Enderle et al. 2014; Kjær et al. 2012; Lenz et al. 2016; Lobo et al. 2014; Marçais et al. 2017; Muñoz et al. 2016). We therefore consider average clonal level of crown damage as a good estimate for the ability of the clones to withstand infections of H. fraxineus.
Our study showed that clones with low levels of ash dieback symptoms (percent damage score) had higher reproductive fitness, but the effect appeared to differ between genders. A strong continuous negative relationship between disease susceptibility and female reproductive success was observed, and the rank in the damage score of the clones therefore corresponded relatively close to the opposite rank in seed productivity. However, the relationship between disease susceptibility and male reproductive success was less straightforward, because some male clones with high damage scores had sired offspring (e.g. clone no. 1 had sired eight ovules; Table 1). The relationship was only significant due to a single male (clone no. 18) that was by far the most effective pollen donor and also the male genotype with lowest ash dieback susceptibility (Fig. 2c, d). The difference between the influence of ash dieback on female and male reproductive fitness probably reflects the differences between maternal and paternal investments in reproduction. Intersexual difference in reproductive investment is a known phenomenon in dioecious plant species, and the cost of reproduction is generally higher for females than males (Antos and Allen 1999; Cipollini and Whigham 1994; Korpelainen 1992; Obeso 2002). For example, in the desert populations of an evergreen dioecious shrub (Simmondsia chinensis), males allocate 10–15% of their resources to reproductive tissues whereas females allocate 30–40% of their resources to reproduction at 100% seed set and female reproductive investment would be equal to that of males at ~ 30% seed set (Wallace and Rundel 1979). In Salix species, females also allocate more resources to reproductive tissues than males and therefore have higher reproductive costs in return (Ueno et al. 2006, 2007). Queenborough et al. (2007) studied intersex costs of reproduction in 16 tropical tree species of Myristicaceae in Amazonian lowland forests and concluded that female trees invested 10 times more biomass than male trees in total reproduction. In ash, crown damage due to ash dieback probably limits the available resources for reproduction in both genders. However, highly susceptible males may be able to produce and release pollen before flushing unlike their highly susceptible female counterparts, which may not be able to produce mature seeds while restoring the crown with epicormic shoots during the growing season. The difference among clones in relative female fitness is likely to be even stronger than suggested by the results presented in Fig. 1 and Table 1, because female reproductive success was scored based on a relative seed score from 0 to 9, where a tree with the crown full of seeds (score 9) most likely had more than nine times more seed compared to a tree with very few seeds in the crown (score 1). As it would be expected, differences among clones in terms of mean reproductive success per living ramet were lower than differences in total success for both males and females (Fig. 2) since differences in total success include the effects of differences in survival among genotypes (number of surviving ramets).
Only a small fraction of Fraxinus excelsior trees in natural populations can be expected to have a low level of susceptibility (1–5%, McKinney et al. 2014), but the findings of the present study suggest that the frequency of individuals with low susceptibility are likely to increase in the following generations due to the higher reproductive fitness of their parents, especially on the maternal side. Several studies have shown high levels of narrow sense heritability in the level of ash dieback susceptibility (as discussed above in the introduction), and the superior reproductive success of the healthy females and males in our study is therefore promising for the future resilience of the European ash forests. Nonetheless, the results indicate that selection against trees with high levels of susceptibility is weaker on the male side because the relationship between susceptibility and male reproductive success is weaker. Flowering in many plant species is regulated by environmental factors and thus stress-induced flowering is not an unknown phenomenon (Wada and Takeno 2010). We did not assess flowering intensity and therefore cannot rule out that diseased trees may have flowered more abundantly than healthy ones. However, we studied successful pollinations and successfully developed seeds, and observed that unhealthy trees were under-represented compared to few healthy male and female clones, which were highly fertile.
Our results present important information from a forest management point of view and suggest landscape managers and foresters to refrain from sanitary clear-cuts of all ash trees in damaged stands, but instead leave healthy trees for natural regeneration. Also, it is important to inform forest owners about the importance of sparing healthy trees in their forests instead of logging them due to concern about timber value. Our results imply that removing unhealthy males would be particularly relevant because unhealthy male trees are more likely to contribute to the next generation than unhealthy female trees. Before the disease outbreak, ash trees could regenerate successfully in European forests with the density of several ten of thousands per hectare (Lygis et al. 2014). Therefore, the potential of ash forests to improve their general level of resistance in the following 1–2 generations should not be underestimated. Still, natural selection and regeneration of ash in forests could be actively supported by the enrichment of damaged stands with offspring from tolerant, healthy ash trees in order to speed up the adaptation processes (Semizer-Cuming et al. submitted). Reliable information on seed and pollen dispersal distances of ash and the factors affecting mating patterns is already at hand (e.g. Semizer-Cuming et al. 2017).
Our results show that the healthiest female clones produced substantially more seeds than the unhealthy female clones. This supports the findings from a recent study on ash regeneration in a Danish forest, where healthy trees were overrepresented as the parents of seeds and young seedlings (Semizer-Cuming et al. submitted). This is indeed good news for European ash forests, because higher contribution from healthy trees at generation turnover followed by lower mortality among the next generation of trees with low levels of susceptibility must gradually give rise to more tolerant ash trees in next generations. We recommend foresters refrain from making complete sanitary clear-cuts of damaged ash stands, but instead leave most resistant individuals, and apply measures for supporting natural regeneration. At the moment, we cannot predict the expected overall progress due to lack of reliable estimates of in situ heritability relevant for the two selection stages. However, the progress may be significant as strong selection seems to be involved in both stages, and effective gene flow is likely to connect surviving trees across large forest landscapes (Bacles and Ennos 2008; Bacles et al. 2005, 2006; Semizer-Cuming et al. 2017). Still, it remains to be revealed whether the process will be fast and efficient enough to ensure the true recovery of the species.
The dataset generated and analysed during the current study is available in the University of Copenhagen–Electronic Research Data Archive (UCPH-ERDA), https://sid.erda.dk/public/archives/8a01d83d895d81575baa3c3147339543/published-archive.html.
Aggarwal RK, Allainguillaume J, Bajay MM, Barthwal S, Bertolino P, Chauhan P, Consuegra S, Croxford A, Dalton DL, den Belder E, Díaz-Ferguson E, Douglas MR, Drees M, Elderson J, Esselink GD, Fernández-Manjarrés JF, Frascaria-Lacoste N, Gäbler-Schwarz S, Garcia de Leaniz C, Ginwal HS, Goodisman MA, Guo B, Hamilton MB, Hayes PK, Hong Y, Kajita T, Kalinowski ST, Keller L, Koop BF, Kotzé A, Lalremruata A, Leese F, Li C, Liew WY, Martinelli S, Matthews EA, Medlin LK, Messmer AM, Meyer EI, Monteiro M, Moyer GR, Nelson RJ, Nguyen TT, Omoto C, Ono J, Pavinato VA, Pearcy M, Pinheiro JB, Power LD, Rawat A, Reusch TB, Sanderson D, Sannier J, Sathe S, Sheridan CK, Smulders MJ, Sukganah A, Takayama K, Tamura M, Tateishi Y, Vanhaecke D, Vu NV, Wickneswari R, Williams AS, Wimp GM, Witte V, Zucchi MI (2011) Permanent genetic resources added to Molecular Ecology Resources Database 1 August 2010–30 September 2010. Mol Ecol Resour 11:219–222. https://doi.org/10.1111/j.1755-0998.2010.02944.x
Altizer S, Harvell D, Friedle E (2003) Rapid evolutionary dynamics and disease threats to biodiversity. Trends Ecol Evol 18:589–596. https://doi.org/10.1016/j.tree.2003.08.013
Antos JA, Allen GA (1999) Patterns of reproductive effort in male and female shrubs of Oemleria cerasiformis: a 6-year study. J Ecol 87(1):77–84. https://doi.org/10.1046/j.1365-2745.1999.00331.x
Bacles CF, Burczyk J, Lowe AJ, Ennos RA (2005) Historical and contemporary mating patterns in remnant populations of the forest tree Fraxinus excelsior L. Evol 59(5):979–990. https://doi.org/10.1111/j.0014-3820.2005.tb01037.x
Bacles CFE, Ennos RA (2008) Paternity analysis of pollen-mediated gene flow for Fraxinus excelsior L. in a chronically fragmented landscape. Hered 101(4):368–380
Bacles CF, Lowe AJ, Ennos RA (2006) Effective seed dispersal across a fragmented landscape. Sci 311(5761):628–628. https://doi.org/10.1126/science.1121543
Bai X, Rivera-Vega L, Mamidala P, Bonello P, Herms DA, Mittapalli O (2011) Transcriptomic signatures of ash (Fraxinus spp.) phloem. PLoS One 6:e16368. https://doi.org/10.1371/journal.pone.0016368
Bakys R, Vasaitis R, Barklund P, Ihrmark K, Stenlid J (2009) Investigations concerning the role of Chalara fraxinea in declining Fraxinus excelsior. Plant Pathol 58(2):284–292. https://doi.org/10.1111/j.1365-3059.2008.01977.x
Baral HO, Queloz V, Hosoya T (2014) Hymenoscyphus fraxineus, the correct scientific name for the fungus causing ash dieback in Europe. IMA Fungus 5(1):79–80
Cipollini ML, Whigham DF (1994) Sexual dimorphism and cost of reproduction in the dioecious shrub Lindera benzoin (Lauraceae). Am J Bot 81(1):65–75. https://doi.org/10.1002/j.1537-2197.1994.tb15410.x
Cleary MR, Andersson PF, Broberg A, Elfstrand M, Daniel G, Stenlid J (2014) Genotypes of Fraxinus excelsior with different susceptibility to the ash dieback pathogen Hymenoscyphus pseudoalbidus and their response to the phytotoxin viridiol—a metabolomic and microscopic study. Phytochem 102:115–125. https://doi.org/10.1016/j.phytochem.2014.03.005
Cleary M, Nguyen D, Marčiulynienė D, Berlin A, Vasaitis R, Stenlid J (2016) Friend or foe? Biological and ecological traits of the European ash dieback pathogen Hymenoscyphus fraxineus in its native environment. Sci Rep 6:21895. https://doi.org/10.1038/srep21895
Enderle R, Nakou A, Thomas K, Metzler B (2014) Susceptibility of autochthonous German Fraxinus excelsior clones to Hymenoscyphus pseudoalbidus is genetically determined. Ann For Sci 72:183–193. https://doi.org/10.1007/s13595-014-0413-1
Gerard PR, Fernandez-Manjarres JF, Frascaria-Lacoste N (2006) Temporal cline in a hybrid zone population between Fraxinus excelsior L. and Fraxinus angustifolia Vahl: structure of an ash hybrid zone population. Mol Ecol 15:3655–3667. https://doi.org/10.1111/j.1365-294X.2006.03032.x
Harper AL, McKinney LV, Nielsen LR, Havlickova L, Li Y, Trick M, Fraser F, Wang L, Fellgett A, Sollars ESA, Janacek SH, Downie JA, Buggs RJA, Kjær ED, Bancroft I (2016) Molecular markers for tolerance of European ash (Fraxinus excelsior) to dieback disease identified using associative transcriptomics. Sci Rep 6:19335. https://doi.org/10.1038/srep19335
Kalinowski ST, Taper ML, Marshall TC (2007) Revising how the computer program Cervus accommodates genotyping error increases success in paternity assignment: Cervus likelihood model. Mol Ecol 16:1099–1106. https://doi.org/10.1111/j.1365-294X.2007.03089.x
Kjaer ED, McKinney LV, Nielsen LR, Hansen LN, Hansen JK (2012) Adaptive potential of ash (Fraxinus excelsior) populations against the novel emerging pathogen Hymenoscyphus pseudoalbidus: adaptive potential of F. excelsior. Evol Appl 5:219–228. https://doi.org/10.1111/j.1752-4571.2011.00222.x
Korpelainen H (1992) Patterns of resource allocation in male and female plants of Rumex acetosa and R. acetosella. Oecologia 89(1):133–139
Lefort F, Brachet S, Frascaria-Lacoste N, Edwards KJ, Douglas GC (1999) Identification and characterization of microsatellite loci in ash (Fraxinus excelsior L.) and their conservation in the olive family (Oleaceae). Mol Ecol 8(6):1088–1089. https://doi.org/10.1046/j.1365-294X.1999.00655_8.x
Lenz H, Bartha B, Straßer L, Lemme H (2016) Development of ash dieback in south-eastern Germany and the increasing occurrence of secondary pathogens. Forests 7:41. https://doi.org/10.3390/f7020041
Lobo A, Hansen JK, McKinney LV, Nielsen LR, Kjær ED (2014) Genetic variation in dieback resistance: growth and survival of Fraxinus excelsior under the influence of Hymenoscyphus pseudoalbidus. Scand J For Res 29(6):519–526. https://doi.org/10.1080/02827581.2014.950603
Lobo A, McKinney LV, Hansen JK, Kjær ED, Nielsen LR (2015) Genetic variation in dieback resistance in Fraxinus excelsior confirmed by progeny inoculation assay. For Path 45:379–387. https://doi.org/10.1111/efp.12179
Lygis V, Bakys R, Gustiene A, Burokiene D, Matelis A, Vasaitis R (2014) Forest self-regeneration following clear-felling of dieback-affected Fraxinus excelsior: focus on ash. Eur J For Res 133(3):501–510. https://doi.org/10.1007/s10342-014-0780-z
Marçais B, Husson C, Cael O, Dowkiw A, Saintonge F- X, Delahaye L, Collet C, Chanderlier A (2017) Estimation of ash mortality induced by Hymenoscyphus fraxineus in France and Belgium. Baltic For 23(1):159–167
Marshall TC, Slate JB, Kruuk LE, Pemberton JM (1998) Statistical confidence for likelihood-based paternity inference in natural populations. Mol Ecol 7(5):639–655. https://doi.org/10.1046/j.1365-294x.1998.00374.x
Matschiner M, Salzburger W (2009) TANDEM: integrating automated allele binning into genetics and genomics workflows. Bioinform 25:1982–1983. https://doi.org/10.1093/bioinformatics/btp303
McKinney LV, Nielsen LR, Hansen JK, Kjær ED (2011) Presence of natural genetic resistance in Fraxinus excelsior (Oleraceae) to Chalara fraxinea (Ascomycota): an emerging infectious disease. Hered 106:788–797
McKinney LV, Nielsen LR, Collinge DB, Thomsen IM, Hansen JK, Kjær ED (2014) The ash dieback crisis: genetic variation in resistance can prove a long-term solution. Plant Pathol 63(3):1–15. https://doi.org/10.1111/ppa.12196
Muñoz F, Marçais B, Dufour J, Dowkiw A (2016) Rising out of the ashes: additive genetic variation for crown and collar resistance to Hymenoscyphus fraxineus in Fraxinus excelsior. Phytopathol 106(12):1535–1543. https://doi.org/10.1094/PHYTO-11-15-0284-R
Nielsen LR, McKinney LV, Hietala AM, Kjær ED (2017) The susceptibility of Asian, European and North American Fraxinus species to the ash dieback pathogen Hymenoscyphus fraxineus reflects their phylogenetic history. Eur J For Res 136(1):59–73. https://doi.org/10.1007/s10342-016-1009-0
Noakes AG, Best T, Staton ME, Koch J, Romero-Severson J (2014) Cross amplification of 15 EST-SSR markers in the genus Fraxinus. Conserv Genet Resour 6(4):969–970. https://doi.org/10.1007/s12686-014-0260-2
Obeso JR (2002) The costs of reproduction in plants. New Phytol 155(3):321–348. https://doi.org/10.1046/j.1469-8137.2002.00477.x
Pliûra A, Heuertz M (2003) EUFORGEN technical guidelines for genetic conservation and use for common ash (Fraxinus excelsior). International Plant Genetic Resources Institute, Rome, p 6
Pliūra A, Lygis V, Suchockas V, Bartkevicius E (2011) Performance of twenty four European Fraxinus excelsior populations in three Lithuanian progeny trials with a special emphasis on resistance to Chalara fraxinea. Baltic For 17(1):17–34
Primack RB, Kang H (1989) Measuring fitness and natural selection in wild plant populations. Annu Rev Ecol Syst 20:367–396. https://doi.org/10.1146/annurev.es.20.110189.002055
Przybył K (2002) Fungi associated with necrotic apical parts of Fraxinus excelsior shoots. For Pathol 32:387–394. https://doi.org/10.1046/j.1439-0329.2002.00301.x
Queenborough SA, Burslem DF, Garwood NC, Valencia R (2007) Determinants of biased sex ratios and inter-sex costs of reproduction in dioecious tropical forest trees. Am J Bot 94(1):67–78. https://doi.org/10.3732/ajb.94.1.67
R Core Team (2017) R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria https://www.R-project.org/. Accessed 14 Jan 2019
Santini A, Ghelardini L, De Pace C, Desprez-Loustau ML, Capretti P, Chandelier A, Cech T, Chira D, Diamandis S, Gaitniekis T, Hantula J (2013) Biogeographical patterns and determinants of invasion by forest pathogens in Europe. New Phytol 197:238–250. https://doi.org/10.1111/j.1469-8137.2012.04364.x
Semizer-Cuming D, Kjӕr ED, Finkeldey R (2017) Gene flow of common ash (Fraxinus excelsior L.) in a fragmented landscape. PLoS One 12(10):e0186757. https://doi.org/10.1371/journal.pone.0186757
Skovsgaard JP, Thomsen IM, Skovgaard IM, Martinussen T (2010) Associations among symptoms of dieback in even-aged stands of ash (Fraxinus excelsior L.). For Pathol 40:7–18. https://doi.org/10.1111/j.1439-0329.2009.00599.x
Sønstebø JH, Vivian-Smith A, Adamson K, Drenkhan R, Solheim H, Hietala A (2017) Genome-wide population diversity in Hymenoscyphus fraxineus points to an eastern Russian origin of European ash dieback. BioRxiv: 154492. https://doi.org/10.1101/154492
Stener L-G (2013) Clonal differences in susceptibility to the dieback of Fraxinus excelsior in southern Sweden. Scand J For Resour 28:205–216. https://doi.org/10.1080/02827581.2012.735699
Ueno N, Kanno H, Seiwa S (2006) Sexual differences in shoot production and leaf dynamics in a dioecious tree, Salix sachalinensis. Can J Bot 84:1852–1859. https://doi.org/10.1139/b06-142
Ueno N, Suyama Y, Seiwa K (2007) What makes the sex ratio female-biased in the dioecious tree Salix sachalinensis? J Ecol 95(5):951–959. https://doi.org/10.1111/j.1365-2745.2007.01269.x
Wada KC, Takeno K (2010) Stress-induced flowering. Plant Signal Behav 5(8):944–947. https://doi.org/10.4161/psb.5.8.11826
Wallace CS, Rundel PW (1979) Sexual dimorphism and resource allocation in male and female shrubs of Simmondsia chinensis. Oecologia 44(1):34–39
Wickham H (2017) Tidyverse: easily install and load ‘Tidyverse’ packages. R package version 1.1.1. https://CRAN.R-project.org/package=tidyverse
We thank Lene Hasmark Andersen for the help with lab work and Lars Nørgaard Hansen and Lea Vig McKinney for help with fieldwork. We thank Oliver Gailing for his comments on an earlier version of the manuscript. We are grateful to the two anonymous reviewers and the editors, Erwin Dreyer and Benoit Marçais, for their comments and suggestions.
This study was supported by the European Commission under the FONASO Erasmus Mundus Joint Doctorate Program and Villum Foundation (Grant no. VKR023062).
Conflict of interest
The authors declare that they have no conflict of interest.
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Contribution of co-authors
Devrim Semizer-Cuming: formulation of the hypotheses, data collection, performing the experiments, data analysis, data presentation, writing the original draft, reviewing, and editing
Reiner Finkeldey: reviewing and editing
Lene Rostgaard Nielsen: formulation of the hypotheses, data collection, reviewing, and editing
Erik Dahl Kjær: formulation of the hypotheses, data analysis, reviewing, and editing
Handling Editor: Benoit Marçais
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Cite this article
Semizer-Cuming, D., Finkeldey, R., Nielsen, L.R. et al. Negative correlation between ash dieback susceptibility and reproductive success: good news for European ash forests. Annals of Forest Science 76, 16 (2019). https://doi.org/10.1007/s13595-019-0799-x
- Ash dieback
- Fraxinus excelsior
- Gene pool
- Reproductive success