Complementary descriptions of two species of the subgenus Anthoseius De Leon (Parasitiformes: Phytoseiidae) collected from wild and cultivated apple trees in the western Himalayas, India

Crop plant domestication can change plant resistance to herbivores leading to differences in pest pressure experienced by crop plants and their wild relatives. To compare resistance to herbivores between domesticated and wild fruit trees, we quantified direct resistance and indirect resistance to a pest insect, the florivorous apple blossom weevil Anthonomus pomorum (Coleoptera: Curculionidae), for the cultivated apple Malus domestica and two wild apple species, the European crab apple M. sylvestris and the exotic M. kirghisorum. We measured weevil infestation and performance (weight, sex ratio), and weevil parasitism by parasitoid wasps for different cultivars of M. domestica and for the two wild apple species. To explain weevil and parasitoid responses to different apple species, we quantified tree characteristics including nitrogen content, size of flower buds, bark roughness, tree size, tree phenology and tree position. We found significant differences in susceptibility to weevil infestation between apple species, with lowest infestation (highest apple resistance) in M. domestica and highest infestation in M. kirghisorum. The suitability of apple species also varied significantly: weevils emerging from M. sylvestris were significantly lighter than those from M. kirghisorum. Parasitism of A. pomorum by different parasitoid species was significantly higher in M. sylvestris than in M. domestica. Infestation, weevil weight and parasitism were positively related to tree characteristics: infestation to bud nitrogen content and bark roughness, weevil size to nitrogen content and bud size, and parasitism to tree height and bud density. Our study revealed marked differences between apple species in susceptibility and suitability for the pest herbivore, but also for antagonistic parasitoids. Whereas direct resistance appeared to be higher in cultivated apple, indirect resistance via parasitoids was apparently higher in wild apple trees. Our findings suggest that wild and cultivated apple trees possess different resistance traits that may be combined to optimize resistance in commercial apple cultivars.

Secondary contact between crops and their wild relatives poses a threat to wild species, not only through gene flow between plants, but also through the dispersal of crop pathogens and genetic exchanges involving these pathogens, particularly those that have become more virulent by indirect selection on resistant crops, a phenomenon known as "pestification." Joint analyses of wild and domesticated hosts and their pathogens are essential to address this issue, but such analyses remain rare. We used population genetics approaches, demographic inference and pathogenicity tests on host-pathogen pairs of wild or domesticated apple trees from Central Asia and their main fungal pathogen, Venturia inaequalis, which itself has differentiated agricultural and wild-type populations. We confirmed the occurrence of gene flow from cultivated (Malus domestica) to wild (Malus sieversii) apple trees in Asian forests, potentially threatening the persistence of Asian wild apple trees. Pathogenicity tests demonstrated the pestification of V. inaequalis, the agricultural-type population being more virulent on both wild and domesticated trees. Single nucleotide polymorphism (SNP) markers and the demographic modelling of pathogen populations revealed hybridization following secondary contact between agricultural and wild-type fungal populations, and dispersal of the agricultural-type pathogen population in wild forests, increasing the threat of disease in the wild apple species. We detected an SNP potentially involved in pathogen pestification, generating an early stop codon in a gene encoding a small secreted protein in the agricultural-type fungal population. Our findings, based on joint analyses of paired host and pathogen data sets, highlight the threat posed by cultivating a crop near its centre of origin, in terms of pestified pathogen invasions in wild plant populations and introgression in the wild-type pathogen population.

Soluble sugars, malic acid, and ascorbic acid in 17 apple cultivars (Malusdomestica Borkh.) and three wild forms (M. pumila ‘Saiwaihong’, M. prunifolia (Willd.) Borkh. and M. micromalus Makino) from three major apple cultivation regions in China were quantified using gas chromatography equipped with flame ionization detector (GC-FID). Fructose was the most abundant sugar, followed by sucrose, glucose, and sorbitol. Wild apples contain more sorbitol and less sucrose and were significantly more acidic than cultivated fruits. The total sugar content varied from 110 to 160 mg/g fresh fruits, total acid content from 2 to 6 mg/g, with a strong influence of genetic background and growth location. Overall, ‘Gala’, ‘Xiali’, ‘Liuyuehong’, ‘Lihong’, ‘Starking Delicious’, and ‘Starkrimson’ were characterized by higher sugar/acid ratio indicating sweeter taste compared to other cultivars. The wild apples had the highest content of ascorbic acid (0.6–0.96 mg/g). Compared to other cultivars, ‘Zhongqiuwang’, ‘Qinguan’, and ‘Nagafu No. 2′ were richer in ascorbic acid. The ascorbic acid content in the commercial cultivars was highly dependent on growth location. The content of malic acid and sucrose positively correlated to altitude, and that of glucose negatively. Malic acid positively correlated with ascorbic acid and sucrose, glucose content with ascorbic acid.

Identifying DNA sequence variations is a fundamental step towards deciphering the genetic basis of traits of interest. Here, a total of 20 cultivated and 10 wild apples were genotyped using specific-locus amplified fragment sequencing, and 39,635 single nucleotide polymorphisms with no missing genotypes and evenly distributed along the genome were selected to investigate patterns of genome-wide genetic variations between cultivated and wild apples. Overall, wild apples displayed higher levels of genetic diversity than cultivated apples. Linkage disequilibrium (LD) decays were observed quite rapidly in cultivated and wild apples, with an r2 -value below 0.2 at 440 and 280 bp, respectively. Moreover, bidirectional gene flow and different distribution patterns of LD blocks were detected between domesticated and wild apples. Most LD blocks unique to cultivated apples were located within QTL regions controlling fruit quality, thus suggesting that fruit quality had probably undergone selection during apple domestication. The genome of the earliest cultivated apple in China, Nai, was highly similar to that of Malus sieversii, and contained a small portion of genetic material from other wild apple species. This suggested that introgression could have been an important driving force during initial domestication of apple. These findings will facilitate future breeding and genetic dissection of complex traits in apple.

Malus sylvestris is the only apple species native to Central Europe. Its genetic integrity may be threatened by hybridization with the cultivated apple (Malus × domestica). A total of 883 genotypes, 477 putative wild apples in Germany and Luxembourg and 406 old to modern cultivars has been investigated. Wild apples growing in Germany originated from Rhineland-Palatinate, North Rhine-Westphalia, Saxony-Anhalt and Saxony. The genetic structure was analysed at ten isozyme marker loci, and morphology was studied by fruit size, leaf pubescence, and a complex morphological description performed in advance. A model-based cluster analysis applied to all nuclear data resulted in two clearly differentiated gene pools for putative wild and cultivated apples with moderate proportions of admixture in the wild group on average (0.138 total, 0.111 German sample). At the individual level, the percentages of both hybrids and feral cultivars together ranged from 2.3% in Rhineland-Palatinate to 28.8% in Luxembourg. The intraspecific variability in fruit diameter ranged from 21 to 40 mm, and that in leaf pubescence in autumn ranged from score 0 to score 1. No single morphological trait of a specimen appeared to be sufficient for identification. Even the correspondence between the complex morphological and complex genetic determinations for individuals did not exceed 93% in pure wilds and 64% in hybrids. Genetic variation in pure wild apple is high (species level: P = 90%, A/L = 3.1, He = 0.369). Allelic differentiation & was 0.089, pairwise genetic distance (d0) ranged from 0.065 to 0.148 among five samples. Correspondence between genetic and geographic distance of populations was observed to a certain extent.

Malus sieversii (wild apple trees) is a tertiary relict tree species and a key ancestor of the cultivated apple trees today. In recent years, serious growth decline and individual death have occurred for M. sieversii . Whether growth decline would lead to the change in twig (composed of leaves and one-year stem) functional traits and their associations of wild apple trees remains unclear. Two long-term monitoring plots (1 hm 2 ) for severely declined and relatively healthy wild apple trees, respectively, were set up in Yili Valley, China. Based on two-year investigations, the differences in tree functional traits and interrelations between the two plots and the influencing factors were analyzed. The crown projective cover, dead branch percentage, and fruit yield were markedly different between two sites and two years. Leaf nitrogen and phosphorous concentrations of declined trees were generally less than healthy trees, although the soil of the former was more fertile. The nitrogen was more susceptible to the stressing, especially for declined trees. Concentrations of stem nitrogen, phosphorous, and potassium and leaf potassium were relatively stable in both plots. The plant trait network analysis indicated that leaf nitrogen (per unit area) that promoted plant growth was the hub trait (meaning that it is correlated with most traits) for healthy trees, while leaf potassium (per unit mass) that related to resistance enhancement became the hub trait for declined trees. Meanwhile, the covariant characteristics of twig traits between the two plots differed obviously, and the soil-trait and growth-trait correlations became more negative for declined trees than healthy trees, indicating that intra-trait and environment-trait associations had changed greatly when trees declined. The ordination analysis revealed that total and available soil nitrogen, phosphorous, and total potassium, pH and plant growth parameters significantly influenced twig traits. Our results demonstrated that twig functional traits and their interrelations of wild apple trees can indicate tree decline trend. We strongly recommend that managers should take measures to prevent the decline of healthy wild apple trees as soon as possible to realize the sustainable existence of this precious germplasm resource. The results provide a theoretical support for scientific management of wild apple populations in the Tianshan Mountains in Central Asia.

Erwinia amylovora is a dangerous pathogen that causes fire blight in fruit trees. As this pathogen has started to spread in the territory of Kazakhstan, it poses a significant threat to cultivated and wild apple (Malus sieversii) forests. Currently there is no effective treatment for widespread types of fire blight, it can be prevented only by early diagnosis. Although much research has focused on the detection of fire blight in cultivated apple orchards, little attention has been paid to controlling wild apple populations. Therefore, a comprehensive four-year monitoring study was conducted in four protected areas of Kazakhstan to determine the spread of fire blight on wild apple trees. During several years of inspections, Malus sieversii trees showed no obvious signs of fire blight. These observations were further confirmed by lateral flow immunochromatography as well as by conventional and real-time polymerase chain reaction analysis performed on asymptomatic samples. The results of this study are of great importance for nature protection, providing invaluable information about the current state of fire blight in wild apple forests of Kazakhstan. Therefore, these data provide a basis for formulating science-based recommendations to government agencies to take proactive measures aimed at limiting the spread of fire blight in these ecologically important ecosystems.

Samples from apple trees in old commercial and abandoned orchards as well as in wild apple trees in the outskirts of Prague in the Czech Republic were surveyed for the presence of Apple mosaic virus (ApMV), Apple stem grooving virus (ASGV), Apple stem pitting virus (ASPV) and Apple chlorotic leaf spot virus (ACLSV) using conventional and quantitative RT-PCR. Results revealed that ACLSV was the most frequent virus (70%) in old apple cultivars. ASPV together with ASGV were present in approximately half of the tested trees, and ApMV was found at the lowest frequency. Unexpectedly, the occurrence of ApMV, ASGV and ASPV was confirmed in wild-growing apple trees although the incidence was low. Sequence of the coat protein coding gene from several of these apple virus isolates provided insights into their population structure in Central Bohemia. Together, our molecular findings extend our knowledge on the distribution of apple viruses in populations of old cultivated apple trees in the Czech Republic and provide the first evidence of the presence of ApMV, ASGV and ASPV in wild apple trees. The latter finding raises the question on the origin of viruses in wild apple trees because their spread without human intervention is not known.

BackgroundFreezing temperatures are an abiotic stress that has a serious impact on plant growth and development in temperate regions and even threatens plant survival. The wild apple tree (Malus sieversii) needs to undergo a cold acclimation process to enhance its freezing tolerance in winter. Changes that occur at the molecular level in response to low temperatures are poorly understood in wild apple trees.ResultsPhytohormone and physiology profiles and transcriptome analysis were used to elaborate on the dynamic response mechanism. We determined that JA, IAA, and ABA accumulated in the cold acclimation stage and decreased during freezing stress in response to freezing stress. To elucidate the molecular mechanisms of freezing stress after cold acclimation, we employed single molecular real-time (SMRT) and RNA-seq technologies to study genome-wide expression profiles in wild apple. Using the PacBio and Illumina platform, we obtained 20.79G subreads. These reads were assembled into 61,908 transcripts, and 24,716 differentially expressed transcripts were obtained. Among them, 4410 transcripts were differentially expressed during the whole process of freezing stress, and these were examined for enrichment via GO and KEGG analyses. Pathway analysis indicated that “plant hormone signal transduction”, “starch and sucrose metabolism”, “peroxisome” and “photosynthesis” might play a vital role in wild apple responses to freezing stress. Furthermore, the transcription factors DREB1/CBF, MYC2, WRKY70, WRKY71, MYB4 and MYB88 were strongly induced during the whole stress period.ConclusionsOur study presents a global survey of the transcriptome profiles of wild apple trees in dynamic response to freezing stress after two days cold acclimation and provides insights into the molecular mechanisms of freezing adaptation of wild apple plants for the first time. The study also provides valuable information for further research on the antifreezing reaction mechanism and genetic improvement of M. sieversii after cold acclimation.

The increasing fragmentation of forest habitats and the omnipresence of cultivars potentially threaten the genetic integrity of the European wild apple (Malus sylvestris (L.) Mill). However, the conservation status of this species remains unclear in Europe, other than in Belgium and the Czech Republic, where it has been declared an endangered species. The population density of M. sylvestris is higher in the forests of the upper Rhine Valley (France) than in most European forests, with an unbalanced age-structure, an overrepresentation of adults and a tendency to clump. We characterize here the ecology, age-structure and genetic diversity of wild apple populations in the Rhine Valley. We use these data to highlight links to the history of this species and to propose guidelines for future conservation strategies. In total, 255 individual wild apple trees from six forest stands (five floodplain forests and one forest growing in drier conditions) were analysed in the field, collected and genotyped on the basis of data for 15 microsatellite markers. Genetic analyses showed no escaped cultivars and few hybrids with the cultivated apple. Excluding the hybrids, the genetically “pure” populations displayed high levels of genetic diversity and a weak population structure. Age-structure and ecology studies of wild apple populations identified four categories that were not randomly distributed across the forests, reflecting the history of the Rhine forest over the last century. The Rhine wild apple populations, with their ecological strategies, high genetic diversity, and weak traces of crop-to-wild gene flow associated with the history of these floodplain forests, constitute candidate populations for inclusion in future conservation programmes for European wild apple.

Evaluation of chlorogenic acid accumulation in cultivated and wild apples

Malus sieversii, a Tertiary relict and primary progenitor of the cultivated apple, is experiencing severe habitat degradation in China’s Tianshan Mountains. To understand how soil ecosystem functions respond to tree vigor decline, we monitored surface soils beneath the canopy of wild apple trees monthly from April to October. Trees were classified into three vigor classes based on the percentage of dead branches: Vigor Class I (<20%), Vigor Class II (40–60%), and Vigor Class III (>80%). Soil multifunctionality (SMF) and temporal variability of nutrients (TVN) were derived from seven key nutrient indicators. Soils under Vigor Class II trees exhibited the lowest SMF and highest TVN, indicating maximal functional instability during intermediate degradation. While SMF peaked and TVN reached its seasonal minimum in October, Vigor Class II showed a consistent decline in TVN over time, unlike the irregular fluctuations in Vigor Classes I and III. A significant negative SMF–TVN correlation in Vigor Classes II and III suggests a trade-off between functionality and stability. Partial least squares path modeling revealed that soil organic carbon, total nitrogen, and total phosphorus were the dominant direct driver of both SMF and TVN, with climate exerting no significant direct effects once tree vigor and soil conditions were accounted for. These results suggest that Vigor Class II represents a critical early-warning stage: soil functional capacity begins to deteriorate before visible signs of severe tree decline or mortality. Targeted ecological restoration of Vigor Class II trees is essential to prevent irreversible ecosystem degradation. Therefore, while continued protection of healthy Vigor Class I trees remains essential, conservation efforts should place greater emphasis on restoring Vigor Class II trees to disrupt degradation feedbacks before irreversible ecosystem decline occurs.

There is growing attention given to gene flow between crops and the wild relatives as global landscapes have been rapidly converted into agricultural farm fields over the past century. Crop-to-wild introgression may advance the extinction risks of rare plants through demographic swamping and/or genetic swamping. Malus sieversii, the progenitor of the apple, is exclusively distributed along the Tien Shan mountains. Habitat fragmentation and hybridization between M. sieversii and the cultivated apples have been proposed to be the causal mechanism of the accelerated extinction risk. We examined the genetic diversity pattern of eleven wild and domesticated apple populations and assessed the gene flow between M. sieversii and the cultivated apples in Kazakhstan using thirteen nuclear microsatellite loci. On average, apple populations harbored fairly high within-population diversity, whereas population divergences were very low suggesting likely influence of human-mediated dispersal. Assignment results showed a split pattern between the cultivated and wild apples and frequent admixture among the apple populations. Coupled with the inflated contemporary migration rates, the admixture pattern might be the signature of increased human intervention within the recent past. Our study highlighted the prevalent crop to wild gene flow of apples occurring in Kazakhstan, proposing an accelerated risk of genetic swamping.

Malus sieversii (Ledeb.) M. Roem is a tertiary relict tree species and a rare and valuable resource for germplasm conservation. Since 1995, its wild forest has been severely destroyed by a devastating wood-boring beetle Agrilus mali Matsumura (Coleoptera: Buprestidae) in Xinjiang Uygur Autonomous Region, China. Where it invaded, this beetle infested more than 95% of the forests, and 80% of wild apple trees were reported dead in the hotspots. The physiological damage by A. mali infestation and their causality to tree death remain unclear. In this study, we attempted to explain the wild apple dieback from plant physiological perspectives, based on the hypothesis that the more damage M. sieversii suffered from the infestation of A. mali, the less water and fewer nutrients it could utilize. The study was conducted on trees with different extents of damage in wild apple forests over a large scale during 2016 and 2017. The stable carbon isotope ratio in leaves was analyzed to indicate tree water stress status. Total N, total P, total K, Ca2+ and Mg2+ were analyzed to reflect plant mineral nutrient status. The extent of damage was significantly associated with the leaf stable carbon isotope ratio in the drier year of 2016, but not significantly in 2017 with heavy rainfall in spring. The mineral nutrient contents of leaves were not significantly different among the four damage rankings in either year. The water stress experienced by M. sieversii was aggravated by the damage caused by A. mali, especially in a drought year, and indicates that the long-term water deficit caused by A. mali infestation may be the key factor leading to the decline of wild apple forests. The finding suggests the necessity of aerial irrigation for sustainable integrated pest management in wild apple trees.

The apple genus (Malus Mill.) comprises approximately 50 species, native to various temperate and subtropical climates in Europe, Asia, and the Americas. East Asia was the apple's original homeland, where the largest number of wild apple species are found. Through Central Asia, modern-day Iran, and the Transcaucasus, the apple tree reached Asia Minor in prehistoric times, and then spread throughout Europe via Ancient Greece and Rome. When North America was still connected to Asia, the apple tree also reached the Americas, where it has its own wild apple species. The foothills of the Altai Mountains, in what is now Kazakhstan and Kyrgyzstan, are considered the birthplace of the domesticated apple tree. It is there that the wild apple tree, Malus sieversii, which gave rise to the domesticated apple, still grows. Wild apple trees retain their attractiveness throughout the growing season. The pleasantly fragrant flowers range in color from white to deep pink and carmine, while autumn foliage varies from light green to anthocyanin-brown. The fruits, which remain fleshy on the branches until mid-winter, also exhibit a variety of shapes and colors and provide excellent material for floral arrangements. Optimized micropropagation technology for wild apple trees provides the basis for obtaining planting material for landscaping, as well as for creating in vitro gene banks to preserve the apple gene pool and subsequently use various varieties in breeding. The results of an experiment to optimize in vitro apple propagation technology demonstrated the feasibility of subculturing microshoots on a medium with a reduced BAP content (0.5 mg/L) before rooting. This is consistent with data from other authors who suggest reducing the hormone content to 0.7-1.0 mg/L. At the same time, well-leafed micro-shoots of a larger size are formed (1.5 - 2.3 cm in our experiment).