Malus Sylvestris: Genetic Issues and Conservation
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Research Scotland’s native wild apple – Malus sylvestris: genetic issues and conservation By Rick Worrell, Markus Ruhsam, James Renny, Will Jessop and Graeme Findlay Summary Wild apple trees (Malus sylvestris (L.) Mill.) have been ‘off the radar’ for foresters and ecologists, to the extent that they generally go unnoticed and many professionals are unsure how to identify them. This paper confirms that wild apple is a natural component of ancient woodland and wood pasture across much of Scotland and northern England. Wild apple has been unobtrusively hybridising with domestic apple such that 27% of apparent wild apples are in fact hybrids. There are still Figure 1. Wild apple (Malus sylvestris) with a 17m crown span coming into flower in ancient wood relatively pure populations of wild pasture, Cree Valley, Dumfries and Galloway. Note the dense, complex crown. apple in several upland regions, with strongholds in the Southern measures by using appropriate seed the fact that nothing useful appears Highlands, Galloway and the Lake sources. to be known about the species, either District; in lowland, populated areas in Scotland or the UK. There are no of Scotland where domestic apples Introduction and aims papers published in the UK on Malus are common about 50% of trees are This project came about as a result of sylvestris and UK forestry books either hybrids. Asiatic apples have also been having encountered impressive wild ignore the species or make a hash of planted in forests, and are now apples in ancient woodlands, in the topic. Of course, most people are naturalising and starting to unexpected places (Figure 1). It familiar with the name ‘crab apple’ hybridise with other apples. The turned out to be surprisingly and think of small, scruffy trees paper gives guidance on how to difficult to verify whether these were (‘scroggies’) planted in hedges, and identify wild apples and how to the native apple (Malus sylvestris (L.) on that basis it seems that both organise genetic conservation Mill.) and then we were puzzled by foresters and ecologists have dismissed the tree as unimportant. Overturning that perception is one of the aims of this paper and a THE AUTHORS companion paper describing the Rick Worrell* has been a self-employed forestry consultant since 1987, specialising in native species’ ecology (Worrell et al, in woodland and growing broadleaves for quality timber press). Markus Ruhsam is a plant molecular ecologist at the Royal Botanic Gardens Edinburgh (RBGE) with a particular interest in how plant populations change over time Aims James Renny is a naturalist based in Grandtully, Perthshire The aims of this work were to Will Jessop was an MSc student at the RBGE and is now studying for a PhD at the University investigate: of York Graham Findlay is Planning and Conservation Forester at Forestry and Land Scotland, • Which apple species are growing Dunkeld Office in the countryside in Scotland *Corresponding author: [email protected] (native or introduced) and what are their origins (natural origin, Upper Park, Aberfeldy, Perthshire PH15 2EH feral, planted)? Volume 72 No. 2 Autumn 2019 | 33 Research • Is hybridisation occurring between wild apple (M. sylvestris) and domestic apples (M. domestica Borkh.), as has been recorded on the continent; and if so, what effects might this have? • Can we use our new evidence to develop genetic conservation guidance for wild apple? History of wild and domestic apples Malus sylvestris is the ‘forest apple’ according to its Latin name, and is generally called the ‘wild apple’ in other European countries, and we adopt this name here. It is a characterful, rare and ecologically significant tree that grows widely across central and western Europe, as Figure 2. Distribution of wild apple (Malus sylvestris) in Europe with Scotland near its northern limit far north as 67o N in coastal (Stephan et al, 2003). Norway (Figure 2, Stephan et al, 2003). Interestingly, recent genetic are now grown the world over and are recently been shown to make up a studies have shown that the lowly one of humankind’s most significant proportion (7–36%) of wild (crab) apple is a major economically and culturally populations of wild crab apples across contributor to the genetic make-up of important tree crops, with annual a range of European countries domestic (eating) apples, via sales in the UK alone worth £846 (Cornille et al, 2014). This has historical hybridisation with sweet million in 20151 – for comparison the contributed to the species being listed apples from Asia (Cornille et al, 2012, GVA2 of forestry in the UK in 2014 on IUCN red data lists in several 2014). The main progenitor of our was £540 million and sawmilling £356 European countries (IUCN, 2011), domestic apple is Malus sieversii, million (Forestry Commission, 2017). and raises the question of what is which grows in spectacular So by providing a critical part of the happening in Scotland. Are the apple apple-dominated forests in the Tian genome of eating apples, the wild trees we see in Scottish woods and Shan mountain foothills bordering apple made a very significant hedgerows really Malus sylvestris or China, Kyrgyzstan and Kazakhstan, contribution to human wellbeing. actually hybrids that have developed where it was domesticated at least Wild apple is thought to have over the centuries? Since M. sylvestris 4000 years ago (Cornille et al, 2014). contributed several valuable traits, and M. domestica cannot be such as later flowering, climate distinguished reliably by adaptation, resistance to pests and morphological characteristics, DNA “Interestingly, recent genetic diseases, and capacity for longer markers have been developed in storage of fruits (Cornille et al, 2012). recent years to differentiate between studies have shown that the them and their hybrids (Cornille et al, lowly wild (crab) apple is a Genetic issues 2013). This paper gives a non- major contributor to the The rampant success of the technical account of work carried out genetic make-up of domestic domestic apple – which has been on this project, and published (eating) apples, via historical grown in many parts of the world for recently in Forestry (Ruhsam et al, centuries – now appears to be starting 2019), which addresses the issue of hybridisation with sweet to threaten the long-term survival hybridisation. apples from Asia” of the wild apple as a species. This is We also set out to discover which because the two species (M. sylvestris other apple species are present in and M. domestica) interbreed, raising woods in Scotland, either due to This made its way towards Europe the possibility of Malus sylvestris being planting, or due to the spread of over the millennia, with the help of threatened by hybridisation – in a feral apples. There are roughly 30–40 trade along the Silk Road. It then similar way to the fate of the Scottish species of crab apples worldwide hybridised with M. orientalis in the wildcat (e.g. Larsen et al, 2006, Reim (Mabberley, 2008), and some of these Middle East and on arrival in Europe et al, 2012, Cornille et al, 2014, have been planted quite widely in it also crossed with the European wild Ruhsam et al, 2019). Hybrid apples Scotland. (crab) apple M. sylvestris, forming (M. domestica × sylvestris) have our domestic apple (Malus domestica) between 4,500 and 1,500 years ago 1 https://www.statista.com/statistics/650348/fruit-vegetable-popular-sales-uk/ (Cornille et al, 2012). Domestic apples 2 GVA is Gross Value Added 34 | SCOTTISH FORESTRY Research Table 1. Apple Leaf Hairiness Scoring and provisional assignment to species Genetic sampling and DNA analysis method APPLE LEAF HAIRINESS SCORING A detailed account of the genetics Scores by inspecting the underside of leaves with a hand lens Species work in this project has been 0 Both petiole and lower leaf surface without hairs, or only a published in Forestry (Ruhsam et al, few sparse hairs on petiole and leaf margin at very base. Hairs 2019) and the methods and results stiff and spaced out. from that paper are summarised Probable wild apple 1 Some hairs, generally sparse, on petiole, lowest parts of midrib Malus sylvestris below. A single leaf was collected (typically lower 20% of blade length) and lower lateral veins from each tree and dried in silica gel. near leaf base. None on leaf blade surface. Hairs generally DNA was extracted using the DNeasy stiff and spaced out. Plant kit (Qiagen, Hilden, Germany). 2 Petiole and lower midrib and lower veins obviously hairy to All samples were assayed for 14 about 30% of leaf length. On most trees there will also be some hairs on lower leaf surface, mainly in vein axils. Probable hybrid apple microsatellite loci using the four Malus sylvestris × do- multiplex reactions MP1, MP2, MP3 3 Petiole and veins hairy to about 50% of leaf length. Lower leaf mestica and MP4 from Cornille et al (2012). surfaces also hairy. Only upper leaf free of hairs. Hairs thin, downy, and frequent. STRUCTURE v.2.3.3 and NEWHYBRIDS v1.1 were used to 4 Petiole, veins and leaf surface largely covered with felted Probable domestic apple hairs. Hairs thin, downy, and frequent. Malus domestica investigate the population structure and levels of admixture of M. sylvestris and M. domestica. Using the term ‘wild apple’ feral domestic apple trees and planted The term ‘crab apple’ is decorative species. Trees were Results problematic because it sweeps up all located by the authors and some were Contacting professionals sorts of apples with small fruits and volunteered by land owners. Contacting professionals for help confusing parentage, many of which Several datasets proved helpful i.e. to locate apple trees revealed some are unrelated to M. sylvestris. Some the Native Woodland Survey of intriguing insights.