The Apple Doesn’t Fall Far from the Tree…or Does It?
In some ancient Greek and Nordic mythologies this fruit grants immortality. In others, it is portrayed as a fruit of discord, a culprit of world conflict and quarrel among the Olympians. Hundreds of years later, it took on a symbolic role personifying knowledge, lure, and violation of God’s will. Arguably no fruit is depicted in as many works of art and shrouded in as much ambiguity. This is the story of the origins, variety, and proliferation of the beloved apple.
What is an “apple”?
Before we dive in, it is necessary to determine what we mean by the word “apple” itself. This is done because the description “apple” is not freestanding on its own. From this point forward, what I refer to as an apple is a specie of the Rosacea family known as Malus domestica. More likely than not, every apple that you have had in your life belongs to M. domestica. The different types of apples at your local supermarket or fruit stand — like Red Delicious or Granny Smith — are known as cultivars. Cultivars are intraspecific collection of apple types that share particular characteristics; color, taste, texture, disease resistance are some of these. While there are more than 7,500 identified cultivars, only 10 of them account for roughly 80% of all produced apple fruit.
History of Malus domestica
Genealogy of an apple is determined through a combination of phylogenic and genetic evidence. Recent improvements in genetic sequencing provide a powerful framework for validating previous suppositions. I seek to outline some of these below.
Genealogy
Genealogical history of domestica is murky. The apple shares the most characteristics (genetic and otherwise) with a wild apple specie of the name Malus sieversii. Sieversii is native to Tian Shan mountains of southern Kazakhstan and northwest China. The fruit reach up to 6 cm in diameter and tend to be more palatable for human taste buds than other wild Malus species. Wild sieversii has a lifespan of under 100 years, and this relatively rapid turnover (in scope of a larger ecological system) lead to more variety and an accelerated selection process. In the wild it does not benefit Malus to maintain long lifespan; found near riverbanks, among pines and oaks, sieversii is vulnerable to seasonal changes, floods, earthquakes — natural phenomena which in evolutionary terms favored shorter juvenile cycles (i.e. the time it takes to reach adulthood). Juniper identifies two others “engines of evolution” — the bear and the wild horse — that in combination with environment, selected for apples preferred by humans.
The Eurasian brown bear, native to Tian Shan, unconsciously selected for larger neo-apples. The larger fruit provides a greater source of calories for the bear, while sweeter taste appeals more to the Ursa’s palette. Small fruit, often observed intact in bear’s fecal matter, would not give rise to descendants (as the seeds would never germinate), placing negative evolutionary pressure on smaller size.
In contrast, the undomesticated horse was not a major driver of selection, but played a pivotal role in spreading sieversii throughout the plains of Eurasia. Once the horse has been domesticated (also in Kazakhstan), sieversii spread along trade routes and human settlements, where it encountered other species of wild proto-apples.
Foreign Encounters
Along what would later be named Silk Routes, sieversii first encountered other wild apple species. Malus orientalis, native to Turkey and mountainous region of Russian Kavkaz, was encountered first. Introduction of orientalis genes to sieversii likely increased the disease resistance of the new hybridized species, improving the genetic pool.
Malus sylvestris, widespread throughout Europe — particularly France, Spain, and Germany — favorites forest edges and limy soils. Local evolutionary adaptations are common, and sylvestris has been observed to grow in rocky, rough soils, at altitudes in excess of 1 km. Genetic analysis has shown that sylvestris has heavily contributed to the domestica’s genome. Sylvestris was previously thought to have been the progenitor of a modern apple, but genetic tests have shown that this is almost certainly not the case. Chloroplast DNA analysis (to be discussed in more detail later) shows that sylvestris is missing duplications in the matK gene (present in domestica and sieversii), supporting the theory that the European proto-apple was an important but secondary progenitor.
While the proto-apple has spread and encountered changes in the West, domestication of horses almost certainly also exposed sieversii to its Northern neighbors. The most important of these is Malus baccata, also known as the Siberian crab apple. Bearing fruit about 1 cm in dimeter, baccata is known for its strong resistance to cold. Introduction of baccata’s genetic material into sieversii pool resulted in greater adaptability of the fruit to harsh climates of Northern Eurasia.
Genetic Evidence
All plants contain three sets of DNA: nuclear or ribosomal DNA (rDNA), chloroplast DNA (cpDNA), and mitochondrial DNA. In the study of apples, the first two — rDNA and cpDNA — have proved useful.
Chloroplast DNA propagates genetic information along the maternal line. One particular gene of interest in cpDNA, matK, has been shown to duplicate in both sieversii and domestica. Statistical analysis based on cpDNA in general and matK specifically produced taxonomies presented in the related figure.
Ribosomal DNA analysis up to date has focused on internal transcribed spacer gene (ITS) to group various apple varieties. In other studies, Cornell biologists Simon and Weeden identified 45S rDNA family as of utility for assessment of domestica’s genetic ancestry. The rDNA analysis generally coincides with the results of cpDNA studies.
Among the most important reasons to study the genetic make-up and ancestry of apples is to help distinguish between thousands of varieties of Malus. The results of these activities help to minimize collection sizes and determine which parts of the genome express fruit characteristics useful in production. Different regional populations and varieties exhibit variable degrees of disease and temperature resistance, juvenile period, taste and color properties, etc. From these building blocks, we can then build a fruit suitable for our needs and wants.
Breeding Practices
A well-known idiom — the apple doesn’t fall far from the tree — usually implies that kids more often than not resemble their parents. The irony is that the idiom ignores the biology of the apple fruit; it is precisely the case that apple seeds produce fruit different than those they came from. Unlike peaches, apricots, and sour cherries, apple trees cannot pollinate themselves; pollen from other apple trees is required to create seeds. This property (commonly abbreviated as SI for self-incompatibility) has been identified as originating on the S-gene, which controls the female receptivity to pollination. Study by Broothaerts and Van Nerum identified 15 alleles of the S-gene which together expressed 14 different incompatibility phenotypes (or characteristics).
The important question is if seeds don’t produce the same fruit, how can we ever grow apples that we like in manageable quantities? And what happens after an apple tree has withered away?
Grafting
Today, all domestica trees are propagated by grafting. Grafting is a practice in which a section of a stem with lead buds (the scion) is attached to the rooted layer. The rooted layer (a grown tree) will supply the required nutrients to the scion, thereby producing the desired fruit. Typically the rooted layer is grown from a dwarfing rootstock. These rootstocks are specifically selected for their smaller size, greater productivity, as well as the ability to grown in particular soil and weather conditions. The scion is attached to the rooted layer with asphalt grafting compounds; the compound can last for a year or more, preventing drying out.
The practice of drafting dates back 3800 years ago to Ancient Mesopotamia. The evidence for apple drafting is primarily soft, as cuneiform tablets from the time period only discuss imported grape vines. Concrete evidence of grafting is first found in Greece dating back to 300 BC, describing a primitive version of the practice.
There are four grafting techniques that are used most frequently for apple propagation: (1) the whip graft, (2) the cleft graft, (3) the side graft, and (4) budding. Other variations and techniques exist, but these four are generally regarded as fundamental basis elements.
The practice of grafting heavily relies on having the right rootstock. Today it is believed that dwarfing rootstocks were probably discovered by chance in Asia. As the name suggests, these rootstocks are smaller than wild Malus, and are catered towards growing in specific soils and climates. Over the course of centuries plethora of rootstocks have been identified, and breeders ran into significant trouble keeping up with the available variety.
In 1912, scientists at East Malling Research Station in Kent, England embarked upon a journey of classifying and standardizing rootstocks. The primary purpose was to identify unique rootstocks, remove duplicates, and standardize naming conventions. Hatton and Wellington, the two scientists who led the research effort, identified nine rootstocks, forming the first Malling Series. Later the Malling Series was expanded, running from M.1 — M.16. Today, only two of the original rootstocks — Malling 7 and Malling 9 — are heavily used in commercial application. The latest Malling rootstock, M116, has been released in 2001.
In 1917, Merton Research Station near London, England joined the efforts at Malling. The collaborative work produced Merton Immune Series (M.I. 778 — M.I. 793). Merton Immune 793 is used in commercial production to this day. Malling-Merton series, another product of collaboration, is designated as M.M., running from M.M. 101 — M.M. 115.
Chance Seedlings and Controlled Breeding
We now know that grafting combined with wise rootstock selections allows us to reproduce apples that are found pleasantly palatable. The question of how one obtains these types of apples is still open. Before 19th century, this was primarily left up to chance — hence the name — chance seedling. In this way, the two varieties presently most consumed in the US have been discovered; Red Delicious was discovered by chance in Iowa in 1872, and Golden Delicious in West Virginia in 1905.
Controlled breeding, a selection of apple varieties based on specific properties — such as size, color, texture, flavor, and disease resistance — has proven difficult, and as a result hasn’t been implement with any sustained degree of success until the 20th century. The basic problem of controlled breeding is introducing a desired phenotype (i.e. characteristic) to a parent variety; under the surface, this implies introduction of a gene or a set of genes (hence changing the genotype) that are responsible for expressing the said phenotype.
Introduction of a phenotype has historically been done via backcrossing, a practice of growing successive generations of apples where a base fruit with a desired trait (say, crispness) is crossed with a high quality parent (for instance, fruit possessing thin skin and disease resistance). Over successive generations, one hopes that the genes from the parent and the base coalesce to form a cultivar possessing the desired traits. Backcrossing can be accelerated by exposing each generation to a different high quality parent. Albeit the many advances, the process of introducing a new cultivar takes around 15 years. Ironically, many chance seedlings are still the best sellers around the world, but the practice of backcrossing will play a greater role, especially when used in combination with genetic breeding.
Genetic Breeding
Genetic breeding is a complicated process, which cannot be fully encompassed in few sentences. We can attempt to simplify the techniques to two methodologies — marker-assisted selection and genomic selection. A marker is a short sequence of DNA within the genetic code. When used properly, it can tell us what phenotypes a seedling carries. For example, if we identify that a particular marker is responsible for a skin thickness of 0.5 mm, the presence of the said marker in a seedling’s DNA likely implies that the fruit will have the set thickness.
The process of marker-assisted selection identifies these markers, and seeks to extract, introduce, and combine them to create transgenic fruit of desired characteristics. It is best suited for cases where the genes that map to a given trait are identified and understood. Typically, model plants (simple plants which are better understood) are used for these studies. Espley and Hellens, for instance, used the plant Arabidopsis thaliana to identify the gene sequence responsible for color expression. Using the identified pattern, they found a similar gene in domestica — MYB10 –which has been shown to be responsible for expression of color red.
A palatable apple, however, possesses multiple desired traits. The more traits and the more complex the trait, the more markers are involved. Genomic selection builds on marker-assisted selection, and attempts to use thousands of markers spread across the entire DNA. Data analysis techniques are used to determine which combinations and configurations of genes will result in the expression of preferred characteristics. Powerful techniques such as machine learning will likely revolutionize the field, allowing for more complicated, sought-after fruit.
Marketability
Today, the apple is the second most produced fruit in the United States. The largest market is for the desert apples, and is driven primarily by three variables — appearance, price, and overall quality. The quality is a complicated metric, and genetics only represent a part of the input. Production, storage, weather, soil type are all statistically significant in influencing the overall quality of an apple. Below is a brief history and some characteristics of the most sough-after cultivars:
Summary
In 2014, an average American consumed nearly 120 pounds of fresh and processed fruit. If we ignore juice consumption, an apple is the most consumed fresh fruit in the United States. Given the many health benefits, the relatively inexpensive sticker price, and the numerous variety of apples out there, there is room for growth. I hope that by reading this article you have learned about and developed a deeper appreciation for my favorite fruit.
