cultivars

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Apriums and Plumcots and Pluots, Oh My!

I was once a teenage paper carrier in small town Idaho. One of my stops was an apartment complex, and for much of the year, this was an uneventful stop. But for a few weeks in the summer, the purple-leaved plum trees out front had ripe fruit on them, and each time I was there, I would stop and take a few. In general, I don’t get that excited about fruit, but I enjoyed eating these plums. This variety of plum is typically planted for its looks rather than its fruit, and it may even be the tree that recently received a pitifully low score on an episode of Completely Arbortrary. Ornamental plum or not – and low cone score or not – I thought the fruit was good.

Many of the things we eat are a result of crosses between two related species, and plums are a great example of this. Species are species because they are reproductively isolated. A species does not typically mate with a member of another species and create viable offspring, except this happens all the time both naturally and artificially. In many cases, the offspring isn’t actually viable, but there is offspring nonetheless, and in the case of plants, that offspring can then reproduce asexually – by leaf, stem, or root cuttings or by some other means – and the resulting hybrid can exist indefinitely. One species mating with another species (specifically two species that are members of the same genus) is called interspecific hybridization, and there is a good chance that you’ve eaten something recently that is a result of this.

One of the most widely grown species of plum, Prunus domestica (commonly known as European plum), is a result of interspecific hybridization that occurred many centuries ago. A paper published in Horticulture Research (2019) confirmed that P. domestica originated as a cross between Prunus cerasifera and Prunus spinosa, the latter of which may have also been a result of interspecific hybridization. There are over 400 species in the genus Prunus that are distributed across temperate regions in the northern hemisphere. Within this genus is the subgenus Prunus (or Prunophora), a group that includes dozens of familiar species such as the plums, apricots, peaches, and almonds. Due to their close relationship, both natural and artificial hybridization among members of this subgenus is common, which explains the origin of Prunus domestica, as well as the majority of the plums we grow today.

Current commercial production of plums in North America is largely thanks to work done by Luther Burbank in the late 19th to early 20th centuries. Burbank was obsessed with plant breeding and released hundreds of new varieties of all kinds of different plants during his decades long career. He seemed particularly interested in plums, developing 113 different cultivars, which account for more than half of all his fruit releases. Probably his most well known plum variety is ‘Santa Rosa,’ which thanks to modern day genetics has been determined to be a cross between at least four different species of plum.

apriums

Early colonizers to the American continent were mainly growing varieties of the European plum they had brought over from Europe. North America is also home to several species of plums, which are used by indigenous populations. Shortly before Burbank began working with plums on his farm in California in 1881, Asian plum species were imported to the U.S., and breeders began using them in crosses with both European and North American plum species. Burbank became particularly engulfed in these efforts. In an article published in HortScience (2015), David Karp writes, “In the history of horticulture it is rare to find an individual who almost single-handedly created a new commercial industry based on a novel fruit type as Luther Burbank did for Asian-type plums in the United States.” Most Asian-type plums sold in stores today are hybrids of several different plum species due to the numerous complex crosses that Burbank made.

Burbank is also said to be the first to cross plums and apricots, creating the first of many cultivars of the plumcot. Plum and apricot crosses didn’t really catch on for a few more decades, and when they did, it was thanks to the work of Floyd Zaiger of Zaiger Genetics who developed and released numerous varieties. Apriums and Pluots are Zaiger Genetics trademarks, along with a few other unlikely crosses with plums and their related counterparts.

plumcots

A plumcot is the simplest cross. It is said to be 50% Asian plum (Prunus salicina) and 50% apricot (Prunus armeniaca). However, due to all the breeding of Asian plums carried out by Burbank and others, the Asian plum involved in the cross is typically a hybrid with other plum species, as discussed in a recent paper published in Plants (2022). An aprium is the result of a cross between a plumcot and an apricot, making it 75% apricot and 25% plum, while a pluot is a cross between a plumcot and a plum, making it 75% plum and 25% apricot. There is typically much more that goes into making these crosses, but that’s the general idea. If you’re lucky, you can find all three of these intraspecific crosses in a produce section near you, but it may not be clear what cultivar you’re purchasing. Myriad cultivars have been released of each of these hybrids – each one varying in color, size, flavor, disease resistance, etc. – and unfortunately most grocery stores don’t include cultivar names on their products, so it’s difficult to know what you’re getting.

At Awkward Botany Headquarters, there is a plum tree growing in our front yard. We didn’t plant it, so at this point I have no idea what species or cultivar it is. The plums are delicious though, and the leaves aren’t purple like the plums I used to eat on my paper route. Considering all of the intraspecific crossing that has gone on with plums, it’s quite likely that it is a combination of different species, which isn’t going to make it easy to figure out. But I’ll do my best.

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Apples and Genetic Bottlenecks

This is the eleventh in a series of posts reviewing the 17 articles found in the October 2014 Special Issue of American Journal of Botany, Speaking of Food: Connecting Basic and Applied Science.

Genetic Diversity in Malus x domestica (Rosaceae) through Time in Response to Domestication by Briana L. Gross, Adam D. Henk, Christopher M. Richards, Gannara Fazio, and Gayle M. Volk

Domestication is a selection process. Plants with desirable traits are selected (consciously or unconsciously) and removed from the larger population to be grown out and selected from again. Over time, this series of selections results in a cultivated variety that differs substantially from the larger, origin population. This process, while yielding crop varieties that feed a growing population of humans, also results in a series of genetic bottlenecks, meaning they experience a reduction in genetic variation compared to their wild relatives.

There are two points were bottlenecks occur in the domestication process. The first takes place “during the initial domestication event as a subset of the wild population is brought into a cultivated setting.” This is called a “domestication bottleneck.” The second, known as an “improvement bottleneck,” happens when “modern, elite cultivars are selected from the broad variety of landraces [locally adapted varieties]” that were developed during the original domestication event. This stepwise reduction in genetic diversity “limits the options of plant breeders, even as they face the need to increase crop productivity and sustainability” in today’s changing climate.

Most of what we know about genetic bottlenecks during domestication is derived from studies of annual fruit and grain crops. However, “non-grain crops, and perennials in particular, respond to domestication or are domesticated in ways that are fundamentally different.” For this reason, the authors investigated genetic bottlenecks in apple (Malus x domestica), “one of the most widely distributed perennial fruit crops.” They then compared what they learned to other published studies of annual and perennial fruit crops in order to gain more insight into how genetic diversity is affected in these types of crops during domestication.

The common apple was domesticated in central Asia around 4,000 years ago and is a hybrid of at least three species: Malus sieversii, Malus orientalis, and Malus sylvestris. Today, apples are grown throughout the world, and there are more than 7,500 known cultivars with new cultivars being released regularly. Despite this impressive diversity, just fifteen cultivars make up 90% of apple production in the U.S. The authors of this study analyzed DNA from 11 of the 15 major cultivars as well as DNA from the three main wild progenitor species.

Malus x domestica 'Gala' - One of the top 15 apple varieties produced in the U.S. (photo credit: wikimedia commons)

Malus x domestica ‘Gala’ – One of the top 15 apple varieties produced in the U.S. (photo credit: wikimedia commons)

Perennial fruit crops typically experience “mild genetic bottlenecks” compared to annual fruit crops, and the authors confirmed this to be the case with domesticated apples, finding “no significant reduction in genetic diversity through time across the last eight centuries.” Because apple cultivars are maintained by clonal propagation, they can often be traced back to when they were originally developed, making bottlenecks easier to observe. The authors found that “the most recently developed or described cultivars of apples show little to no reduction in genetic diversity compared with the most ancient cultivars.” Cultivars developed since the 1950’s show increased diversity, which may partly be the result of plant breeders introducing genes from another wild species, Malus floribunda.

After a review of the literature, the authors found that apples have retained the highest amount of genetic diversity through the domestication process compared to other fruits, both annual and perennial. More studies are needed in order to confirm the accuracy and extent of these findings; however, the unique story of apple domestication may help explain why it has been “particularly prone to retaining diversity through time.” First, it was widely distributed across Eurasia during its early days of domestication. Second, it experienced “admixture with cultivars” as it expanded its range. For example, after being introduced to North America, it became naturalized, resulting in gene flow occurring between naturalized individuals and cultivated varieties. Offspring of these populations (“chance seedlings”), were then selected, cloned, and became named cultivars.

Despite the mild genetic bottleneck observed in apples, the authors warned that a “dependence on a small number of cultivars” for the majority of U.S. apple production may be resulting in some loss of genetic variation. Relying on so few cultivars may leave apple production vulnerable to pests, diseases, and climate change. “Careful management” is advised as “the continued genetic resilience of the crop is dependent on the genetic diversity of cultivars that are present in living and cryopreserved collections around the world.”

Malus sylvestris (common crabapple) - One of the three main players involved in the apple domestication story (photo credit: www.eol.org)

Blossoms of Malus sylvestris (common crabapple) – One of three main species involved in the history of apple domestication (photo credit: www.eol.org)