The Dispersal of Ancient and Modern Apples by Humans and Other Megafauna

Crop domestication often involves selection for larger fruits. In some crops, humans took plant species with relatively small fruits and, over many generations of artificial selection, developed a plant with much larger fruits. Consider giant pumpkins as an extreme example. Yet in the case of apples, relatively large fruits already existed in the wild. Producing larger apples happened quickly and, perhaps even, unconsciously. Apples were practically primed for domestication, and as Robert Spengler explains in a paper published last year in Frontiers in Plant Science, looking back in time at the origins of the apple genus, Malus, can help us understand how the apple we know and love today came to be.

Apples are members of the rose family (Rosaceae), a plant family that today consists of nearly 5000 species. According to the fossil record, plants in the rose family were found in large numbers across North America as early as the Eocene (56 – 33.9 million years ago). They were present in Eurasia at this time as well, but Spengler notes, “there is a much clearer fossil record for Rosaceae fruits and seeds in Europe and Asia during the Miocene and Pliocene (20 – 2.6 million years ago).” Around 14 million years ago, larger fruits and tree-form growth habits evolved in Rosaceae subfamilies, giving rise to the genera Malus and Pyrus (apples and pears). Small, Rosaceae fruits were typically dispersed by birds, but as Sprengler writes, “it seems likely that the large fruits [in Malus and Pyrus] were a response to faunal dispersers of the late Miocene through the Pliocene of Eurasia.” Larger animals were being recruited for seed dispersal in a changing landscape.

Glacial advances and retreats during the Pleistocene (2.6 million – 11,700 years ago) brought even more changes. Plants with effective, long distance seed dispersal were favored because they were able to move into glacial refugium during glacial advances. Even today, these glacial refugium are considered genetic hot spots for Malus, and could be useful for future apple breeding. As the Pleistocene came to a close, many megafauna were going extinct. This continued into the Holocene. Large-fruited apple species lost their primary seed dispersers, and their ranges became even more contracted.

Humans have had an extensive relationship with apples, which began long before domestication. Foraging for apples was common, and seeds were certainly spread that way (perhaps even intentionally). Favorable growing conditions were also created when forests were cleared and old fields were left fallow. Apple trees are early successional species that easily colonize open landscapes, gaps in forests, and forest edges, so human activity that would have created such conditions “could have greatly promoted the spread and success of wild Malus spp. trees during the Holocene.”

The earliest evidence we have of apple domestication (in which “people were intentionally breeding and directing reproduction”) occurred around 3000 years ago in the Tian Shan Mountains of Kazakhstan, where Malus sieversii – a species that is now facing extinction – was being cultivated. This species was later brought into contact with other apple species, a few of which were also being cultivated, including M. orientalis, M. sylvestris, and M. baccata. These species easily hybridized, giving us the modern, domesticated apple, M. domestica. As Spengler writes, “the driving force of apple domestication appears to have been the trans-Eurasian crop exchange, or the movement of plants along the Silk Road.” Continued cultivation and further hybridization among M. domestica cultivars over the past 2000 years has resulted in thousands of different apple varieties.

The unique thing about domesticated apples is that their traits are not fixed in the same way that traits of other domesticated crops are. Growing an apple from seed will result in a very different apple than the apple from which the seed came. Apple traits instead have to be maintained through cloning, which is accomplished mainly through cuttings and grafting. Apples hybridize with other apple species so readily that most apple trees found in the wild are hybrids between wild and cultivated populations.

Spengler considers the study of apple domestication to be “an important critique of plant domestication studies broadly, illustrating that there is not a one-size fits-all model for plant domestication.” The “key” for understanding apple domestication “rests in figuring out the evolutionary driver for large fruits in the wild – seed dispersal through megafaunal mammals – and the process of evolution for these large fruits – hybridization.” He notes that “domestication studies often ignore evolutionary processes leading up to human cultivation,” which, in the case of apples, involves “hybridization events in the wild” that led to the evolution of large fruits “selected for through the success in recruiting large megafaunal mammals as seed disperses.” Many of those mammals went extinct, but humans eventually assumed the role, selecting and propagating “large-fruiting hybrids through cloning and grafting – creating our modern apple.”

Excerpt from Fruit from the Sands by Robert N. Spengler:

Indeed, the relationship between apples and people is close and complex, spanning at least five millennia. The story of the apple begins along the Silk Road… In recent years genetic studies have resolved much of the debate over these origins. Nevertheless, the ancestry of the apple is highly complex. Cloning, inbreeding, and reproduction between species have created a genealogy that looks more like a spider’s web than a family tree. To growers, the beauty of the apple lies not in its rosy skin but in its genetic variability and plasticity, its ability to cross with other species of Malus and other distant lines of M. domestica, and the ease with which it can be grafted onto different rootstocks and cloned.

See Also: Science Daily – Exploring the Origins of the Apple

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Interested in learning more about how plants get around. Check out the first issue of our new zine Dispersal Stories.

Camel Crickets and the Dust Seeds of Parasitic Plants

A common way for plants to disperse their seeds is to entice animals to eat their seed-bearing fruits – a strategy known as endozoochory. Undigested seeds have the potential to travel long distances in the belly of an animal, and when they are finally deposited, a bit of fertilizer joins them. Discussions surrounding this method of seed dispersal usually have birds and mammals playing the starring roles – vertebrates, in other words. But what about invertebrates like insects? Do they have a role to play in transporting seeds within themselves?

Certain insects are absolutely important in the dispersal of seeds, particularly ants. But ants aren’t known to eat fruits and then poop out seeds. Instead they carry seeds to new locations, and some of these seeds go on to grow into new plants. In certain cases there is an elaisome attached to the seed, which is a nutritious treat that ants are particularly interested in eating. Elaisomes or arils have also been known to attract other insects like wasps and crickets, which may then become agents of seed dispersal. But endozoochory in insects, at first, seems unlikely. How would seeds survive not being crushed by an insect’s mandibles or otherwise destroyed in the digestion process?

camel crickets eating fruits of parasitic plants (via New Phytologist)

While observing parasitic plants in Japan, Kenji Suetsugu wanted to know how their seeds were dispersed. Many parasitic plants rely on wind dispersal, thus their seeds are minuscule, dust-like, and often winged. However, the seeds of the plants Suetsugu was observing, while tiny, were housed in fleshy fruits that don’t split open when ripe (i.e. indehiscent). This isn’t particularly unusual as other species of parasitic plants are known to have similar fruits, and Suetsugu was aware of studies that found rodents to be potential seed disperers for one species, birds to be dispersers of another, and even one instance of beetle endozoochory in a parasitic plant with fleshy, indehiscent fruit. With this in mind, he set out to identify the seed dispersers in his study.

Suetsugu observed three achlorophyllous, holoparisitic plants – Yoania amagiensis, Monotropastrum humile, and Phacellanthus tubiflorus. While their lifestyles are similar, they are not at all closely related and represent three different families (Orchidaceae,  Ericaceae, and Orobanchaceae respectively). All of these plants grow very low to the ground in deep shade below the canopy of trees. Air movement is at a minimum at their level, so seed dispersal by wind is not likely to be very effective. Using remote cameras, Suetsugu captured dozens of hours of footage and found camel crickets and ground beetles to be the main consumers of the fruits, with camel crickets being “the most voracious of the invertebrates.” This lead to the next question – did the feces of the fruit-eating camel crickets and ground beetles contain viable seeds?

Monotropastrum humile via wikimedia commons

After collecting a number of fecal pellets from the insects, Suetsugu determined that the seeds of all three species were “not robust enough to withstand mastication by the mandibles of the ground beetles.” On the other hand, the seeds passed through the camel crickets unscathed. A seed viability test confirmed that they were viable. Camel crickets were dispersing intact seeds of all three parasitic plants via their poop. The minuscule size of the seeds as well as their tough seed coat (compared to wind dispersed seeds of similar species) allowed for safe passage through the digestive system of this common ground insect.

In a later study, Suetsugu observed another mycoheterotrophic orchid, Yoania japonica, and also found camel crickets to be a common consumer of its fleshy, indehiscent fruits. Viable seeds were again found in the insect’s frass and were observed germinating in their natural habitat. Seutsugu noted that all of the fruits in his studies consumed by camel crickets are white or translucent, easily accessible to ground dwelling insects, and give off a fermented scent to which insects like camel crickets are known to be attracted. Camel crickets also spend their time foraging in areas suitable for the growth of these plants. All of this suggests co-evolutionary adaptations that have led to camel cricket-mediated seed dispersal.

Yoania japonica via wikimedia commons

Insect endozoochory may be an uncommon phenomenon, but perhaps it’s not as rare as we once presumed. As mentioned above, an instance of endozoochory by a beetle has been reported, as has one by a species of cockroach. Certainly the most well known example involves the wetas of New Zealand, which are large, flightless insects in the same order as grasshoppers and crickets and sometimes referred to as “invertebrate mice.” New Zealand lacks native ground-dwelling mammals, and wetas appear to have taken on the seed dispersal role that such mammals often play.

Where seeds are small enough and seed coats tough enough, insects have the potential to be agents of seed dispersal via ingestion. Further investigation will reveal additional instances where this is the case. Of course, effective seed dispersal means seeds must ultimately find themselves in locations suitable for germination in numbers that maintain healthy populations, which for the dust seeds of parasitic plants is quite specific since they require a host organism to root into. Thus, effective seed dispersal in these scenarios is also worth a more detailed look.

Further Reading:

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For more stories of seed dispersal check out the first issue of my new zine, Dispersal Stories.

Out Now! Dispersal Stories #1

Before I started this blog, I had spent 16 years publishing zines at a steady clip and sending them to all corners of the world through the mail. I had never really meant to abandon zines altogether, and in some ways, putting all my writing efforts into a blog felt a little like a betrayal. My intention had always been to one day put together another zine. Now, six and a half years later, I’m happy to report that day has come.

Rather than bring an old zine back from the grave, I decided to make a new zine. Thus, Dispersal Stories #1. It’s quite a bit different from zines I’ve made in the past, which were generally more personal and, I guess, ranty. In fact, Dispersal Stories is very much like this blog, largely because it is mostly made up of writing that originally appeared here, but also because its main focus (for now) is plants. What sets it apart is that, unlike this blog, it zeroes in on a specific aspect of plants. As the title suggests, it’s all about dispersal. For much of their life, plants are essentially sessile. Once they are rooted in place, they rarely go anywhere else. But as seeds, spores, or some other sort of propagule they are actually able to move around quite a bit. The world is their oyster. What’s happening during this period of their lives is the focus of Dispersal Stories.

But why do a zine about this? Apart from just wanting to do another zine after all these years, my hope is that Dispersal Stories will be the start of a much more ambitious project. A book perhaps. My interest in dispersal was born out of my interest in weeds, and there is so much that I would like to learn and share about both of these subjects – so much so that the blog just doesn’t really cut it. So, I’m expanding the Awkward Botany empire. First a zine, then a book, then … who knows? I’m an oyster! (Or something like that.)

Dispersal Stories #1 is available in our etsy shop, or you can contact me here and we can work something out. While you’re at it, check out our new sticker.

If you love looking at plants and learning their names, then you probably enjoy doing it any chance you get. Usually it’s an activity you do while walking, but who says you can’t botanize while riding a bike? This sticker is inspired by a friend who once said that while mountain biking you get to “see three times as many flowers in half the time!” Stick it on your bike or in some other prominent location to remind yourself and others that we can botanize anytime anywhere.

Your purchase of one or both of these items helps support what we do. You can also support us by buying us a ko-fi or putting money in our donorbox. Sharing these posts also helps us out. If you get a copy of the zine, let us know what you think by sending us an email, a message on twitter or facebook, or by leaving a comment below. As always, thanks for reading.

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