Mulling the Origin of Mulberry Fruit

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We rely on plants for almost all of our most basic needs: the air we breathe, the food we eat, shelter from the elements, and even medicines. Learning more about the biology, evolution, and cultivation of the nearly 400,000 known plant species that cover our planet will help us better conserve and utilize our essential green neighbors.

Plant biologist Nyree Zerega studies the past, present and future of one of these groups, the mulberry family. Her research on two of its most prominent members, the jackfruit and the breadfruit, will not only bring new insights into where these species originated and how they spread, but also ways they might help address food security issues in one of our world’s poorest countries. We asked Zerega, director of the Graduate Program in Plant Biology and Conservation at Northwestern University and the Chicago Botanic Garden, for the details.

Your work focuses on the mulberry family of plants. Why were you drawn to these? 
I was always interested in plants that people use. I do love plants for the sake of plants, but I’m drawn to how we interact with them. Nyree Zerega

The mulberry family has a lot of very useful plants from a human perspective. It has about 1,100 species, and more than 700 of those are figs. (Only one of these is an edible fig found in markets.) Then there’s the mulberry. [Mulberry] fruit is edible, and the leaves of the white mulberry are actually the primary food source of silkworms, so it is a really big part of the silk industry. There’s paper mulberry—[its bark] makes very high-quality paper, which is often used for origami. And, of course, [there are] the breadfruit and the jackfruit. There are many other species in that group of plants that people use on a more regional scale, but those are the big ones.

You’re interested specifically in the breadfruit and the jackfruit trees. What do you want to learn about them?
I look at the genus they belong to—the group of most closely related plants. There are about 60 of them. I’m interested in general how that group evolved: what their wild relatives are, what kind of diversity exists among them, and their interactions with humans. Understanding their origins and diversity has implications for conservation of germplasm (collection of seeds/genetic material for an organism). We conserve germplasm of wild crop relatives—it’s a really important thing that many countries focus on.

My work on jackfruit mainly has to do with finding the center of origin, understanding how it spread from there, and in particular I’m focused on Bangladesh.

Why Bangladesh?
It started because someone emailed me from Bangladesh [who had seen] my work on breadfruit, and wondered if I would be interested in looking at jackfruit. He had a master’s student who was looking at the morphological diversity in Bangladesh. He thought Bangladesh was a likely area of origin of jackfruit, or a secondary center of diversity, but no one had really studied it. That will be considered as we have data from more places and can put it in a broader context.

But what we ended up looking at there was the impacts of different cultivation methods of jackfruit on diversity in Bangladesh. One of the other reasons he contacted me was [because of his concerns about] the way people were growing and cultivating it. [He was concerned that] they were only focusing on a small percentage of the genetic diversity that might exist throughout Bangladesh, and that in the process they were actually losing valuable genetic diversity.

Bangladesh is one of the poorest and most densely populated countries in the world. It’s basically the size of Illinois and has about 150 million people living there. We have about 13 million. So they don’t have a lot of natural landscape left. Most of the areas that are left are there because they’re just too difficult for people to cultivate or live in because of flooding. About a third of their arable land floods every year.

So there aren't really natural populations of [jackfruit] in the wild. The government has been interested in trying to reforest Bangladesh, but using useful plants. They’ve promoted nurseries throughout the country that grow saplings of different useful trees, jackfruits among them, and then they’re sold at a low price.

We’re looking at the genetic diversity levels of self-planted [jackfruit] versus nursery stock. We went to different villages throughout Bangladesh and found out about the jackfruit that they grow.

What have you found?
So far, it looks like there is a decreased level of diversity among the nursery stock. The concern was, when we started looking at this, that the nurseries may only be focusing on a smaller percentage of different types of jackfruit, as opposed to if [seeds are shared] within a village, or with your neighbor, or with another village. This is still preliminary. We will be investigating further and communicating with our collaborator [in Bangladesh] to communicate this to the farmers and the government.

Why is genetic diversity so important?
The potato blight is the typical example of why it can be detrimental to not have genetic diversity in a crop. You can have a whole field or a whole country basically planted with clones of one another. Then a disease hits and they’re all gone quickly. Maintaining diversity is basically hedging your bets.

You’ve also traced the spread of breadfruit from its likely origin in New Guinea throughout the Pacific Islands. What have you learned?
The first part of the work was to identify what the wild relatives were, how breadfruit was domesticated from those wild relatives, and how they spread to their current range.  [Breadfruit has] been introduced throughout other parts of the tropics, but it is clear that where it’s diverse and where it came from is the Pacific Islands. We used DNA fingerprinting, and it turns out that there are two different wild relatives: one probably came from New Guinea, and the other from some islands in Micronesia.

The Pacific Islands, like Polynesia, were some of the last places colonized by humans on the Earth. There was a group of people that we think came from somewhere in southeast Asia that are the ancestors of the Polynesians. They came to New Guinea but kept moving on after that. [In New Guinea], they would have encountered this wild relative and taken it with them on their ocean voyages to colonize new islands. They were really accomplished seafarers, and traveled further east, into the islands out there. 

Because the wild relatives of breadfruit have big seeds, and they [don’t] have any dormancy period, it’s not practical to carry the seeds on long trips because they don’t last very long. It’s likely that what they had to do was vegetatively propagate [the breadfruit] through root cuttings—[bringing] just pieces of the root that you can store and plant when you get to a place where you can plant them. That kind of vegetative propagation can lead to various mutations, which eventually led to seedlessness. The seedless varieties, for whatever reason, were preferentially selected. It’s called breadfruit, but it’s not really cooked like a fruit; it’s more like a starch, like a potato. Without the seeds, it was easier to prepare, so that might be one reason it was preferentially selected.

By the time you get way out into eastern Polynesia, you don’t really find any breadfruit with seeds. You actually see a gradual reduction in seeds from New Guinea eastward to Polynesia. But, it looks like there were also migrations up to Micronesia, where the other wild relative was, and hybridizations occurred, so in Micronesia you get a whole different set of diversity from two different species.

Breadfruit is recognized as one of the priority underutilized crops that has potential for feeding a lot more people than it does. There are projects in Jamaica, Haiti and Ghana to introduce good cultivars of breadfruit to help with local food supply and food security there, because it grows well in those environments, and it’s pretty nutritious. Right now, I’m working on a grant to do the same thing in Bangladesh, and introduce breadfruit to help with food security issues. The nice thing about [breadfruit] is that you don’t have to worry about it being invasive, because it’s seedless.

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