Lettuce Gone Wild, part two

The lettuce we eat is a close relative to the lettuce we weed out of our gardens. Last week we discussed the potential that wild relatives may have for improving cultivated lettuce. But if wild lettuce can be crossed with cultivated lettuce to create new cultivars, can cultivated lettuce cross with wild lettuce to make it more weedy?

Because so many of our crops are closely related to some of the weeds found along with them or the plants growing in nearby natural areas, the creation of crop-wild hybrids has long been a concern. This concern is heightened in the age of transgenic crops (also known as GMOs), for fear that hybrids between weeds and such crops could create super weeds – fast spreading or highly adapted weeds resistant to traditional control methods such as certain herbicides. To reduce this risk, extensive research is necessary before such crops are released for commercial use.

flowers of prickly lettuce (Lactuca serriola)

There are no commercially available, genetically modified varieties of cultivated lettuce, so this is not a concern when it comes to crop-wild hybrids; however, due to how prevalent weedy species like prickly lettuce (Lactuca serriola) are, hybridization with cultivated lettuce is still a concern. So, it is important to understand what the consequences might be when hybridization occurs.

In a paper published in Journal of Applied Ecology in 2005, Hooftman et al. examined a group of second-generation hybrids (L. sativa x L. serriola), and found that the hybrids behaved and appeared very similarly to non-hybrid prickly lettuce. They also found that the seeds produced by the hybrids had a significantly higher germination rate than non-hybrid plants. This is an example of hybrid vigor. Thus, “if hybridization does occur, this could lead to better performing and thus potentially more invasive (hybrid) genotypes.” However, the authors cautioned that “better performing genotypes do not automatically result in higher invasiveness,” and that much depends on the conditions they are found in, the level of human disturbance, etc.

Another thing to consider is that hybrids are not stable. In an article published in Nature Reviews Genetics in 2003, Stewart et al. adress the “misunderstanding that can arise through the confusion of hybridization and … introgression.” It is wrong to assume that hybrids between crops and wild relatives will automatically lead to super weeds. For this to occur, repeated crosses with parental lines (also known as backcrossing) must occur, and “backcross generations to the wild relative must progress to the point at which the transgene [or other gene(s) in question] is incorporated into the genome of the wild relative.” That is what is meant by “introgression.” This may happen quickly or over many generations or it may never happen at all. Each case is different.

prickly leaf of prickly lettuce (Lactuca serriola)

In a paper published in Journal of Applied Ecology in 2007, Hooftman et al. observe the breakdown of crop-wild lettuce hybrids. They note that “fitness surplus through [hybrid vigor] will often be reduced over few generations,” which is what was seen in the hybrids they observed. One possible reason why this occurs is that lettuce is predominantly a self-crossing species; outcrossing is rare, occurring 1 – 5% of the time thanks to pollinating insects. But that doesn’t mean that a stable, aggressive genotype could never develop. Again, much depends on environmental conditions, as well as rates of outcrossing and other factors relating to population dynamics.

A significant expansion of prickly lettuce across parts of Europe led some to hypothesize that crop-wild hybrids were partly to blame. In a paper published in Molecular Ecology in 2012 Uwimana et al. ran population genetic analyses on extensive data sets to determine the role that hybridization had in the expansion. They concluded that, at a level of only 7% in wild habitats, crop-wild hybrids were not having a significant impact. They observed greater fitness in the hybrids, as has been observed in other studies (including the one above), but they acknowledged the instability of hybrids, especially in self-pollinating annuals like lettuce.

seed head of prickly lettuce (Lactuca serriola)

It is more likely that the expansion of prickly lettuce in Europe is due to “the expansion of favorable habitat as a result of climate warming and anthropogenic habitat disturbance and to seed dispersal because of transportation of goods.” Uwimana et al. did warn, however, that “the occurrence of 7% crop-wild hybrids among natural L. serriola populations is relatively high [for a predominantly self-pollinating species] and reveals a potential [for] transgene movement from crop to wild relatives [in] self-pollinating crops.”

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Lettuce Gone Wild, part one

Lettuce, domesticated about six thousand years ago in a region referred to as the Fertile Crescent, bears little resemblance to its wild ancestors. Hundreds of years of cultivation and artificial selection eliminated spines from the leaves, reduced the latex content and bitter flavor, shortened stem internodes for a more compact, leafy plant, and increased seed size, among several other things. The resulting plant even has a different name, Lactuca sativa (in Latin, sativa means cultivated). However, cultivated lettuce remains closely related to its progenitors, with whom it can cross to produce wild-domestic hybrids. For this reason, there is great interest in the wild relatives of lettuce and the beneficial traits they offer.

image credit: wikimedia commons

Crop wild relatives are a hot topic these days. That’s because feeding a growing population in an increasingly globalized world with the threat of climate change looming requires creative strategies. Utilizing wild relatives of crops in breeding programs is a potential way to improve yields and address issues like pests and diseases, drought, and climate change. While this isn’t necessarily a new strategy, it is increasingly important as the loss of biodiversity around the globe threatens many crop wild relatives. Securing them now is imperative.

There are about 100 species in the genus Lactuca. Most of them are found in Asia and Africa, with the greatest diversity distributed across Southwest Asia and the Mediterranean Basin. The genus consists of annual, biennial, and perennial species, a few of which are shrubs or vines. Prickly lettuce (L. serriola), willowleaf lettuce (L. saligna), and bitter lettuce (L. virosa) are weedy species with a wide distribution outside of their native range. Prickly lettuce is particularly common in North America, occurring in the diverse habitats of urban areas, natural areas, and agricultural fields. It is also the species considered to be the main ancestor of today’s cultivated lettuce.

In a paper published in European Journal of Plant Pathology in 2014. Lebeda et al. discuss using wild relatives in lettuce breeding and list some of the known cultivars derived from crosses with wild species. They write that in the last thirty years, “significant progress has been made in germplasm enhancement and the introduction of novel traits in cultivated lettuce.” Traditionally, Lactuca serriola has been the primary source for novel traits, but breeders are increasingly looking to other species of wild lettuce.

bitter lettuce (Lactuca virosa) – image credit: wikimedia commons

Resistance to disease is one of the main aims of lettuce breeders. Resistance genes can be found among populations of cultivated lettuce, but as “extensive screening” for such genes leads to “diminishing returns in terms of new resistance,” breeders look to wild lettuce species as “sources of new beneficial alleles.” The problem is that there are large gaps in our knowledge when it comes to wild lettuce species and their interactions with pests and pathogens. Finding the genes we are looking for will require “screening large collections of well defined wild Lactuca germplasm.” But first we must develop such collections.

In a separate paper (published in Euphytica in 2009), Lebeda et al. discuss just how large the gaps in our understanding of the genus Lactuca are. Beginning with our present collections they found “serious taxonomic discrepancies” as well as significant redundancy and unnecessary duplicates in and among gene banks. They also pointed out that “over 90% of wild collections are represented by only three species” [the three weedy species named above], and they urged gene banks to “rapidly [acquire] lettuce progenitors and wild relatives from the probable center of origin of lettuce and from those areas with the highest genetic diversity of Lactuca species” as their potential for improving cultivated lettuce is too important to neglect.

Lactuca is a highly variable genus; species can differ substantially in their growth and phenology from individual to individual. Lebeda et al. write, “developmental stages of plants, as influenced through selective processes under the eco-geographic conditions where they evolved, can persist when plants are cultivated under common environmental conditions and may be fixed genetically.” For this reason it is important to collect numerous individuals of each species from across their entire range in order to obtain the broadest possible suite of traits to select from.

One such trait is root development and the related ability to access water and nutrients and tolerate drought. Through selection, cultivated lettuce has become a very shallow-rooted plant, reliant on regular irrigation and fertilizer applications. In an issue of Theoretical and Applied Genetics published in 2000, Johnson et al. demonstrate the potential that Lactuca serriola, with its deep taproot and ability to tolerate drought, has for developing lettuce cultivars that are more drought tolerant and more efficient at using soil nutrients.

willowleaf lettuce (Lactuca saligna) – image credit: wikimedia commons

Clearly we have long way to go in developing improved lettuce cultivars using wild relatives, but the potential is there. As Lebeda et al. write in the European Journal of Plant Pathology, “Lettuce is one of the main horticultural crops where a strategy of wild related germplasm exploitation and utilization in breeding programs is most commonly used with very high practical impact.”

Coming Up in Part Two: Can cultivated lettuce cross with wild lettuce to create super weeds?

Charles Darwin and the Phylogeny of State Flowers and State Trees

This is a guest post by Rachel Rodman. Photos by Daniel Murphy.

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Every U.S. state has its own set of symbols: an official flower, an official tree, and an official bird. Collectively, these organisms form the stuff of trivia and are traditionally presented in the form of a list.

But, lists…well. As charming as lists can sometimes be, lists are rarely very satisfying.

So I decided to try something different.

I am not, of course, the first person to be unhappy with the eclectic, disordered nature of many biological assemblages. In the 18th century, Linnaeus developed a classification system in order to make sense of that untidiness. Kingdom, Phylum, Class, and so on.

In the 19th century, Darwin set biodiversity into an even more satisfying intellectual framework, outlining a model that linked organisms via descent from a series of common ancestors. And, as early as 1837, he experimented with a tree-like structure, in order to diagram these relationships.

Following Darwin’s lead, I’ve worked to reframe the state flowers and state trees in terms of their evolutionary history (*see the methods section below). And today, in honor of Darwin’s 209th birthday, I am delighted to present the results to you.

Let’s start with the state flowers.

In this tree, Maine’s “white pine cone and tassel” forms the outgroup. Among all the state “flowers,” it is the only gymnosperm—and therefore, in fact, not actually a flower.

Notice, also, that the number of branches in this tree is 39—not 50. Most of this stems from the untidy fact that there is no requirement for each state to select a unique flower. Nebraska and Kentucky, for example, share the goldenrod; North Carolina and Virginia share the dogwood.

With the branch labeled “Rose,” I’ve compressed the tree further. The state flowers of Georgia, Iowa, North Dakota, New York, and Oklahoma are all roses of various sorts; with my data set (*see methods below), however, I was unable to disentangle them. So I kept all five grouped.

This is a rich tree with many intriguing juxtapositions. Several clades, in particular, link geographical regions that are not normally regarded as having a connection. Texas’ bluebonnet, for example, forms a clade with Vermont’s red clover. So, similarly, do New Hampshire’s purple lilac and Wyoming’s Indian paintbrush.

Texas bluebonnet (Lupinus texensis) – the state flower of Texas

The second tree—the tree of state trees—is similarly rewarding. This tree is evenly divided between angiosperms (19 species) and gymnosperms (17 species).

Iowa’s state tree is simply the “oak”—no particular species was singled out. To indicate Iowa’s selection, I set “IA” next to the node representing the common ancestor of the three particular oak species: white oak, red oak, and live oak, which were selected as symbols by other states.

Arkansas’ and North Carolina’s state tree, similarly, is the “pine,”—no particular species specified. I’ve indicated their choice in just the same way, setting “AR” and “NC” next to the node representing the common ancestor of the eight particular pine species chosen to represent other states.

In this tree of trees, as with the tree of flowers, several clades link geographical regions that are not usually linked—at least not politically. Consider, for example, the pairing of New Hampshire’s white birch with Texas’ tree, the pecan.

Another phylogenetic pairing also intrigued me: Pennsylvania’s eastern hemlock and Washington’s western hemlock. It evokes, I think, a pleasing coast-to-coast symmetry: two states, linked via an east-west cross-country bridge, over a distance of 2,500 miles

The corky bark of bur oak (Quercus macrocarpa). Oak is the state tree of Iowa.

In this post, I’ve presented the U.S. state flowers and U.S. state trees in evolutionary framework. The point in doing that was not to denigrate any of the small, human stories that lie behind these symbols—all of the various economic, historical, and legislative vagaries, which led each state to select these particular plants to represent them. (Even more importantly, I have no wish to downplay the interesting nature of any of the environmental factors that led particular plants to flourish and predominate in some states and not others.)

The point, instead, was to suggest that these stories can coexist and be simultaneously appreciated alongside a much larger one: the many million year story of plant evolution.

With Darwin’s big idea—descent with modification—the eclectic gains depth and meaning. And trivia become a story—a grand story, which can be traced back, divergence point by divergence point: rosids from asterids (~120 mya); eudicots from monocots (~160 mya); angiosperms from gymnosperms (~300 mya), and so on and so on.

So today, on Darwin’s 209th, here, I hope, is one of the takeaways:

An evolutionary framework really does make everything—absolutely everything: U.S. state symbols included—more fun, more colorful, more momentous, and more intellectually satisfying.

Thanks, Darwin.

*Methods:

To build these two trees, I relied on a data set from TimeTree.org, a website maintained by a team at Temple University. At the “Load a List of Species” option at the bottom of the page, I uploaded two lists of species in .txt format; each time, TimeTree generated a phylogenetic tree, which served as a preliminary outline.

Later, once I’d refined my outlines, I used the “Get Divergence Time For a Pair of Taxa” feature at the top of the page in order to search for divergence time estimates. As I reconstructed my trees in LibreOffice, I used these estimates to make my branch lengths proportional.

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Rachel Rodman has a Ph.D. in Arabidopsis genetics and presently aspires to recontextualize all of history, literature, and popular culture in the form of a phylogenetic tree. Won’t you help her?

Dischidia and Its Self-contained Watering System

This is a guest post by Jeremiah Sandler.

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I was doing some sunday reading in a plant biology textbook, a section about leaves. It was highlighting leaf-specific adaptations and other cool leaf specializations. I came across a paragraph about a “flower-pot” leaf, and my mind was so blown after reading it I had to literally stand up.

It reads:

Some leaves of the Dischidia [genus], an epiphyte from Australasia, develop into urnlike pouches that become the home of ant colonies. The ants carry in soil and add nitrogeneous wastes, while moisture collects in the leaves through condensation of the water vapor coming from the mesophyll through stomata. This creates a good growing medium for roots, which develop adventitiously from the same node as the leaf and grow down into the soil contained in the urnlike pouch. In other words, this extraordinary plant not only reproduces itself by conventional means but also, with the aid of ants, provides its own fertilized growing medium and flower pots and then produces special roots, which “exploit” the situation.

Naturally I had to look up images of this plant because that’s amazing.

Illustration of Dischidia major (image credit: wikimedia commons)

Dischidia major – cross section of “flower-pot” leaf (photo credit: eol.org)

Dischidia vidalii– cross section of “flower-pot” leaf (photo credit: eol.org)

In shorter words, the plant grows modified leaves that form a little cavity, within which ants live. The ants incidentally carry soil into the cavity, while fertilizing that soil with their waste. The stomata are located on the insides of these cavities, which expel water from the leaves, where it then waters the soil that is located inside the leaves. Not to mention, the outside of those cavities are photosynthesizing all the while.

So, with the help of ants, an epiphytic Dischidia has evolved leaves to bring the soil to itself up in the trees, where it fertilizes and waters itself? SAY WHAT?! That is so damn cool.


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