Fern Stems Reveal How Evolutionary Constraints Create New Forms in Nature

Jacob S. Suissa, University of Tennessee There are few forms of the botanical world as readily identifiable as fern leaves. These often large, lacy fronds lend themselves nicely to watercolor paintings and tricep tattoos alike. Thoreau said it best: “Nature made ferns for pure leaves, to show what she could do in that line.” But ferns are not just for art and gardens. While fern leaves are the most iconic part of their body, these plants are whole organisms, with stems and roots that are often underground or creeping along the soil surface. With over 400 million years of evolutionary history, ferns can teach us a lot about how the diversity of planet Earth came to be. Specifically, examining their inner anatomy can reveal some of the intricacies of evolution.

Sums of parts or an integrated whole?

When one structure cannot change without altering the other, researchers consider them constrained by each other. In biology, this linkage between traits is called a developmental constraint. It explains the limits of what possible forms organisms can take. For instance, why there aren’t square trees or mammals with wheels. However, constraint does not always limit form. In my recently published research, I examined the fern vascular system to highlight how changes in one part of the organism can lead to changes in another, which can generate new forms.

Before Charles Darwin proposed his theory of evolution by natural selection, many scientists believed in creationism – the idea that all living things were created by a god. Among these believers was the 19th-century naturalist Georges Cuvier, who is lauded as the father of paleontology. His argument against evolution was not exclusively based in faith but on a theory he called the correlation of parts. Cuvier proposed that because each part of an organism is developmentally linked to every other part, changes in one part would result in changes to another. With this theory, he argued that a single tooth or bone could be used to reconstruct an entire organism. He used this theory to make a larger claim: If organisms are truly integrated wholes and not merely sums of individual parts, how could evolution fashion specific traits? Since changes in one part of an organism would necessitate changes in others, he argued, small modifications would require restructuring every other part. If the individual parts of an organism are all fully integrated, evolution of particular traits could not proceed. However, not all of the parts of an organism are tethered together so tightly. Indeed, some parts can evolve at different rates and under different selection pressures. This idea was solidified as the concept of quasi-independence in the 1970s by evolutionary biologist Richard Lewontin. The idea of organisms as collections of individually evolving parts remains today, influencing how researchers and students think about evolution.

Fern vasculature and the process of evolution

Ferns are one of four lineages of land plants that have vascular tissues – specialized sets of tubes that move water and nutrients through their bodies. These tissues are composed of vascular bundles – clusters of cells that conduct water through the stem. How vascular bundles are arranged in fern stems varies substantially. Some have as many as three to eight or more vascular bundles scattered throughout their stem. Some are arranged symmetrically, while others such as the tobacco fern – Mickelia nicotianifolia – have bundles arranged in a whimsical, smiley-face pattern.

For much of the 20th century, scientists studying the pattern and arrangement of vascular bundles in fern stems thought these broad patterns may be adaptive to environmental conditions. I set out in my own research to test whether certain types of arrangements were more resistant to drought. But contrary to my initial hypotheses – and my desire for a relationship between form and function – the arrangement of vascular bundles in the stem did not seem to correlate with drought tolerance. This may sound counterintuitive, but it turns out the ability of a fern to move water through its body has more to do with the size and shape of the water-conducting cells rather than how they’re arranged as a whole in the stem. This finding is analogous to looking at road maps to understand traffic patterns. The patterning of roads on a map (how cells are arranged) may be less important in determining traffic patterns than the number and size of lanes (cell size and number). This observation hinted at something deeper about the evolution of the vascular systems of ferns. It sent me on a journey to uncover exactly what gave rise to the varying vascular patterns of ferns.

Simple observations and insights into evolution

I wondered how this variation in the number and arrangement of vascular bundles relates to leaf placement around the stem. So I quantified this variation in vascular patterning for 27 ferns representing roughly 30% of all fern species. I found a striking correlation between the number of rows of leaves and the number of vascular bundles within the stem. This relationship was almost 1-to-1 in some cases. For instance, if there were three rows of leaves along the stem, there were three vascular bundles in the stem. What’s more, how leaves were arranged around the stem determined the spatial arrangement of bundles. If the leaves were arranged spirally (on all sides of the stem), the vascular bundles were arranged in a radial pattern. If the leaves were shifted to the dorsal side of the stem, the smiley-face pattern emerged. Importantly, based on our understanding of plant development, there was a directionality here. Specifically, the placement of leaves determines the arrangement of bundles, not the other way around.

This may not sound all that surprising – it seems logical that vasculature should link up between leaves and stems. But it runs counter to how scientists have viewed the fern vascular system for over 100 years. Many studies on fern vascular patterning have tended to focus on individual parts of the plant, removing vascular architecture from the context of the plant as a whole and viewing it as an independently evolving pattern. However, this new work suggests that the arrangement of vascular bundles in fern stems is not able to change in isolation. Rather, like Cuvier’s idealized organisms, vascular patterning is linked to and explicitly determined by the number and placement of leaves along the stem. This is not to say that vascular patterns could not be adaptive to environmental conditions, but it means that the handle of evolutionary change in the number and arrangement of vascular bundles is likely changes to leaf number and placement.

From parochial to existential

While this study on ferns and their vascular system may seem parochial, it speaks to the broader question of how variation – the fuel of evolution – arises, and how evolution can proceed. While not all parts of an organism are so tightly linked, considering the individual as a whole – or at least sets of parts as a unit – can help researchers better understand how, and if, observable patterns can evolve in isolation. This insight takes scientists one step closer to understanding the minutia of how evolution works to generate the immense biodiversity on Earth. Understanding these processes is also important for industry. In agricultural settings, plant and animal breeders attempt to increase one aspect of an organism without changing another. By taking a holistic approach and understanding which parts of an organism are developmentally or genetically linked and which are more quasi-independent, breeders may be able to more effectively create organisms with desired traits.

Constraint is often viewed as restricting, but it may not always be so. The Polish nuclear physicist Stanisław Ulam noted that rhymes “compel one to find the unobvious because of the necessity of finding a word which rhymes,” paradoxically acting as an “automatic mechanism of originality.” Whether from the literary rules of a haiku or the development of ferns, constraint can be a generator of form. Fern stems reveal secrets of evolution – how constraints in development can lead to new forms Jacob S. Suissa, Assistant Professor of Plant Evolutionary Biology, University of Tennessee This article is republished from The Conversation under a Creative Commons license. Read the original article.

Thousands of genomes reveal the wild wolf genes in most dogs’ DNA

Audrey T. Lin, Smithsonian Institution and Logan Kistler, Smithsonian Institution Dogs were the first of any species that people domesticated, and they have been a constant part of human life for millennia. Domesticated species are the plants and animals that have evolved to live alongside humans, providing nearly all of our food and numerous other benefits. Dogs provide protection, hunting assistance, companionship, transportation and even wool for weaving blankets. Dogs evolved from gray wolves, but scientists debate exactly where, when and how many times dogs were domesticated. Ancient DNA evidence suggests that domestication happened twice, in eastern and western Eurasia, before the groups eventually mixed. That blended population was the ancestor of all dogs living today. Molecular clock analysis of the DNA from hundreds of modern and ancient dogs suggests they were domesticated between around 20,000 and 22,000 years ago, when large ice sheets covered much of Eurasia and North America. The first dog identified in the archaeological record is a 14,000-year-old pup found in Bonn-Oberkassel, Germany, but it can be difficult to tell based on bones whether an animal was an early domestic dog or a wild wolf. Despite the shared history of dogs and wolves, scientists have long thought these two species rarely mated and gave birth to hybrid offspring. As an evolutionary biologist and a molecular anthropologist who study domestic plants and animals, we wanted to take a new look at whether dog-wolf hybridization has really been all that uncommon.

Little interbreeding in the wild

Dogs are not exactly descended from modern wolves. Rather, dogs and wolves living today both derive from a shared ancient wolf population that lived alongside woolly mammoths and cave bears. In most domesticated species, there are often clear, documented patterns of gene flow between the animals that live alongside humans and their wild counterparts. Where wild and domesticated animals’ habitats overlap, they can breed with each other to produce hybrid offspring. In these cases, the genes from wild animals are folded into the genetic variation of the domesticated population. For example, pigs were domesticated in the Near East over 10,000 years ago. But when early farmers brought them to Europe, they hybridized so frequently with local wild boar that almost all of their Near Eastern DNA was replaced. Similar patterns can be seen in the endangered wild Anatolian and Cypriot mouflon that researchers have found to have high proportions of domestic sheep DNA in their genomes. It’s more common than not to find evidence of wild and domesticated animals interbreeding through time and sharing genetic material. That wolves and dogs wouldn’t show that typical pattern is surprising, since they live in overlapping ranges and can freely interbreed. Dog and wolf behavior are completely different, though, with wolves generally organized around a family pack structure and dogs reliant on humans. When hybridization does occur, it tends to be when human activities – such as habitat encroachment and hunting – disrupt pack dynamics, leading female wolves to strike out on their own and breed with male dogs. People intentionally bred a few “wolf dog” hybrid types in the 20th century, but these are considered the exception.

Tiny but detectable wolf ancestry

To investigate how much gene flow there really has been between dogs and wolves after domestication, we analyzed 2,693 previously published genomes, making use of massive publicly available datasets. These included 146 ancient dogs and wolves covering about 100,000 years. We also looked at 1,872 modern dogs, including golden retrievers, Chihuahuas, malamutes, basenjis and other well-known breeds, plus more unusual breeds from around the world such as the Caucasian ovcharka and Swedish vallhund. Finally, we included genomes from about 300 “village dogs.” These are not pets but are free-living animals that are dependent on their close association with human environments. We traced the evolutionary histories of all of these canids by looking at maternal lineages via their mitochondrial genomes and paternal lineages via their Y chromosomes. We used highly sensitive computational methods to dive into the dogs’ and wolves’ nuclear genomes – that is, the genetic material contained in their cells’ nuclei. We found the presence of wild wolf genes in most dog genomes and the presence of dog genes in about half of wild wolf genomes. The sign of the wolf was small but it was there, in the form of tiny, almost imperceptible chunks of continuous wolf DNA in dogs’ chromosomes. About two-thirds of breed dogs in our sample had wolf genes from crossbreeding that took place roughly 800 generations ago, on average. While our results showed that larger, working dogs – such as sled dogs and large guardian dogs that protect livestock – generally have more wolf ancestry, the patterns aren’t universal. Some massive breeds such as the St. Bernard completely lack wolf DNA, but the tiny Chihuahua retains detectable wolf ancestry at 0.2% of its genome. Terriers and scent hounds typically fall at the low end of the spectrum for wolf genes.

We were surprised that every single village dog we tested had pieces of wolf DNA in their genomes. Why would this be the case? Village dogs are free-living animals that make up about half the world’s dogs. Their lives can be tough, with short life expectancy and high infant mortality. Village dogs are also associated with pathogenic diseases, including rabies and canine distemper, making them a public health concern. More often than predicted by chance, the stretches of wolf DNA we found in village dog genomes contained genes related to olfactory receptors. We imagine that olfactory abilities influenced by wolf genes may have helped these free-living dogs survive in harsh, volatile environments.

The intertwining of dogs and wolves

Because dogs evolved from wolves, all of dogs’ DNA is originally wolf DNA. So when we’re talking about the small pieces of wolf DNA in dog genomes, we’re not referring to that original wolf gene pool that’s been kicking around over the past 20,000 years, but rather evidence for dogs and wolves continuing to interbreed much later in time. A wolf-dog hybrid with one of each kind of parent would carry 50% dog and 50% wolf DNA. If that hybrid then lived and mated with dogs, its offspring would be 25% wolf, and so on, until we see only small snippets of wolf DNA present. The situation is similar to one in human genomes: Neanderthals and humans share a common ancestor around half a million years ago. However, Neanderthals and our species, Homo sapiens, also overlapped and interbred in Eurasia as recently as a few thousand generations ago, shortly before Neanderthals disappeared. Scientists can spot the small pieces of Neanderthal DNA in most living humans in the same way we can see wolf genes within most dogs.

Our study updates the previously held belief that hybridization between dogs and wolves is rare; interactions between these two species do have visible genetic traces. Hybridization with free-roaming dogs is considered a threat to conservation efforts of endangered wolves, including Iberian, Italian and Himalayan wolves. However, there also is evidence that dog-wolf mixing might confer genetic advantages to wolves as they adapt to environments that are increasingly shaped by humans. Though dogs evolved as human companions, wolves have served as their genetic lifeline. When dogs encountered evolutionary challenges such as how to survive harsh climates, scavenge for food in the streets or guard livestock, it appears they’ve been able to tap into wolf ancestry as part of their evolutionary survival kit. Audrey T. Lin, Research Associate in Anthropology, Smithsonian Institution and Logan Kistler, Curator of Archaeobotany and Archaeogenomics, National Museum of Natural History, Smithsonian Institution This article is republished from The Conversation under a Creative Commons license. Read the original article.