Oak Trees extraordinary evolutionary history is revealed by their genomes and fossils.
Preface
Oaks, which are all members of the genus Quercus, underwent 56 million years of evolution to become the about 435 species that may now be found on five continents, from Canada to Borneo and from Colombia to Norway. Oaks are important tree species that are essential to the health of the forests they create throughout the Northern Hemisphere. They promote diversity among creatures at all levels of the food chain, including fungi, wasps, birds, and mammals.
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| Gigantic Oak Tree. Credit: Unsplash |
By capturing carbon dioxide and absorbing airborne contaminants, they contribute to air purification. And by providing us with food in the form of acorns and wood for the construction of our houses, furniture, and ships, they have influenced human society. Indeed, oaks have proven to be so helpful to humans that they have been revered in myths and stories for ages.
The Americas are home to many oak species. Here, 60 percent of the species of Quercus are found. The oaks in this area represent more forest tree biomass than any other genus of woody plant in North America and Mexico, and their remarkable variety makes them the most significant group of trees in the continent's forests. Therefore, one must comprehend how oaks evolved to dominate forests in order to comprehend their biodiversity, food webs, and contributions to human well-being.
Due to gaps in their fossil record and limitations of the bio-molecular techniques used to infer evolutionary events from the DNA of live organisms, scientists were only able to make educated guesses about most of the evolutionary history of oaks for many years. However, new developments in genome sequencing and analysis have made it possible for us and our colleagues to piece together a thorough picture of the origin, evolution, and spread of oaks. It is a remarkable example of evolutionary success, and it will have significant ramifications for forecasting how these crucial trees will fare in the face of climate change—and for creating management strategies to guarantee their survival.
Red and White Oaks
Figure - 01: Red Oaks Leaves
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| Red Oaks have bristle-tipped Leaves. Credit: Illinois Wildflowers |
Figure - 02: White Oaks Leaves
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| Leaves of White Oaks Lack Bristles. Credit: Yoksel Zok Unsplash |
Even a casual observer can easily discern the distinctions between the main groupings of oaks. You may already be aware of the two evolutionary lineages that dominate oaks in the Americas. The red oak group, one of these, consists of species with bristly leaf tips. Acorns, the fruits of these trees, are pollinated in one year, but only ripen in the following year in the majority of red oak group species because pollen takes a full year from the time it settles on the female flower to fertilize the seed. White oak species don't have bristles on their leaves, and when they fall, their leaves often carry more nutrients that replenish the soil than red oak leaves do. Additionally, the majority of white oak acorns ripen the same year they are fertilized, with some even germinating before they fall. Red oak acorns are less likely to spoil before gray squirrels can consume them than white oak acorns, hence they are preferred by gray squirrels to store for later consumption.
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| Acorns or Oak nut. Credit: Nicolas Ruiz Unsplash |
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| Squirrel with Acorn. Credit: Onder Ortel Unsplash |
White oaks can also jam water-conducting, tubelike cells called vessels in its wood with tyloses, balloonlike structures that seal the vessels as a barrier against severe fungal diseases like oak wilt. Red oaks defend more slowly and sloppily. As a result, white oaks have traditionally been used as timber for ships and wine barrels because their blocked vessels store water more effectively than red oaks'. Chewing insects can distinguish between red and white oaks, and the majority of them have evolved to prefer one of these two species over the other. Even mycorrhizal fungi, which link plant roots to soil nutrients, seem to be aware of the distinctions between the two lineages of oaks since many of them prefer symbiotic associations with one over the other.
However, when it comes to species, it might be challenging to distinguish between closely related oaks. The genetic variety between individuals and plastic reactions of the trees to their environment both contribute to the variance within species, which frequently resembles the genetic variation between species. And whether it's the white, red, or any of the six other major lineages of oaks globally, oaks frequently hybridize within their own species. High variety within species and ongoing cross-species hybridization confound classification.
Because individual genes frequently trace diverse histories, hybridization can also make it challenging to reconstruct the evolutionary history of oaks using conventional biomolecular approaches, which entail sequencing one or a few genes. A single oak species may have crossed over with multiple other species, causing various genes to record various facets of this history throughout the species' range. Thus, speciation and hybridization have produced the mosaic-like genome of the oak. The complete history of oak speciation cannot be deduced from the sequences of just one or a few genes.
Only a few nuclear genes and the DNA sequences from chloroplasts, the cell organelles that perform photosynthesis, were available to researchers 20 years ago. The oak tree of life's main branching structure could be clearly seen, but the arrangement of its outermost branches was not visible. The three of us discovered in 2008 that new molecular methods could be used to determine the evolutionary history of oaks in addition to the limits of species in the red oak group and hybridization. Since then, we have used a technique called restriction-site associated DNA sequencing to read short segments of DNA from all throughout the genome in cooperation with colleagues from around the globe. We use statistical techniques to assess this data in order to recreate the sequence of species' divergence from common ancestors and the order of their subsequent hybridization. We may determine the most likely ages of significant events in the evolutionary history of oaks by combining these analyses with fossil data. We have been able to infer a large portion of the history of speciation in this group dating back to the root of the oak tree of life, despite the complex genetic history of oaks.
Southward Bound
We may never be able to accurately determine the exact moment or location when the first oak trees appeared, but about 56 million years ago, a group of oak trees living close to Salzburg, Austria, left some of the copious amounts of pollen they produced each spring in the mud. These pollen grains, which have a rugby ball-like form, three longitudinal grooves, and surface textures that differ depending on ancestry, are the earliest conclusive fossil evidence of oaks yet discovered. North America and Eurasia were linked by land bridges that crossed the Atlantic and Pacific Oceans in the early Eocene. Animals and plants traveled freely between the two continents. Oaks were probably a component of a massive forest that covered North America, Europe, and Asia. Because of this, it is challenging to determine whether oaks originated in Eurasia and spread to the Americas via a branch, or the opposite. Perhaps "in the north" is the appropriate response to the question of where modern oaks originated.
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| Oak Tree. Credit: Unsplash |
In any way, very soon after they emerged, oaks began to split into two main branches, one primarily restricted to the Americas and the other primarily restricted to Europe, Asia, and North Africa. Initially, the division of the continents was not perfect. For instance, based on the concentric rings created by the woody scales on its acorn crown, the oldest fossil attributed to the ring-cupped oaks was deposited in Oregon about 48 million years ago. Today, only Southeast Asia retains this lineage. Additionally, red oaks, a modern American species, have been found in fossilized remains from places in Europe that date back to 35 million years. However, oaks were gradually pushed southern and away from the land bridges that had periodically connected Eurasia and North America for the previous 50 million years when global temperatures began their lengthy drop about 52 million years ago. The boundaries between the two continents became very clear as cooling wiped off populations of northern oaks; just two branches of the American clade appeared in Eurasia, and no species from the Eurasian clade did.
The eight
main lineages that we can identify in contemporary forests were further
separated into subgroups before oaks could be pushed too far to the
south. The red, golden cup, and southern live oaks are the only three
that grow only in the Americas. One lineage, that of the white oaks,
began in the Americas, underwent diversification there, and then sent a
branch back to Eurasia. Because the oldest American oak fossil is a
45-million-year-old white oak from Axel Heiberg Island in Nunavut,
Canada, which can be separated from red oaks and all other major
lineages of oaks, we know these major lineages emerged early in the
evolution of oaks. We rely on genomic evidence to determine when the
other oaks split into independent lineages because fossils from this
early stage of diversification are difficult to ascribe to any one
lineage. The eight lineages of the world split up early, according to
the integration of DNA data with a few specific fossils. It recounts
what happened when the North American oaks suffered their own surge of
variety, making it a crucial component of the narrative.
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