Flowering Plants: The Aim of Natural Selection

2018-01-15 00:00:00 ​Cedric Dent 기자

Genetics experts at the University of Oxford have recently found that plant genes are safeguarded by a cellular mechanism against mutation. They published a new study that proves relatively groundbreaking by illustrating that certain genome regions are targeted by DNA Mismatch Repair — the mechanism corrects mutations that manifest while cell division causes genome replication. They conclude that natural selection likely privileged DNA MMR’s targeting of genes like over non-genic regions. Natural selection has proven to be a sort of motif in several of the most recent plant genome studies.

Nicholas Harberd was lead researcher on the project, and he drew the study to its conclusions in collaboration with scientists at Lahore University of Management Studies, which is based in Pakistan, as well as from Zhejiang University based in China. They looked at 9,000 mutations that spanned five generations of Arabidopsis thanliana, which is a model plant species. This strain in particular is MMR-deficient. They compared the mutations with the ones that popped up in different strain that was MMR-proficient.

“We were surprised to see that whilst mutations are more or less randomly spread throughout the genome of the MMR-deficient strain, they are not randomly spread throughout the genome of the MMR-proficient strain,” according to Harberd. MMR-proficient pants actually showed preferential targeting of genes that resulted from mutation over others. Harberd deems it “understandable that natural selection may have favored the relative targeting of MMR to genes rather than non-genic regions. The challenge now is to understand how that targeting works.”

The study informs a firmly established, bio-philosophical question of what natural selection really is and how it functions in nature. That DNA MMR has this much influence over how natural selection ostensibly works suggests that the function of natural selection is intended to be, at least in some part, corrective. In other words, it may actually be a trial-and-error method in nature to figure out what the ideal living organism on earth is or should be. Another study that was recently published in PLoS Biology builds on this to some extent as well. Two scientists researched angiosperms, which are flowering plants like grasses, wheat, tulips, and orchids, and they’ve found genome size to actually be what determines the parameters of evolution.

It was the emergence of angiosperms that first engendered the explosion of biodiversity among general, terrestrial plant life. There are now 350,000 different species of angiosperms; all of which have thrived in most of Earth’s environments, even serving as the backbone of many ecosystems in fact. Beyond that, they account for 90 percent of terrestrial plants. Biologists have endeavored for as long as the theory of evolution has served as the basis for investigating the history of living organisms to figure out how it was possible for angiosperms to become as predominant as they are.

Historically in studying these things, though, most experts have concentrated on searching for answers on the basis of the physiological, which the authors of the PLoS publication suggest is the most likely reason for the problem. Adam Roddy, a Yale University postdoctoral fellow, and San Francisco State University plant biologist, Kevin Simonin, co-authored the paper. They contend that experts should’ve been focusing on the genome sizes all along. Different species have widely differing genome sizes regardless of how complex the living organisms in question are, and most biologists tend to explain this with the example of the onion having five times the DNA content of the Homo sapiens.

The study shows how much genome size variability matters with regard to broad biodiversity. They collected copious data on cell density and size as well as genome size and rates of photosynthesis for lots of different angiosperms, gymnosperms, and ferns. From there, they were able to track correlations between traits throughout history and thereby piece together a fairly involved, evolutionary timeline. Basically, the “when” of angiosperms showing up could be triangulated using several other events in the plants’ timeline in which entire genomes were duplicated.

The duplications yielded extra gene copies that could adapt to new functionality, but the sheer abundance of genetic material also presents a physiological burden. As a result, natural selection typically dictated that these duplications be succeeded by extreme pruning of unnecessary DNA sequences, which the study refers to as “genome downsizing.” It usually left less DNA content for angiosperms than the parent plants had. Moreover, Simonin and Roddy were able to trace angiosperm family trees back to their base, and they pontificate that base angiosperms had particularly small genomes.

“We now know that this not only contributed to their diversity but may have given angiosperms the metabolic advantage to out-compete the other plant groups,” Simonin personally explained. Natural selection, therefore, may very well be solving toward certain ideal organisms, for lack of a better term, that might suddenly dominate when realized or nearly realized.

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