We are told that the mistletoe species lacks genes found in all other complex organisms.
A discovery made during an analysis of a species of mistletoe whose apparent ability to survive without key genes involved in energy production could make it one of the most unusual plants on Earth.
“This loss of genes very likely corresponds to the loss of an entire respiratory complex,” said Jeff Palmer, IU Distinguished Professor in the IU Bloomington College of Arts and Sciences’ Department of Biology, who led the study. “This is something that hasn’t been reported before for any multicellular organism.”
The genes that have been lost from V. scurruloideum typically reside in the mitochondrial genomes of plants and animals. Mitochondria play an important role in the overall metabolism of cellular organisms, serving as generators that produce energy to power other functions of the cell through respiration.
Contained within all eukaryotic cells, mitochondria possess their own DNA due to their evolutionary history as an “enslaved” bacterium swallowed up billions of years ago by a primitive eukaryotic organism.
Biologists regard the mitochondrial genomes, or mitogenomes, of flowering plants as “extreme and often perplexing” in their wide range of sizes, structures and mutation rates, Palmer said. But this genetic diversity has never before been shown to include the loss of genes involved in the mitochondria’s primary genetic function of respiration. More.
In short, evolution is a science that can make no predictions as to what to expect.
It is relevant that mistletoe is a semi-parasite, typically on deciduous trees:
“This plant has taken a unique overall tack in evolution, presumably related to its parasitic lifestyle,” said Elizabeth Skippington, a postdoctoral fellow at IU who is first author on the study. “Whether cause or cure, parasitism is most likely at the root of its extremely small mitochondrial genome, which shows clear signs of both rapid and degenerative evolution.”
How do we know that the mistletoe’s development direction is a “unique overall tack” in evolution, as opposed to the first example discovered?
Also, what does “degenerative” mean if the plant has been around successfully for millions of years and not expected to die out naturally soon?
When we use the term “degenerative disease,” we mean a disease a disease that will probably kill or fatally weaken the patient. Maybe “degenerative” is still correct here, but we need to be clear how we are using the term. What if, in its niche, the mistletoe is better off without the extra programming? How is that degeneration?
Despite their familiar role in Christmas traditions, however, Palmer said the broader ecological importance and prevalence of mistletoes across the world is commonly unrecognized.
So it appears to be a niche that works for the mistletoe.
We have only begun to find out enough to have the right questions. Fortunately, with Darwin in charge, we already have all the answers in place to jam the questions into, and hammer them down hard if they don’t fit.
Here’s the abstract:
Despite the enormous diversity among parasitic angiosperms in form and structure, life-history strategies, and plastid genomes, little is known about the diversity of their mitogenomes. We report the sequence of the wonderfully bizarre mitogenome of the hemiparasitic aerial mistletoe Viscum scurruloideum. This genome is only 66 kb in size, making it the smallest known angiosperm mitogenome by a factor of more than three and the smallest land plant mitogenome. Accompanying this size reduction is exceptional reduction of gene content. Much of this reduction arises from the unexpected loss of respiratory complex I (NADH dehydrogenase), universally present in all 300+ other angiosperms examined, where it is encoded by nine mitochondrial and many nuclear nad genes. Loss of complex I in a multicellular organism is unprecedented. We explore the potential relationship between this loss in Viscum and its parasitic lifestyle. Despite its small size, the Viscum mitogenome is unusually rich in recombinationally active repeats, possessing unparalleled levels of predicted sublimons resulting from recombination across short repeats. Many mitochondrial gene products exhibit extraordinary levels of divergence in Viscum, indicative of highly relaxed if not positive selection. In addition, all Viscum mitochondrial protein genes have experienced a dramatic acceleration in synonymous substitution rates, consistent with the hypothesis of genomic streamlining in response to a high mutation rate but completely opposite to the pattern seen for the high-rate but enormous mitogenomes of Silene. In sum, the Viscum mitogenome possesses a unique constellation of extremely unusual features, a subset of which may be related to its parasitic lifestyle. (paywall) – Elizabeth Skippingtona, Todd J. Barkmanb, Danny W. Ricea, and Jeffrey D. Palmera. Miniaturized mitogenome of the parasitic plant Viscum scurruloideum is extremely divergent and dynamic and has lost all nad genes. PNAS, 2015 DOI: 10.1073/pnas.1504491112
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