Year: 2022

X-INACTIVATION. “In mammals, X inactivation avoids the dosage differences of X-linked genes between males and females, as one of the two copies of the X chromosome present in female cells is inactivated by Xist. This gene is located on the X chromosome and is transcribed as a long non-protein-coding RNA with a transcript size of 17 kb in humans.” http://www.mdpi.com/1422-0067/16/2/3251/htm View in LinkedIn

FRAGMENT FROM NATURE continues the meditation of the loss and reactivation of the vitamin C gene during evolution. It is a case study of a gene mutation that has only one consequence, and is not under selective pressure when there are ample dietary sources of vitamin C. Indeed, both consumption levels and tissue acquisition are greater than expected, suggesting that we still have more to learn about its function. View in LinkedIn

ALTERNATIVE PATHWAYS. “Many animal lineages, including primates, have become ascorbate auxotrophs due to the loss of the terminal enzyme in their biosynthetic pathway, L- gulonolactone oxidase (GULO). The alternative pathways found in land plants and Euglena use a different terminal enzyme, L-galactonolactone dehydrogenase (GLDH).” https://lnkd.in/d9X8VWN View in LinkedIn

PLANT GENE SHAKEUPS. “Photosynthetic eukaryotes arose following the endosymbiotic acquisition of a cyanobacterial ancestor by a non-photosynthetic eukaryote in the Archaeplastida (Plantae) lineage. Several other eukaryote lineages, including the diatoms, haptophytes and euglenids, subsequently gained plastids through a secondary endosymbiosis with either a red or green alga. These plastid endosymbioses were accompanied by lateral gene transfer on a massive scale from the symbiont to the host nuclear genome (known as endosymbiotic gene transfer or EGT), giving rise to the complex physiologies of photosynthetic eukaryotes.” https://lnkd.in/d9X8VWN View in LinkedIn

PLANT SYNTHESIS. “Here we present molecular and biochemical evidence demonstrating that GULO was functionally replaced with GLDH in photosynthetic eukaryote lineages following plastid acquisition. GULO has therefore been lost repeatedly throughout eukaryote evolution. The formation of the alternative biosynthetic pathways in photosynthetic eukaryotes uncoupled ascorbate synthesis from hydrogen peroxide production and likely contributed to the rise of ascorbate as a major photoprotective antioxidant.” https://lnkd.in/d9X8VWN View in LinkedIn

VITAMIN C GENE LOSS. “Given its high daily requirement and important functions, it is surprising that many species, such as teleost fishes, anthropoid primates, guinea pigs, as well as some bat and Passeriformes bird species, have lost the capacity to synthesize it.” https://lnkd.in/d66RD9Z View in LinkedIn

HIGHLY SPECIFIC MUTATIONS. “Interestingly, all the known losses are the result of mutations in the L-gulono-γ-lactone oxidase gene (GLO; EC number 1.1.3.8) which codes for the enzyme that catalyzes the final step of vitamin C biosynthesis. Therefore, losing this gene only impacts the ability to make vitamin C.” https://lnkd.in/d66RD9Z View in LinkedIn

UPSTREAM IMPACT. “In contrast, losing genes for other enzymes in this synthetic pathway would affect the production of many other molecules. Compared to other genes, the GLO gene is therefore “predisposed” to being lost because it makes a single compound unnecessary for other pathways.” https://lnkd.in/d66RD9Z View in LinkedIn

NO LOSS OF FITNESS. “Explaining the frequent loss of GLO genes by saying that it only affects the production of a single compound also implies that losing the capacity to make this compound is not selected against, i.e., that such a loss does not cause any selective disadvantage.” https://lnkd.in/d66RD9Z View in LinkedIn

DIETARY ALTERNATIVE. “Since all species which have lost the capacity to synthesize vitamin C have a vitamin C-rich diet, this is the most common explanation brought forward to explain its frequent occurrence.” https://lnkd.in/d66RD9Z View in LinkedIn