Passerines and Vitamin C
There is an enzyme known as L-gulonolactone oxidase (GLO) that is thought to be crucial in the role of synthesising vitamin c (ascorbic acid) which has been lost in a number of avian and mammalian lineages. With mammals, the enzyme is found within all the primary and major lineages but not in bats as well as anthropoid primates and guinea pigs. The presence of GLO in mammals seems to be a trait that is conservative that isn’t likely to be lost and difficult to recover when it is lost. GLO is found in all non-passerine birds that were examined but have been discovered to be absent in several other passerines. Some early research and analysis makes a suggestion that GLO was lost and gone from a phylogenetic event in the most highly evolved passerines.
It may be prudent then to reassess the two conclusions previously made in other studies; that GLO has been lost repeatedly and regained in passerine birds; and that with current available data, the presence of GLO in passerines doesn’t represent a possibility in prediction from other phylogenetic diet or affiliations. The presence or absence of GLO in passerines seems to be indicative of being specified within the avian genera. With little information or research available, it doesn’t allow the opportunity to provide an identification as to whether the presences of GLO is ancestral in nature or is it deemed to be a derivative in passerines. At the very least, GLO appears to have come back into existence in passerines that have two independent lineages. If you could make the assumption that the disappearance of GLO in ancestral lines of passerines, then with parsimony tracing, it is suggested that GLO expression has been lost once and reattained four times. To contrast, the appearance of GLO expression is to be assumed as ancestral in nature in passerines and the analysis will suggest two reattainments and two losses.
The data and research available on the GLO expression with birds is limiting, so any inferences made from further analysis should be considered tentatively. With that in mind, there are two conclusions that can be made: the idea that one single moment of evolutionary significance brought about the loss of GLO in the most evolved species of passerines is untenable; that there could be a considerable and significant liability of evolution in the expression of GLO within passerines.
The pattern that can be observed of GLO expression within passerines can be varied when compared to those displayed in mammals in which the trait is not presented as phylogenetically conservative. This then leads to considerations of whether the expression of GLO is related in any way or correlated to diet. A study conducted by Diamond in 1986 suggested two answers to the question of why it is that enzymes happen to lose their genetic basis or become repressed. The first one being if a specific gene or enzyme is neutered from non-use, then there will be mutations that occur to remove this character. The second being that with the development of a character, there may be costs involved of a biosynthetic and energetic nature.
Characters that are not in use may not be placed neutral but could be disadvantageous selectively. These theories hold the prediction that with enzyme loss, it can be discovered in lineages where the metabolic pathway disuse can be identified. GLO deficiency presents with cases of scurvy with animals on a diet free from vitamin c. With this in mind, a theory can be formed that there is a correlation or association between the presence and absence of GLO with dietary habits. Some researchers have suggested that animals lost the ability to synthesize vitamin c from evolving in environments where vitamin c is readily available in their food sources.
Source Information for Vitamin C Facts
The research and information on the presence of GLO in avian species is limited and only based on a small sample of phylogenetically biased passerines, this makes it a risky decision to attempt to predict GLO expression in these birds and therefore any reliance on the correlation between dietary habits and intake of vitamin c and GLO expression. At this point in time with the data available, the ability to synthesize vitamin c from passerines remains unpredictable.
© Copyright 2016 Michael Lam, M.D. All Rights Reserved.