F Rosa Rubicondior: Carrot Evolution And Creationism

Tuesday 10 May 2016

Carrot Evolution And Creationism

A high-quality carrot genome assembly provides new insights into carotenoid accumulation and asterid genome evolution:

Can there be any other group of people who cling to a belief so tenaciously in spite of all the evidence against it as creationists?

I suppose, to be fair, there are climate change deniers, anti-vaxxers, moon landing conspiracy nut-jobs and other associated wack-a-doodle cults, but there can be few who do it with such fanaticism as creationists. There just doesn't appear to be any connection between their fanatically-held beliefs and what the evidence shows to be true. For example, a central dogma of creationsm is that mutaions are always harmful and therefore evolution due to natural selection from variations due to mutations is impossible. The claim is that the mutant simply wouldn't survive to be selected.

This belief is held with absolute conviction, despite the evidence that variation does arise by mutation, that this variation can be neutral or advantageous as well as deleterious. Here, for example, is a paper published in Nature Genetics by a team of researchers led by University of Wisconsin-Madison horticulture professor and geneticist Phil Simon, which shows how the evolution of the humble domestic carrot was hugely influenced by not one but two mutations.

The team has just succeeded in analysing the entire carrot genome.

We report a high-quality chromosome-scale assembly and analysis of the carrot (Daucus carota) genome, the first sequenced genome to include a comparative evolutionary analysis among members of the euasterid II clade. We characterized two new polyploidization events, both occurring after the divergence of carrot from members of the Asterales order, clarifying the evolutionary scenario before and after radiation of the two main asterid clades. Large- and small-scale lineage-specific duplications have contributed to the expansion of gene families, including those with roles in flowering time, defense response, flavor, and pigment accumulation. We identified a candidate gene, DCAR_032551, that conditions carotenoid accumulation (Y) in carrot taproot and is coexpressed with several isoprenoid biosynthetic genes. The primary mechanism regulating carotenoid accumulation in carrot taproot is not at the biosynthetic level. We hypothesize that DCAR_032551 regulates upstream photosystem development and functional processes, including photomorphogenesis and root de-etiolation.

The first cultivated carrots appeared in Central Asia about 1,100 years ago although there is evidence from the occurrence of wild carrot seeds in campsites from 3-5000 years ago that carrots were part of the European diet before they were domesticated. So far as carrot cultivation in Europe is concerned they first make their appearance, in Germany in the 1500s, as purple and yellow-rooted varieties. The familiar orange-rooted varieties did not appear until later. The wild carrot has white roots.

It is not clear why human agriculturalists selected for coloured-rooted varieties. It was probably not because of flavour because the genes for colour and flavour are not linked and are the same in the wild and domestic varieties. The assumption therefore is that it was either for novelty value of for purely aesthetic reasons that we ended up with coloured-rooted carrots and not white-rooted ones. It is also unlikely that the nutritional benefits of the carotenoids in carrots was appreciated at that time.

However, this is where is gets interesting from a human nutritional point of view as well as a genetic and evolutionary perspective. The accumulation of carotenoids in carrot roots is not a natural situation but is due to mutations in two genes, in both of which the mutations are recessive, so the carrot needs to be homozygous for two recessive genes to develop and accumulate the pigments which humans have selected for. Not only that but the pigments are not needed in the carrot root either for storage or for a physiological function. They are actually associated with photosynthesis so are normally present in the stems and leaves. In the roots, underground, they can play no part in photosynthesis.

But, they do not harm the carrot either so are not naturally deleterious. Instead, in the presence of human agriculturalists in the carrot's environment, these mutations were hugely beneficial to the other carrot genes, so there are now vastly more orange-rooted carrots in the world than there are white-rooted carrots.

The nutritional benefits to humans in the form of a good source of the precursors of vitamin A in our diet appears to be entirely fortuitous and due to these mutations some time in the Middle Ages, and to the aesthetic appreciation of our German forebears.

Now, all it needs is a creationist to explain why none of this was possible and how we don't actually have orange-rooted carrots because mutations are invariably deleterious and don't ever survive, so can't have played any part on carrot evolution, despite the evidence in their DNA that this is exactly what actually happened.

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