I remember as a child being fascinated by the two different forms of primroses, which, unless you're aware of it, you probably wouldn't notice unless you looked especially closely. What I didn't realise until much later was that this had evolved to overcome a problem potentially faced by all hermaphrodite species and especially plants - the problem of losing the benefits of sexual reproduction by self-fertilisation.
Such is the advantage of mixing genes with those of another individual and so being able to 'try out' new combinations, or to acquire more advantageous alleles and pass these on to your offspring, that there is evolutionary pressure to ensure it happens frequently.
The strategy the primrose, Primula vulgaris, and other members of the primula family, have adopted also depends on the relationship between the plant and pollinating insects, as Darwin suggested in one of the key pieces of evidence of how natural selection works and how it creates these examples of mutually beneficial alliances.
The primrose has evolved two forms of the flower. The difference is in the relative positions of the stamens and the stigma. The so-called pin-eye form has the stigma above the stamens, and looking like the head of a pin sitting in the middle of the ring of petals. The stamens are low down fixed to the base of the corolla tube. The thrum-eyed form has the stigma low down in the flower and the stamens form a ring around the top of the corolla tube.
So, when a pollinating insect visits the flower, pollen is deposited on two different locations which correspond to the positions of the stigma in the other form of the flower. The strategy depends entirely on the fact that pollen is transferred from flower to flower by insects.
Now scientists at the University of East Anglia, working at the John Innes Centre, have identified exactly which part of these species’ genetic code made them that way, through an event that occurred more than 51 million years ago. The condition is termed 'heterosyly' and is controlled by a supergene, the S locus. In pinpointing this gene the team also discovered that not only can the mutation which gave rise to it occurred 51.7 million years ago but that it arose in a duplicated gene; a duplication of the gene responsible for the identity of petals in a Primula flower.
Darwin's studies on heterostyly in Primula described two floral morphs, pin and thrum, with reciprocal anther and stigma heights that promote insect-mediated cross-pollination. This key innovation evolved independently in several angiosperm families. Subsequent studies on heterostyly in Primula contributed to the foundation of modern genetic theory and the neo-Darwinian synthesis. The established genetic model for Primula heterostyly involves a diallelic S locus comprising several genes, with rare recombination events that result in self-fertile homostyle flowers with anthers and stigma at the same height. Here we reveal the S locus supergene as a tightly linked cluster of thrum-specific genes that are absent in pins. We show that thrums are hemizygous not heterozygous for the S locus, which suggests that homostyles do not arise by recombination between S locus haplotypes as previously proposed. Duplication of a floral homeotic gene 51.7 million years (Myr) ago, followed by its neofunctionalization, created the current S locus assemblage which led to floral heteromorphy in Primula. Our findings provide new insights into the structure, function and evolution of this archetypal supergene.
Jinhong Li, Jonathan M. Cocker, Jonathan Wright, Margaret A. Webster, Mark McMullan, Sarah Dyer, David Swarbreck, Mario Caccamo, Cock van Oosterhout & Philip M. Gilmartin
Genetic architecture and evolution of the S locus supergene in Primula vulgaris
Nature Plants 2, Article number: 16188 (2016) doi:10.1038/nplants.2016.188
Copyright © 2016 Nature Publishing Group. Reprinted with kind permission under license #4001270926815
What we have here then is a beautiful example of how evolutionary alliances work to produce solutions to problems, how the structure of an organ such as a flower can be controlled from a single locus and of how, despite creationists' almost hysterical insistence otherwise, gene duplication and mutation can produce entirely new information without any deleterious effects. It also shows how the environment produces meaning from that information because this mutation would have been entirely meaningless in the absence of pollinating insects. Of course, the fact that this happened 51.7 million years ago is also a problem for creationists but they have been living with the abundant evidence of an old Earth in a very old Universe for so long now that just a little more evidence is barely going to be noticeable.
So any creationist willing to explain why this isn't an example of co-evolution between the Primula genus and it's pollinating insects and why it isn't an example of new information being produced by gene duplication and subsequent mutation?
Answers below, please... Or not...
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