The Primula S locus: gene function and the maintenance and breakdown of heterostyly

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Pollination is important, ~70% of our food results directly from pollination as seeds, grains, fruits and berries. It is important to plants too for reproduction, which not only produces the next generation but creates variation upon which natural selection can act. This variation enables plants to adapt to changing environments and colonize new habitats. Most plants produce hermaphrodite flowers, but plants cannot move or actively choose a partner, instead they have evolved intriguing strategies to prevent self-pollination and promote cross-pollination. One of the most remarkable of these strategies is heterostyly, which uses insect pollinators (hetero=different; style=female structure). Charles Darwin observed that Primroses have two forms of flower, pin with a long style and short stamens, and thrum with a short style and longs stamens. These reciprocal positions facilitate pollen transfer by insect visitors between each flower type. A group of genes, known collectively as the S locus, controls development of the two forms of flower. A rich history of scientific research on Primroses by early botanists and geneticist made heterostyly an important textbook example of a plant breeding system. Our study will add to this work by using the latest research tools to explore how heterostyly arose, how it is controlled and why it occasionally goes wrong.We sequenced the Primrose (Primula vulgaris) genome and identified a group of five genes that control development of its two flower types. These genes are found only in thrum and are absent in pin plants. Only those individuals that inherit this gene cluster can produce thrum flowers. Our studies show the gene which we believe is responsible for elongating stamens in thrum flowers arose ~51.7million years ago in an ancestor of all modern Primula species. We have also identified the gene responsible for controlling style length. The gene cluster contains a further three genes and in this study we will characterize their function to determine how they work together to control the floral architectures of pin and thrum flowers that facilitate insect-mediated pollination in the Primrose. Over the past 51.7 million years, 30 different species of Primula have lost the ability to produce two types of flower, they all have long styles and long stamens and never produce short styled flowers. These homostyles (Homo = same) do not use insect pollinators, but self-fertilize. Our studies suggest the gene that controls style length has been lost or damaged in these species.These homostyle species provide an exciting opportunity to explore how one gene has, over time, on multiple separate occasions, been lost or damaged during the evolution of the different Primula species. This is important because understanding how development has gone wrong can help explain how it normally works. We will use our Primrose genome sequence to identify and characterize the corresponding S locus gene clusters in these different Primula species to discover the gene mutation that has resulted in the loss of heterostyly. Most plant and animal genes are present in two copies, one set from the mother, one from the father; this is useful, if one copy is damaged, the second copy acts as backup so function is retained, it also enables repair of the damaged copy using the other as a template. This arrangement provides genetic stability from generation to generation. One very surprising finding from our studies of genes controlling heterostyly in Primula is that they are present only as a single copy, there is no back up copy. It is therefore surprising that heterostyly is stable as any genetic damage to the key genes cannot be repaired. We do not yet have an explanation for this conundrum but our project will initiate studies to seek an explanation.