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

授粉很重要，我们70%的食物直接来自于授粉，如种子、谷物、水果和浆果。这对植物的繁殖也很重要，因为繁殖不仅能产生下一代，还能产生自然选择作用的变异。这种变异使植物能够适应不断变化的环境，并在新的栖息地定居。大多数植物都能开出雌雄同体的花，但植物不能移动或主动选择伴侣，相反，它们进化出了有趣的策略来防止自花授粉和促进异花授粉。这些策略中最显著的一种是异质传粉，它利用昆虫传粉媒介（异质=不同；花柱=雌性结构）。查尔斯·达尔文观察到报春花有两种形式的花，花柱长而雄蕊短，花柱短而雄蕊长。这些相互的位置有助于昆虫在每种花类型之间传递花粉。一组基因，统称为S基因座，控制着这两种花的发育。早期植物学家和遗传学家对报春花进行了丰富的科学研究，使异花柱成为植物育种系统的重要教科书范例。我们的研究将通过使用最新的研究工具来探索异质性是如何产生的，它是如何被控制的，以及为什么它偶尔会出错。我们对报春花（Primula vulgaris）基因组进行了测序，并鉴定出一组控制其两种花类型发育的5个基因。这些基因只在线秆中发现，而在针叶植物中不存在。只有那些遗传了这个基因簇的个体才能产生线头花。我们的研究表明，我们认为负责线头花雄蕊延长的基因在5170万年前出现在所有现代报春花物种的祖先身上。我们还确定了控制花柱长度的基因。该基因簇包含另外三个基因，在本研究中，我们将描述它们的功能，以确定它们如何共同控制针状花和线状花的花结构，从而促进报春花中昆虫介导的授粉。在过去的5170万年里，30种报春花失去了两种花的能力，它们都有长花柱和长雄蕊，从不开出短花柱的花。这些同源植物（Homo = same）不使用昆虫传粉者，而是自我受精。我们的研究表明，控制花柱长度的基因在这些物种中已经丢失或受损。这些同源种提供了一个令人兴奋的机会来探索一个基因是如何随着时间的推移，在多个不同的场合，在不同报春花物种的进化过程中丢失或损坏的。这很重要，因为了解发展是如何出错的，可以帮助解释它通常是如何工作的。我们将使用我们的报春花基因组序列来鉴定和表征这些不同报春花物种中相应的S位点基因簇，以发现导致杂种优势丧失的基因突变。大多数植物和动物的基因都有两个拷贝，一组来自母亲，一组来自父亲；这是有用的，如果一个副本损坏，第二个副本作为备份功能保留，它也使修复损坏的副本使用另一个作为模板。这种排列提供了遗传代代相传的稳定性。在我们对报春花中控制异型的基因的研究中，有一个非常令人惊讶的发现，那就是它们只以一个拷贝的形式存在，没有备份拷贝。因此，当关键基因的任何遗传损伤都无法修复时，异种花柱是稳定的，这是令人惊讶的。对于这个难题，我们还没有一个解释，但我们的项目将启动研究来寻求一个解释。

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.