Why so mixed up? Exploring the history and adaptive significance of pervasive large chromosomal inversions in sunflowers.

点击翻译按钮获取中文摘要

Across the planet we find species adapted to a wide range of habitats. This is true for species as well as individual populations that are locally adapted to their home environment. How adaptation is achieved and maintained is one of evolutionary biology's fundamental questions. Research has shown that for populations to be locally adapted - i.e., do better in their home environment - it requires the combined action of multiple mutations. This can be challenging when there is ongoing migration between environments bringing in non-adaptive alleles and breaking up combinations of adaptive alleles through recombination. Thus, adaptation depends not just on what alleles are present, but how they are packaged together in the genome. Chromosomal inversions occur when stretches of DNA invert. As a by-product, this prevents recombination between inverted and uninverted segments and can effectively lock together alleles. Theoretical work has predicted that inversions can be used to facilitate adaptation when there is migration from different environments, but this hasn't been empirically tested because of a lack of ecological knowledge or known inversions in studied taxa. Recently, we discovered that sunflowers (genus Helianthus) were polymorphic for 37 large inversions and that these inversions were associated with locally adaptive traits (Todesco, Owens et al. Nature, 2020). This provides an unprecedented opportunity to expand our understanding of why inversions appear and what they are doing now. One surprising fact is that sunflower inversions are very old, often older than the species they are found in. One possibility is that they were transferred in from other species through hybridization. We will test this by sequencing the genomes of potential donor species and comparing them to known inversion sequences. This raises the question, are these ancient inversions also found in other species? To answer this, we will develop a new method to detect ancient inversions by comparing reference genome sequences. Shifting sand dunes are a harsh landscape for any plant, but in sunflowers, some populations have adapted and thrive in this treacherous environment. We found that this adaptation was achieved through the use of seven ancient inversions. Were these inversions always important in sand dune adaptation or were they later co-opted for this function? We will answer this question using quantitative genetics to trace when the adaptive functions originated. Lastly, genome sequencing of other sand dune adapted species will find out if the same inversions are reused across the genus. Modern genome sequencing technology is increasingly revealing genome structure variation across the tree of life and the work proposed here will develop sunflowers as an ecological and genomic model for understanding the evolutionary role of inversions more broadly.