Plant invasions are prime opportunities for studying hybridization and the nature of species boundaries, but hybrids also complicate the taxonomic treatment and management of introduced taxa. In this study, we use population genomics to estimate the extent of genomic admixture and test for its association with morphology and genome size in a hybrid complex of knapweeds invasive to North America: meadow knapweed (Centaurea × moncktonii) and its parental species (C. jacea and C. nigra). We sampled 20 populations from New York and Vermont, USA, and used genotyping by sequencing to identify single nucleotide polymorphisms in order to estimate genome-wide ancestry and classify individuals into hybrid genotype classes. We then tested for association between degree of genomic introgression and variation in a subset of traits diagnostic for the parental taxa, namely capitula morphology and monoploid genome size. Genomic clustering revealed two clearly defined lineages, as well as many admixed individuals forming a continuous gradation of introgression. Individual assignments to hybrid genotype classes revealed many advanced generation intercrosses and backcrosses, suggesting introgression has been extensive and unimpeded by strong reproductive barriers between taxa. Variation in capitula traits between the two unadmixed, presumed parental, lineages exhibited continuous, and in some cases transgressive, segregation among introgressed hybrids. Genome size was also divergent between lineages, although advanced generation hybrids had smaller genomes relative to additive expectations. Our study demonstrates deep introgression between the porous genomes of a hybrid invasive species complex. In addition to strong associations among genomic ancestry, genome size and morphology, hybrids expressed more extreme phenotypic values for capitula traits and genome size, indicating transgressive segregation, as well as a bias towards smaller genomes, possibly due to genomic downsizing. Future studies will apply these results to experimentally test how introgression, transgressive segregation and genome size reduction interact to confer invasiveness.

中文翻译：

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北美东部入侵草甸矢车菊杂交复合体（矢车菊 × moncktonii）的基因组祖先、基因组大小和头状花序形态之间的关联。

植物入侵是研究杂交和物种边界性质的主要机会，但杂交也使引入类群的分类处理和管理复杂化。在这项研究中，我们利用群体基因组学来估计基因组混合的程度，并测试其与入侵北美的矢车菊杂种复合体的形态和基因组大小的关系：草甸矢车菊（Centaurea × moncktonii）及其亲本物种（C. jacea 和 C. nigra）。我们对来自美国纽约和佛蒙特州的 20 个人群进行了采样，并通过测序进行基因分型来鉴定单核苷酸多态性，以估计全基因组祖先并将个体分类为混合基因型类别。然后，我们测试了基因组渗入程度与亲本类群诊断性状子集（即头状花序形态和单倍体基因组大小）变异之间的关联。基因组聚类揭示了两个明确定义的谱系，以及形成连续渐渗分级的许多混合个体。杂交基因型类别的个体分配揭示了许多先进世代的杂交和回交，表明基因渗入是广泛的，并且不受类群之间强大的生殖障碍的阻碍。两个未混合的假定亲本谱系之间头状花序性状的变异表现出在渐渗杂种之间连续的、并且在某些情况下是越界的分离。尽管高级杂交后代的基因组相对于附加预期较小，但不同谱系之间的基因组大小也存在差异。我们的研究证明了混合入侵物种复合体的多孔基因组之间的深度渗入。除了基因组祖先、基因组大小和形态之间的强相关性之外，杂种在头状花序性状和基因组大小方面表达了更极端的表型值，表明越界分离，以及对较小基因组的偏向，可能是由于基因组缩小。未来的研究将应用这些结果来通过实验测试基因渗入、侵入分离和基因组大小减小如何相互作用以赋予侵入性。