PREMISE The importance of hybridization to invasion has been frequently discussed, with most studies focusing on the comparison of fitness-related traits between F1 hybrids and their parents and the consequences of such fitness differences. However, relatively little attention has been given to late-generation hybrids. Different fitness landscapes could emerge in later generations after hybrids cross with each other or backcross with their parents, which may play an important role in plant invasion and subsequent speciation. METHODS In this study, artificial crosses were conducted to generate multiple generations, including F1, F2, and backcrosses between two invasive species: Cakile edentula (self-compatible) and C. maritima (self-incompatible). Putative hybrids were also collected in the sympatric zone and compared with their co-occurring parents for phenotypic and genetic differences. RESULTS Genetic data provided evidence of hybridization happening in the wild, and phenotypic comparisons showed that natural hybrids had intermediate traits between the two species but showed more similarity to C. maritima than to C. edentula. The asymmetry was further identified in artificial generations for several phenotypic characters. Furthermore, backcrosses exhibited different patterns of variation, with backcrosses to C. maritima having higher reproductive output than their counterparts. CONCLUSIONS Our results suggest that hybridization and introgression (backcrossing) in Cakile species is asymmetric and most likely to favor the proliferation of C. maritima genes in the mixed population and thus help its establishment, a finding that could not be predicted by characterizing F1 hybrids.

中文翻译：

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两个入侵物种之间后代杂交体的适应性相关性状的不对称性

前提 杂交对入侵的重要性经常被讨论，大多数研究都集中在 F1 杂交种与其亲本之间的适应性相关性状的比较以及这种适应性差异的后果上。然而，对晚代杂种的关注相对较少。杂种在与亲本杂交或回交后的后代中可能出现不同的适应性景观，这可能在植物入侵和随后的物种形成中起重要作用。方法 在本研究中，进行了人工杂交以产生多代，包括 F1、F2 和两个入侵物种之间的回交：Cakile edentula（自交）和 C. maritima（自交）。假定的杂种也被收集在同域区，并与它们共同出现的亲本进行表型和遗传差异的比较。结果 遗传数据提供了在野外发生杂交的证据，表型比较表明，天然杂交种具有两个物种之间的中间性状，但与 C. maritima 的相似性高于与 C. edentula 的相似性。在人工世代中进一步确定了几种表型特征的不对称性。此外，回交表现出不同的变异模式，与 C. maritima 的回交比其对应物具有更高的繁殖产量。结论我们的结果表明，Cakile 物种的杂交和基因渗入（回交）是不对称的，最有可能有利于 C.