Asymmetry in fitness-related traits of later-generation hybrids between two invasive species

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 its 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 Cakile C. maritima (self-incompatible). Putative hybrids were also collected from 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 study results suggests 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. ASSOCIATED FILES: Genetic and phenotypic data from the first expt is listed in the file "all data file_first expt.xlsx" while data from the second expt is listed in the "ALL DATA SHEET_Second expt.csv".