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

Premise: Most studies on the importance of hybridisation to invasions have focused on the comparison of fitness-related traits between F1 hybrids and their parents whilst different fitness landscapes could emerge in later generations after hybrids cross with each other (i.e. F2) or backcross with their parents.

Methods: In this study, artificial crosses were conducted to generate F1, F2 and backcrosses between two invasive species: Cakile edentula (self-compatible) and Cakile maritima (self-incompatible). Putative hybrids were also collected from the sympatric zone and compared with their co-occurring parents for phenotypic and genetic differences.

Results: Genetic data provided evidence of hybridisation 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 pattern of variation, with backcrosses to C. maritima having high reproductive output than their counterparts.

Conclusions: Our study suggests that hybridisation and introgression (backcrossing) in Cakile species is asymmetric and most likely to favour 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.