Life history traits and phenotypic selection among Helianthus annuus (sunflower) crop-wild hybrids (BCw, F1, and F2) and their wild counterparts: Implications for introgression

Background/Question/Methods

Gene flow and introgression of cultivated alleles into crop wild relative (CWR) populations can reduce the genetic diversity present in CWRs through the processes of demographic swamping and genetic assimilation.  Sunflower is an ideal system to study crop toward wild gene flow and subsequent introgression.  Abundant research has demonstrated that gene flow and introgression between cultivated and wild sunflower occurs.  However, these studies do not provide insight into the process itself.  Differences between sunflower hybrids and their wild counterparts for life history traits and fitness components ultimately determine how introgression of crop alleles proceeds within wild populations.  Multiple studies suggest that life history trait characteristics of F₁ crop-wild hybrids do not provide a complete barrier to the introgression of sunflower crop alleles into wild populations.  Since the process of introgression does not occur until the F₂ or BC_w generations, life history trait and fitness studies including these crosstypes are warranted.  Here we quantify life history trait differences between Helianthus annuus (sunflower) crop-wild hybrid crosstypes (BC_w, F₁, and F₂) and their wild counterparts in an experimental hybrid zone and perform phenotypic selection analysis to determine the likelihood of crop allele introgression into wild sunflower populations. 

Results/Conclusions

We show that all four sunflower crosstypes (BC_w, F₁, and F₂) overwintered, emerged in the spring, and survived until reproduction indicating these life-history stages are not a strong barrier to crop alleles introgressing into wild sunflower populations.  Phenotypic selection favored increased early season plant size and earlier emergence (crop-like traits) due to direct selection on increased early season leaf size.  Crosstype specific selection analysis indicated the intensity of direct selection on early season leaf size a crosstype experienced related to overall selection on other traits.  Furthermore, crosstype selection patterns indicate introgression of cultivated alleles underlying increased early season plant size and early emergence can occur through both the BC_w and F₂ routes.  We conclude that: (1) cultivated alleles underlying increased early season plant size and earlier emergence are likely to introgress into wild sunflower populations following sunflower crop-wild hybridization, which could have consequences on wild sunflower genetic diversity; and (2) other crop-like traits conferring a fitness advantage in wild environments likely maintain the potential to introgress following sunflower crop-wild hybridization.