Replaying the clock in hybrid evolution: A field experimental evolution study using sunflowers (Helianthus)

Background/Question/Methods

Hybridization is hypothesized to fuel adaptation, but experimental tests are lacking.  Hybridization between common sunflower (Helianthus annuus annuus) and a southern congener (H. debilis) is believed to have permitted the southward range expansion of H. annuus into southern Texas (as the stabilized hybrid H. annuus texanus).  Previous experiments using these three taxa identified a suite of ecophysiological and herbivore resistance traits that may have been key to this adaptive introgression.  Here, we have expanded upon that work by exposing experimental hybrid sunflower lineages to seven generations of natural selection in the field and then growing them in common gardens with non-hybrid experimental controls and natural accessions of the three taxa.  We formulated predictions regarding how traits would evolve in the hybrids based on trait means and selection differentials for H. annuus texanus and both parental species from an earlier common garden experiment.  We ask whether traits have changed as predicted following seven generations of natural selection in the experimental hybrids and whether those trait values are converging on H. annuus texanus.

Results/Conclusions

Preliminary analyses indicate that 1) our experimental hybrid lineages have become significantly more fit over seven generations of natural selection and 2)  faster achene development and increased resistance to at least two insect herbivores (the midge Neolasioptera helianthus and a seed-feeding weevil)  may have contributed to that increase.  Although resistance to midge damage was previously identified as an important trait for southward range expansion in H. annuus, few of the other traits identified in our earlier work appear to have changed as predicted over time.  Predicting evolutionary changes in trait values may be challenging due to annual variation in selective pressures and possible epistatic interactions among traits.  Further analyses will assess the extent to which experimental hybrids are converging on natural populations of H. annuus texanus, perhaps clarifying these early patterns.