Although the evolution of the selfing syndrome often involves reductions in floral size, pollen, and nectar, few studies of selfing syndrome divergence have examined nectar. We investigate whether nectar traits have evolved independently of other floral size traits in the selfing syndrome, whether nectar traits diverged due to drift or selection, and the extent to which quantitative trait locus (QTL) analyses predict genetic correlations.

We use F5 recombinant inbred lines (RILs) generated from a cross between Ipomoea cordatotriloba and I. lacunosa. We calculate genetic correlations to identify evolutionary modules, test whether trait divergence was due to selection, identify QTLs, and perform correlation analyses to evaluate how well QTL properties reflect genetic correlations.

Nectar and floral size traits form separate evolutionary modules. Selection has acted to reduce nectar traits in the selfing I. lacunosa. Genetic correlations predicted from QTL properties are consistent with observed genetic correlations.

Changes in floral traits associated with the selfing syndrome reflect independent evolution of at least two evolutionary modules: nectar and floral size traits. We also demonstrate directional selection on nectar traits, which is likely independent of selection on floral size traits. Our study also supports the expected mechanistic link between QTL properties and genetic correlations.

We crossed Ipomoea cordatotriloba by Ipomoea lacunosa and generated a F2 mapping population; F2 individuals were selfed by single seed decent to the F5 generation. Nectaries were collected while phenotyping for nectar volume and nectar sugar concentration and stored in RNA later and in the -20 freezer. They were subsequently imaged using a Leica MZFLIII stereomicroscope set at 1X magnification and connected to a 4D HD digital microscope camera from Microscope Systems, Inc. (https://microscope-systems.com/).