Testing potential selective agents acting on leaf shape in Ipomoea hederacea: predictions based on an adaptive leaf shape cline

Leaf shape is a highly variable phenotype, and is likely influenced by many sources of selection. Ipomoea hederacea exhibits an adaptive latitudinal cline in leaf shape, which is controlled by a single Mendelian locus: lobed individuals dominate the north with entire-shaped individuals mostly in the south. We test if the following candidate selective agents, suggested by the literature, are responsible for the cline: differential insect herbivory, genetic correlations with other clinal traits like flowering time and growth rate, and thermoregulatory differences. We planted 1680 F3 individuals, segregating for leaf shape, in the north of I. hederacea’s range, where we expected lobed genotypes to have higher fitness. Individuals were assigned to insect removal or control treatments, and we scored herbivory, flowering time, growth rate, leaf temperature, and fitness (seed number). Herbivory, flowering, and growth rate had significant fitness effects, but none differed between leaf shapes. Lobed leaves were consistently warmer at night, but no performance advantage was detected. Finally, we detected no overall fitness differences between leaf shape genotypes, whether we controlled for other traits under selection or not. Our data suggest these candidate selective agents may not be important contributors to the cline, and alternative approaches to understanding the mechanisms maintaining the leaf shape cline in I. hederacea may be necessary.