Variation in floral morphology and plant reproductive success in four Ipomoea species (Convolvulaceae) with contrasting breeding systems

Despite the benefits of outcrossing, the transition from outcrossing to self-pollination is common in angiosperms. This study tested the hypothesis that self-compatibility would be associated with floral traits that facilitated autonomous self-pollination to ensure reproduction under low pollinator visitation. In a comparison of two pairs of Ipomoea species with contrasting breeding systems, we expected that self-compatible (SC) species should have smaller, less variable flowers, reduced herkogamy, lower pollinator visitation, and greater reproductive success than their self-incompatible (SI) congeners. We studied closely related sympatric species pairs, I. hederacea (SC) - I. mitchellae (SI), and I. purpurea (SC) - SI I.indica (SI), in Veracruz, Mexico over two consecutive years. We quantified variation in floral traits -including size, herkogamy and nectar production-, documented pollinators in the field, and determined natural fruit and seed set. Hand-pollination and bagging experiments were conducted to determine potential for autonomous self-pollination and apomixis. Self-compatible Ipomoea species had smaller flowers and lower nectar production than SI species; however, floral variation and integration did not vary according to breeding system. All species were primarily pollinated by bees, but visitation rates were seven times lower in the SC than the SI species. The two SC species had a high capacity for autonomous self-pollination due to reduced herkogamy at the highest anther levels. Self-compatible species had two to six times higher fruit set than SI species. Results generally support the hypothesis that self-compatibility and autonomous self-pollination ensure reproduction under low pollinator visitation. However, the high variation in morphological traits of SC Ipomoea species suggests that these species still maintain variation through outcrossing. Our results also indicate that reduced herkogamy is associated with a high potential for autonomous self-pollination, providing a reproductive advantage that possibly underlies transitions to self-compatibility in the genus Ipomoea. This article is protected by copyright. All rights reserved.