Enhancing pollen competition by delaying stigma receptivity: pollen deposition schedules affect siring ability, paternal diversity, and seed production in Collinsia heterophylla (Plantaginaceae).

PREMISE OF THE STUDY: Even though pollen deposition schedules may have profound effects on the evolutionary outcome of pollen competition, few studies have investigated such effects in relation to pistil traits such as delayed stigma receptivity that enhance pollen competition. In Collinsia heterophylla, a largely outcrossing species with delayed stigma receptivity, we performed a series of controlled crosses involving several donors to understand how timing of pollen deposition influences siring ability, paternal diversity, and offspring fitness.
METHODS: Pollen was applied to fully receptive stigmas either as mixtures or consecutively with or without a time lag to mimic cases with early or delayed stigma receptivity. We used a genetic marker to assess offspring paternity. KEY
RESULTS: As expected, siring ability was affected by application order in crosses without a time lag, providing a first-donor advantage for pollen arriving on unreceptive stigmas. However, because pollen donor identity influenced siring ability, delaying stigma receptivity may still favor pollen of high competitive ability. In crosses on fully receptive pistils with a time lag of 24 h, a surprisingly high proportion of seeds (12-47%) were sired by pollen applied last. A novel finding was that pollen applied only once (as a mixture), mimicking delayed stigma receptivity, led to higher paternal diversity within progeny families, which was associated with increased seed production.
CONCLUSIONS: Our results suggest fitness advantages of enhancing pollen competition by delaying stigma receptivity in C. heterophylla, particularly in relation to increased paternal diversity.