26 September, 2022

Consequences of pollinator availability and effectiveness for pollen transfer in a gynodioecious seed crop system

 by R. Waytes, R. Cartar, S. Hoover

A leafcutter bee on canola

Pollen limitation can constrain the number of seeds that a plant sets, and therefore its reproductive success. For plants that depend on animal pollinators, the availability of pollen is determined in part by the availability of pollinators and their ability to effectively deposit pollen (i.e., their efficacy).

We studied pollinator visitation and efficacy in hybrid seed canola (Brassica napus) production fields in Alberta, Canada. These fields consisted of 1 m wide bays (lines) of ‘male’ flowers (hermaphrodites which act as pollen-donors) alternating with 6 m wide bays (lines) of ‘female’ flowers (hermaphrodites with induced male sterility). Honey bees (Apis mellifera) and alfalfa leafcutting bees (‘leafcutter bees’; Megachile rotundata), both introduced European species, are typically placed in the crop for pollination. Native pollinators, including bees and flies, may also contribute. We used a GoPro® camera targeted at an inflorescence of female flowers placed at 1 m distance from the observer to offer flowers to insects visiting the canola and record their responses. We examined how insects responded to female flowers and what behaviours affected pollen deposition. Additionally, we measured pollinator visitation to both male and female bays.

Most flower visitation in this system was by managed pollinators (honey bees and leafcutter bees), who were placed and maintained in the canola fields. Flies and native bees were present, but at much lower numbers. Honey bees and leafcutter bees tended to individually specialize on male or female flowers, and their unwillingness to move between bays represents a potential barrier to pollen transfer.

Female leafcutter bees deposited more pollen than honey bees or flies, although male leafcutter bees and honey bees deposited similar amounts. Bumble bees were similar to female leafcutter bees in pollen deposition, but their low presence in the canola fields implies low contributions to overall pollination. Pollen deposition increased with the amount of time a pollinator spent on a flower.

So what mattered in pollen deposition? Far and away, it was the sex of flower from which the pollinator moved: moving from a male flower transferred more pollen. Also important were pollinator identity (female leafcutter bees were the best) and time on flower (more time meant more pollen deposited). Pollen receipt is a confluence of three factors: from where, who, and how long.

21 September, 2022

Pollination ecology and breeding system of the tropical tree Guaiacum sanctum on two Caribbean islands with contrasting pollinator assemblages

 by José J. Fumero-Cabán, Elvia J. Meléndez-Ackerman and Julissa Rojas-Sandoval

Flowers, details of reproductive structures,
and visitors of Guaiacum sanctum in the Caribbean.

Islands often have plants with reproductive mechanisms allowing for self-compatibility and low species-rich communities of pollinators. Islands are also areas more prone to extinction and vulnerable to invasive species than mainland ones. In this study, we document different reproductive traits of the tropical tree Guaiacum sanctum on two insular populations in the Caribbean with contrasting pollinator assemblages: Guánica on the main island of Puerto Rico where the alien honeybees (Apis mellifera) were first reported in 1994 and Mona Island where honeybees do not occur. On these two islands, we performed a series of field observations and pollination experiments over a period of three years, to assess pollinator species richness, visitation rates, breeding system, and the fitness of selfed- vs. crossed-progenies. We found that flowers are pollinated by insects on both islands, but while the richness of pollinators was higher on Mona, the visitation rates were considerably higher in Guánica where trees are almost exclusively visited by the introduced Apis mellifera. Pollinations experiments show that outcrossing treatment yielded nearly twice the number of fruits and seeds than selfing treatment and these differences are consistent between populations, which might reflect early acting inbreeding depression, partial self-incompatibility, or differences in resource allocation between selfed and outcrossed fruits. Overall, our results suggest that the substantial reduction in pollinator visitors in areas dominated by alien honeybees may add an additional level of vulnerability to these threatened populations.

Read the scientific publication in JPE!