Biological journal of the Linnean Society (1990), 40: 229-243
Cryptic dioecy and insect pollination in Rosa setigera Michx. (Rosaceae), a rare plant of Carolinian Canada

Peter G. Kevan, Department of Environmental Biology, University of Guelph, Guelph, Ontario N1G 2W1, Canada
D. Eisikowitch, Botany Department, Tel Aviv University, Tel Aviv, Israel
John D. Ambrose, The Arboretum, University of Guelph, Guelph, Ontario N1G 2W1, Canada
James R. Kemp, Department of Botany, University of Guelph, Guelph, Ontario N1G 2W1, Canada

Rosa setigera is unique among known roses because it is truly dioecious, yet the plants and flowers are almost impossible to sex. Subtle differences in the inflorescences have been detected, for example, there are more flowers per inflorescence on male plants than on female plants and petal expansion from the day of opening to the next day is greater in females than in males; in addition, pollen from female plants appears somewhat collapsed and does not germinate. Pollinators (Apis mellifera, other bees (Apidae), Eristalis tenax and other Syrphidae: Diptera) visit the blossoms mostly in the morning to collect pollen (there is no nectar) and in choice experiments do not discriminate between male flowers and female flowers on landing, but do spend significantly more time on the former. They do discriminate between freshly opened flowers and day-old flowers, and show marked preference for bouquets of five flowers vs. single flowers. The female plants, with smaller inflorescences and lesser interest to pollen foraging insects, seem to encourage them to forage at more flowers than do their male counterparts. This may be biologically significant for effective pollen flow in a dioecious pollen-only plant with pollenivorous pollinators.

Cole (1917), "R. setigera shows a large percentage of microsporic degeneracy."

Shepherd (1954), "As much as 80 per cent of the pollen of the seed-bearing plants may be sterile".