Mechanistic understanding of perianth traits hindering pollination in Aristolochia contorta Bunge

Abstract

Insects are vital pollinators for angiosperms, playing a crucial role in their reproductive success and fruit production. Aristolochia contorta is a perennial herbaceous vine that occurs in fragmented populations across East Asia. One notable feature of this plant is its trap flower, which employs a unique mechanism to attract, trap, retain, and release insects, ensuring effective pollination. The presence of this trap flower significantly influences the pollination system of A. contorta. Field surveys and pollination experiments were conducted to understand the processes and effectiveness of its pollination mechanism. It was allogamous and was pollinated by the species from Ceratopogonidae. During the insect attraction stage, 11.57% of the flowers contained insects, primarily Ceratopogonidae spp. Most Ceratopogonidae spp. concentrated in few flowers, indicating that although overall attraction might be modest, specific flowers acted as significant focal points for gathering. Trichomes effectively trapped Ceratopogonidae spp. inside flower tubes. In the retention stage, 26.16% of Ceratopogonidae spp. were loaded with pollen grains, but only 7.91% of those exited the flowers in the release stage. The sticky texture of the perianth's internal cavity posed challenges during this release, leading to adhesion and clogging of the narrow perianth tube. Consequently, a significant portion of Ceratopogonidae spp. became trapped on the perianth wall and perished. This highlights that despite the significant energy and resources invested in flower development, the perianth contributes to the low pollination effectiveness. This study revealed additive factors with negative effects on pollination, including the densely clustered distribution of its pollinators within only a few flowers, insufficient pollen loading onto pollinators, hindered release of entrapped pollinators due to the perianth adhesive surface, and a high rate of defective pollen grains in A. contorta. These factors account for the observed phenomenon of low fruit set (7.7%) and contribute to the diminished rate of sexual reproduction in A. contorta populations. This might lead the species to heavily rely on asexual reproduction, which could potentially lead to gene erosion within populations. The implications of these findings extend to the ecological and conservation aspects, emphasizing the need to understand and conserve the unique pollination system of A. contorta.

Keywords: Aristolochia contorta Bunge; Ceratopogonidae spp.; northern pipevine; perianth function; pollinator; sexual reproduction; trap flower.

Figure 1

Structural measurement of perianth of Aristolochia contorta.

Figure 2

Microscopic examination of floral structures and phases of anthesis in Aristolochia contorta. (A) a yellow limb bud, (B) a purple limb bud, (C) inflorescences by order, (D) a yellowish perianth, (E) a purplish perianth, (F) a flower and gynostemium at the female phase of anthesis, (G) trichomes in the tube at the female phase of anthesis, (H) a flower and gynostemium at the male phase of anthesis, (I) trichomes in the tube at the male phase of anthesis, (J) inner utricle (side), (K) inner utricle with glands, (L) glands (scanning electron microscopy), (M) glands (optical microscope). The scale bars of (A–K) = 1 mm, and the scale bars of (the inset I), (L, M) = 100 µm.

Figure 3

Diverse visitors trapped in Aristolochia contorta flowers. (A–C) Ceratopogonidae spp., (D) Cicadellidae sp., (E) Psocoptera sp., (F) Braconidae sp., (G) Scymnus sp. (H) Entomobrya vigintiseta, (I) Tetranychoidea sp., (J) Typhlocybinae sp., (K) Myzus persicae, (L) Frankliniella occidentalis, (M) Ponticulothrips diospyrosi. All the scale bars = 1 mm.

Figure 4

Percentage of the species of visitors to the Aristolochia contorta flowers while investigating the pollinators of the plant. E/: Percentage of visitors exited from flowers in a bottle.

Figure 5

Distributions (%) of visitors to Aristolochia contorta according to (A) the time of day, (B) different developmental stages, (C) height, (D) temperature. 1. Ceratopogonidae spp. 1, 2. Ceratopogonidae spp. 2, 3. Frankliniella occidentalis, 4. Ponticulothrips diospyrosi, 5. Chironomidae spp., 6. others. * p < 0.05, *** p < 0.001.

Figure 6

The percentage of fruit set in the three groups of Aristolochia contorta, including flowers that were enclosed in bags, buds that were cut, and the control.

Figure 7

Visual evidence of the distribution and transfer of pollen grains on the floral reproductive structures and the bodies of potential pollinators, (A), (B) Ceratopogonidae spp., (C) Ceratopogonidae sp. with pollen on their thoraxes, (D) upper side of gynostemium (stigma), (E) pollen grains and their tubes on the stigma (pollination), (F) seeds developing in the ovary (fertilization). (G) stamens on the side of an Aristolochia contorta gynostemium, (H) A. contorta pollen grains in the utricle. All the scale bars = 50 μm.

Figure 8

Pollen-loaded floral visitors and trapped visitors (A–D) Ceratopogonidae spp. with pollen, (E, F) Ceratopogonidae spp. trapped to death on the gynostemium, (G–J) Ceratopogonidae spp. trapped to death on the utricle, (K, L) Ceratopogonidae spp. trapped to death on the tube. All the scale bars = 1 mm.

Figure 9

The pollination mechanism of Aristolochia contorta. The numbers in the flowchart indicate the number of insects or flowers that answered Yes or No to the question.