| Gesneriaceae is a dominant herb in tropical and subtropical regions of Asia and China,and the species specialization caused by habitats has a very high endemism.In the long-term history of evolution,Gesneraceae produced richly varied pollination mechanisms and floral traits.Mirror-image flowers,a highly specialized pollination system was repeatedly occurred in Gesneriaceae,with various types.Firstly,we focused on type and systematic distribution of mirror-image flowers in Gesneriaceae,and then we used method of pollination biology to explore the pollination mechanisms of two types of mirror-image flowers(reciprocal and nonreciprocal)to determine the evolutionary history and specialized mechanism of reciprocal and nonreciprocal mirror-image flowers in Gesneriaceae.(1)Mirror-image flowers found in seven genera in the Gesneriaceae,among which 25 species have reciprocal mirror-image flowers(Rhabdothamnopsis,Hencklia,Didymocarpoideae,Streptocarplla)and other 53 species possess nonreciprocal mirrorimage flowers(Paraboea,Ornithoboea,Saintpaulia).According to the number of species,Gesneriaceae was dominated by nonreciprocal mirror-image flowers with united anthers and obviously floral tube,and at least five independent origins of mirror-image flowers were found.(2)We studied a special form of reciprocal mirror-image flowers,the styly in Rhabdothamnopsis sinensis remains straight throughout the booming period,while the basal part of the floral tube bends either to left or right side,causing the styly leans at the right or left side of the floral tube.Flowering phenology found that the style continued to elongate after flower opening and last 5–6d.Pollination observations found that the bended floral tube and twisted filament can prevent insects entering deeply into the floral tube and select for long-tongued bumblebees as pollinators and transfer pollen with thorax sides.The outcrossing index of R.sinensis was 4,indicating that its breeding system could be interpreted as primarily outcrossed.R.sinensis was self-compatible with homogamy and Pollen-Ovule ratio was strikingly low(37.41 ± 11.90),suggesting a very high level of pollen transfer efficiency.(3)Ornithoboea is a typical nonreciprocal mirror-image flowers,and special barbs in the corolla is the main traits that distinguishes it from other genera in Gesneriaceae.Our studies found that a large amount of pollen was deposition on the barbs of O.wildeana in the day 1 flowers,we speculate that the function of the barbs is secondary pollen presentation.Flowering phenology found that the style continued to elongates form day 2 and last 4–5d.Allobaccha apicalis was the most frequent visitor and legitimate pollinator,and foraged with pollen on the barbs of O.wildeana to the stigma by the abdomen and mouthparts.The resulting P/O ratio per flower was 258.13 ± 94.10,and the plants were found to be selfcompatible.Our result indicated that visiting frequencies of intact flowers was significantly higher than for flowers with barbs cut off.(4)In summary,reciprocal mirror-image flowers of R.sinensis with more accurate method of pollen transfer,greatly enhancing pollen transfer efficiency.However,the nonreciprocal mirror-image flowers of O.wildeana indicated little interference between male and female functions than reciprocal mirror-image flowers.Inadequate pollination and nonreciprocal mirror-image flowers resulted in lower pollination accuracy of O.wildeana,which promotes reproductive success through the secondary pollen presentation to improve male function(increase pollen removal and hermaphrodite flowers).In addition,we found that the larger herkogamy of nonreciprocal mirror-image flowers is more likely to cause pollination isolation,leading to the rapid differentiation of nonreciprocal mirror-image flowers in Gesneriaceae. |
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