Evolutionary Breakdown Of The Specialized Pollination System In Senna Surattensis

Evolutionary breakdown of specialized pollination system is the adaptive transitions of complex floral syndromes and pollination mechanism in the changing environments characterized with few co-evolved partners, e.g. the plant was introduced to a place without suitable pollinator. In this paper, we studied the mirror-image flowers (enantiostyly) with heteranthery (heteromorphic stamens) in Senna surattensis Burm.(Fabaceae) in China to explore the possible evolutionary breakdowns of these highly-specialized pollination system and their effects on the mating systems.Such information is important to facilitate reproduction and new variants breeding of this valuable trees and can be useful in determining its invasive possibility.(1) S. surattensis can produce both left-and right-styled flowers on the same individual (monomorphic enantiostyly, one less-specialized type of enantiostyly). The spatial separation of "pollinating" and "feeding" stamens is obvious,～0.52±0.01cm, while there is no difference in pollen viability between the pollinating and feeding anthers. Stigma, pollinating stamens, feeding stamens have been divided into three groups according to spatial positions, which formed a isosceles triangle—"reproductive triangle" with herkogamy (distance between stigma and pollinating anther) as the base. Such spatial arrangement combines the pollination mechanisms of mirror-image flowers and heteranthery and may facilitate high level of outcrossing.(2)The evolutionary breakdown occursin S. surattensis’m three main variants, i.e. non-heteranthery (the stamens are of similar size and position), selfing (the style curved upward and the stigma close to the feeding anthers), central style (the style located at the central and lower position of the flower and did not reflect to the side of the flower). The selfing variant was the most common variant in the three cities and two types of variants could occur in the same flower. These variants indicated that the highly-specialized pollination systems, i.e. mirror-image flowers and heteranthery, can shift to generalized flower traits that adapt to generalist insects and selfing. Fruit set of S. surattensis in population of Nanning, Guangzhou, Haikou is very low, while seed set is up to50%. Seedlings can be also found in all three cities, suggesting that natural population regeneration is possible.(3) Microsatellite markers were used to explore the mating system (selfing rate) of the variants.Eleven pairs of microsatellite primers were obtained from the literature, but they did not produce expected amplification. Very low level of genetic differentiation were found among variants and the wild type, indicating that mutation occurred recently.In the future, more sensitive molecular markers need to be used to further explore the selfing level among variants, which is important to estimate the effects of evolutionary breakdown of mirror-image flowers and heteranthery on the plant’s mating system and adaptive potential.