Have Highly Specialized Pollination Systems Reached anEvolutionary Dead End?

摘要

Pollination refers to the transfer of pollen grains from the anther to the stigma in angiosperm flower. Nearly 90% of plants use a range of animals to achieve pollination and the remaining species use the wind or water as abiotic agents for this purpose. In animal-pollinated species, there is a continuum from generalized systems in which a plant species is pollinated by a number of animal species to a highly specialized obligate mutualism in which each plant species is pollinated by just one animal species. Figs - fig-wasps, yuccas - yucca-moths and a number of orchids are well-documented examples of obligate mutualism. Although obligate mutualism increases pollination efficiency, it carries a load of disadvantages. Several investigators have considered super-specialized pollination systems as 'evolutionary dead ends', as they are prone to extinction under pollinator constraint. Human-induced 'global pollinator crisis', because of habitat degradation and climate change, has exacerbated this constraint.However, many studies in recent years have shown that a number of plant species engaged in specialized obligate pollination systems have evolved several flexible strategies, to survive under pollinator constraints, such as opting out of obligate mutualism by recruiting additional pollinators and adopting an autogamous mode of pollination. Further, many of them have inbuilt compensatory strategies such as perennial nature and vegetative mode of propagation which enable them to survive for a number of years/generations even in the absence of pollinators. Thus, super-specialized pollination systems may not have reached an 'evolutionary dead end'. Their flexible reproductive strategies and inbuilt compensatory mechanisms enable them not only to survive under pollinator constraint but also sustain them during species diversification until new pollinators evolve and the new mutualisms stabilize.