The crypsis hypothesis: a stenopic view of the selective factors in the evolution of physical dormancy in seeds

Abstract

AbstractPhysical dormancy (PY) in seeds/fruits, which is caused by the water-impermeable palisade layer, has long been considered a mechanism for synchronizing germination to a favourable time for seedling survival and establishment. Recently, a new hypothesis (crypsis hypothesis) was proposed as the main selective factor for the evolution of PY. However, there are some misconceptions in this hypothesis. Our objective is to critically evaluate the crypsis hypothesis and to point out that there are multiple adaptive roles of PY. The fundamental argument in the crypsis hypothesis, that PY evolved as an escape mechanism from predators, is not valid according to the evolutionary theory of Darwin. According to Darwin's hypothesis, variations (dormancy in our case) within a population occur randomly, i.e. there is no direct function of a variation at the time of its origin. Different selection pressures operating in the environment increase or decrease the fitness of individuals with the variation. Water-gap anatomy in seeds/fruits and phylogenetic relationships of species with PY suggest that PY has evolved several times in angiosperms. Thus, we argue that not only predatory pressure but also several other environmental pressures (e.g. proper timing of germination, ultra-drying of seeds, dispersal and pathogens) were involved in increasing the fitness of species producing seeds with PY. The significance of PY in the survival of the species under the above-mentioned environmental pressures and other misconceptions of the crypsis hypothesis are discussed in detail.