ИСТИНА

Plasticity of ontogenetic pathways is an effective mechanism providing the survival of population in non-stable environment. An existence of polymorphic survival strategy in the population of common toad have been revealed in the course of long-term investigations. In the experiments with the spawns of common toad I have studied an influence of initial density of developing eggs on the survival and developmental rate of embryos. It was shown that in some spawns the relationship between survival rate and egg density evaluated at the hatchling stage has a shape of monotone curve. Higher survival rate of embryos is a characteristic feature of these spawns. However, embryonic mortality increases and developmental rate decreases with the increase in the initial egg density. In other spawns, the relationship between survival rate and egg density has a shape of a curve with two maximums (non-monotone). There is an optimal density at which the values of developmental rate are maximal and their variance is minimal. It is also possible to observe the second maximum of embryonic survival rate existing at the high values of density. The level of embryonic mortality is high enough in this group of spawns, however these spawns get advantage in the case of high egg density. Thus, two different developmental strategies of spawns have been observed in the common toad population. Some specific features of toad’s spawn structure (i.e. different types of 3D packing of egg cords) coupling with the ecology of toad’s spawning (i.e. arrangement of ponds) cause very high heterogeneity of developmental conditions within the same spawn. This heterogeneity may be responsible for the evolutionary origin of polymorphism in the strategy of embryonic survival.