Morphological variation of oocytes of initial phases of vitellogenesis in two forms of Baikal grayling <Emphasis Type="Italic">Thymallus baicalensis</Emphasis> (Thymallidae)

The variation of oocytes of initial phases of vitellogenesis in two forms of Baikal grayling Thymallus baicalensis was studied. The dependence of their diameter on the size-age indices of females, as well as differences in the sizes of oocytes between the studied forms, was established. Comparison of oocytes of identical forms of development revealed that oocytes of the white grayling during termination of the phase of cytoplasm vacuolization and at the beginning of the phase of yolk accumulation have a smaller diameter of the cell, of the nucleus, respectively, and of yolk granules and a greater, with respect to cell diameter, size of vacuoli. The revealed differences of morphological characteristics of oocytes of initial phases of vitellogenesis in combination with the well-known, stably smaller (by a factor of 1.5) size of mature eggs of the white Baikal grayling that matures at a later age at larger sizes indicate a profound adaptive diversification in the course of development of the reproductive system of both forms.