Biochemical changes occurring during final maturation of eggs of some marine and freshwater teleosts

Abstract

Biochemical changes which occur during post‐vitellogenic meiotic maturation of oocytes were investigated by measuring the concentrations of various phosphorus‐containing fractions of the ovaries and mature eggs of four marine species with pelagic eggs, plaice, Pleuronecles platessa, cod, Gadus morhua, haddock, Melanogrammus aeglefinus, and whiting, Merlangius merlangus, and three freshwater species with demersal eggs, rainbow trout, Salmo gairdneri, pike, Esox lucius, and powan, Coregonus lavarelus. In the former group, protein phosphate was deposited at normal vertebrate levels in the ovaries during vitellogenesis but was exhausted during maturation, its decrease correlating with the uptake of water which rendered the mature egg buoyant in sea water. In plaice, phospholipid phosphate declined slightly during maturation, while inorganic phosphate increased by an amount slightly less than that which disappeared from the protein fraction. No such changes occurred on maturation in these three fractions in the freshwater species, in which little or no water uptake occurred. In plaice, the approximately five‐fold increase in the water content of the oocyte was accompanied by a corresponding five‐fold increase in the amount of potassium, the major inorganic cation of the oocyte. These pelagic eggs appear to be unique among the eggs of vertebrates in their high water content (c. 92%) and in the fact that the protein phosphate is almost entirely utilized before fertilization, while, in those freshwater fishes and other oviparous vertebrates that have been examined, the egg is of much lower water content (50–70%) and most of the protein phosphate, like the other major yolk constituents, is used during embryonic growth. This utilization at such an early stage in the life cycle constitutes an extreme example of heterochrony.