The role of lampbrush chromosomes in the formation of nucleoli in amphibian oocytes.

Theories concerning the mode of origin of peripheral nucleoli in amphibian oocytes have been examined and tested. In Triturus cristatus the giant fusing loops of the 3 shortest lampbrush bivalents resemble nucleoli when viewed in phase contrast and may be considered as possible sites of production of nucleoli. Giant fusing loops, however, differ from peripheral nucleoli in certain important respects, and animals lacking giant fusing loops on their lampbrush chromosomes nevertheless have normal peripheral nucleoli. Therefore, similarity in appearance between objects attached to lampbrush chromosomes and free peripheral nucleoli may not be significant. In oocytes of T. c. carnifex, T. c. karelinii, and T. c. danubialis, peripheral nucleoli do not increase in number during the lampbrush phase of oogenesis, except by division of pre-existing nucleoli towards the end of oogenesis. There are about 1,000 nucleoli per oocyte nucleus in each of these sub-species. In T. c. cristatus there are more nucleoli in large oocytes than in small ones, and it seems likely that in this sub-species the giant fusing loops add to the existing population of nucleoli in an oocyte by successively growing and shedding new nucleoli. A similar situation probably holds in Plethodon cinereus. Hexaploid oocytes from triploid females of Ambystoma jeffersonianum have 3 times as many nucleoli as diploid oocytes from diploid females of the same species. The number of nucleoli in an amphibian oocyte nucleus is therefore related to the number of sets of chromosomes in the cell. In yolky oocytes from hypophysectomized newts most peripheral nucleoli are firmly attached to the inner surface of the nuclear membrane; whereas in similar oocytes from unoperated or gonadotrophin-treated animals none of the nucleoli is so attached. On the basis of these observations 2 mechanisms are suggested for the formation of amphibian oocyte nucleoli. The first of these mechanisms probably operates in T. c. carnifex, where all peripheral nucleoli are formed before or soon after the chromosomes assume the lampbrush form, and no part of a lampbrush chromosome is involved in a process which adds to the existing population of nucleoli. The second mechanism probably operates in T. c. cristatus, where most of the peripheral nucleoli are formed before the lampbrush phase of oogenesis but a nucleolar organizer on the lampbrush chromosomes continues to grow and detach nucleoli throughout oogenesis. Both these mechanisms are discussed in terms of what is known of the chemical composition and function of peripheral nucleoli.