Quantitative analysis of the formation of nucleoli in Locusta migratoria

Abstract

At meiosis of Locusta migratoria, zero, one, or two nucleoli per long (L), medium (M), or short (S) nucleolar organizer region (NOR) bivalent can be clearly seen using an Ag-NOR technique. Diplotene cells with zero to six nucleoli were observed, whose distribution fitted a normal one. However, the distribution of nucleoli in the particular bivalents was not at random. A mathematical model was developed in an attempt to explain this differential activation of nucleoli. The model is based on the assumption that the activation of nucleoli is sequential following successive rounds, in such a way that the probability of a particular L, M, or S nucleolus being activated depends on the type of nucleoli previously activated. The nucleoli activated per cell tend to be distributed among bivalents, and the activation of a second nucleolus in a bivalent has a lower probability than the first one of any bivalent, generating interference. The model was developed for wild-type individuals and it was also applied to mutants that showed a distribution of nucleoli different from wild types, i.e., individuals carrying a translocation between two NOR chromosomes and asynaptic mutants. In both cases the model fits with the observed results. This suggests its general validity.Key words: nucleolus, Locusta, translocation, asynapsis.