Nucleolus size variation during meiosis and NOR activity of a B chromosome in the grasshopper Eyprepocnemis plorans

The number of nucleoli and nucleolar area were measured in meiotic cells from males of the grasshopper Eyprepocnemis plorans collected in three natural populations. Number of nucleoli per cell showed no significant correlation among cells in different meiotic stages, but there was strong positive correlation for nucleolar area between leptotene and interkinesis cells in individuals from distant populations (Salobrena in Spain, and Smir in Morocco). No correlation was, however, observed for both parameters between the meiotic stages analysed in individuals from the population of Torrox (Spain). The number of nucleoli at leptotene was about double the number at interkinesis, as expected from the double ploidy level at leptotene and the corresponding double number of rDNA clusters. Leptotene nucleolar area, however, was about fourfold that in interkinesis, presumably due to higher requirements for ribosome biogenesis in meiosis I than meiosis II. In Torrox, diplotene cells showed a lower number of nucleoli but larger nucleolar area than in leptotene cells, suggesting an increase in nucleolus size during prophase I. Significant differences were found among populations for nucleolar area but not for number of nucleoli, the smallest nucleolar area being observed in Torrox, which is the population harbouring the most parasitic B chromosome variant. No clear effects on nucleolar area or number of nucleoli were associated with the B-chromosome number. However, B-chromosome effects on the nucleolar area were apparent in the Torrox population when data were analysed with respect to a B-chromosome odd-even pattern in leptotene and interkinesis cells. However, in diplotene cells no odd-even pattern was observed for both nucleolar parameters, suggesting that the increase in nucleolar size from leptotene to diplotene dilutes the leptotene odd-even pattern. The rDNA distally located in the B chromosome was associated with a nucleolus in 6.5% out of the 247 diplotene cells analysed. The implications of these findings are discussed in the context of B chromosomes as stress-causing genome parasites and the nucleolus as a sensor of stress.