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FISH mapping of rDNA loci and nucleolar organizer activity in Allium commutatum Guss.

The position and number of rNDA loci-genes coding for 18S-5.8S-26S and 5S rRNA and nucleolar organizer activity were investigated in six South Adriatic populations of Allium commutatum. All plants studied had diploid chromosome complement (2n = 16) with two NOR-bearing chromosome pairs VII and VIII with secondary concristions and pericentric and median position. FISH confirmed that these were the only sites of 18S-5.8S-26S rRNA genes. Differential banding with flourochromes revealed that NOR-associated heterochromatin is constituted of both GC- and AT-rich DNA segments. The same heteromorphism of CMA/DAPI fluorescent signals and FISH signals between homologous NOR-bearing chromosomes was recorded. The maximum number of active NORs is in correlation with maximum number of nucleoli, but variability in the number and size of active NORs between cells and individuals was observed. DIfferent size of homologous and nonhomologous Ag-NORs are connected with different amount of ribosomal cistrones visible as a different size of FISH signal, but also with the secondary constriction extension. Loci of 5S rRNA genes were located intercolory on chromosome pair VI. Signal strength difference between homologous sites as also present as a size of DAPI-positive bands at the same position.

Allium commutatum Guss.; fluochrome banding; fluorescent in situ hybridization; NOR activity

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