PARACENTRIC INVERSION INVOLVING A NOR-BEARING CHROMOSOME OF RAINBOW-TROUT (ONCORHYNCUS-MYKISS) - ELECTRON-MICROSCOPY STUDIES OF THE SYNAPTONEMAL COMPLEX

Cytogenetic studies conduced on a sample of 19 specimens of rainbow tr out (Oncorhyncus mykiss) showed that 6 presented two chromosomes with a single subterminal AgNOR on the short arms and 13 presented one chro mosome with a single subterminal Ag-NOR and a second chromosome with t wo small subterminal Ag-NORs. Synaptonemal complex studies showed the presence of a pairing failure involving the two lateral elements (LEs) of the NOR-bearing chromosomes only in those specimens with two diffe rent NOR-bearing chromosomes, suggesting that the chromosomes with two Ag-NORs originated from a paracentric inversion involving a terminal segment of the original NOR-bearing chromosome. A comparative analysis between three normal and three heterozygous fishes suggest that in th e latter group the LEs of the NOR-bearing chromosomes initiate the syn aptic process in a more delayed manner, the synapsis develops more qui ckly, and the nucleolus stays associated with chromosomes for a longer time.