Asynchronous chromosome pairing in male meiosis of the rat (Rattus norvegicus)

Premeiotic and meiotic chromosome distribution was studied in rat testes su spensions by a triple-color fluorescent staining protocol which allows simu ltaneous visual inspection of two chromosomal targets highlighted by FISH t ogether with immunostained SCP3 synaptonemal complex (SC) proteins which ar e marked by a third, composite color. Triple labeling with rat chromosome ( RNO) 4q and 19p specific probes and SCP3 staining disclosed that homologs a re separated in premeiotic and leptotene nuclei. Pairing of homologous chro mosome regions commenced during early zygotene, with pairing of the small m etacentric chromosomes 19 preceding that of the distal region of the long a rm of RNO4. Our results show that homolog association occurs during zygoten e of rat spermatogenesis, with small and large chromosomes showing a consid erable asynchrony. Comparison with pairing progression in meiosis of other mammals suggests that asynchronous chromosome pairing reflects size differe nces within a complement.