Abstract: Protein Phosphatase-2A(PP2A) is one of the most important protein serine/threonine phosphatases and plays distinct roles in regulating gene expression,signal transduction,cell proliferation,differentiation,senescence,apoptosis,autophagy,morphogenesis and organogenesis.Our previous studies have established the expression patterns of the catalytic subunits and the regulatory subunits for PP-2A in mouse eye1.To explore the possible functions of PP-2A in various tissues of the lower vertebrates,here,we have analyzed the differential expression pattern,functional status,and the cellular localization of the catalytic subunit for PP-2A using western blot and immunofluorescence cytochemistry on four different ploidy level fish.These include the allotetraploid hybrids and their diploid parents,common carp(♂) and red crucian carp(♀) as well as the triploid crucian carp derived from crossover between the allotetraploids and common carp or between the allotetraploids and the red crucian carp.Our study demonstrated the following results:1) In brain,muscle and liver from the four types of fish,the highest expression level of PP-2Ac was observed in the triploid crucian carp;2) In the kidney of the four types of fish,however,the highest expression level was detected in the allotetraploids,and the lowest level was detected in the diploid common carp and the triploid crucian carp;3) In the gonads of the four types of fish,the highest expression level of PP-2Ac was found in testis of the diploid common carp;4) In the heart of the four kinds of fish,the highest expression level of PP-2Ac was observed in the diploid red crucian carp;5) While both phosphorylated and non-phosphorylated PP-2Ac were found in the heart and muscle tissues from all four types of fish,only phosphorylated PP-2Ac was detected in the brain from these organisms;6) In the kidney,liver and gonads,non-phosphorylated PP-2Ac was only detected in the diploid common carp and red crucian carp;7) PP-2Ac was mainly localized in the cytoplasm of the neurons in the brain tissue,of the cardiomyocytes in the heart,and of the myocytes in the muscle of the four types of fish;certain nuclei from the neurons in both allotetraploid and triploid brain have strong immunofluorescence signal;8)PP-2Ac was found in the cytoplasm but not the nuclei of cuboidal cells of the renal collecting and filtration tubes of the kidney,and in the hepatocytes,sinusoids and the bile canaliculi of the liver of the four types of fish;and finally,PP-2Ac was found in the cytoplasm membrane and nuclei of the growing oocytes from the diploid red crucian carp and triploid crucian carp,in the base membranes covering the seminiferous tubules and the interstitial tissues separating the seminiferous tubules,sertoli cells and some spermatocytes of the diploid common carp.Together,these results lead to the following conclusions:1) PP-2Ac was differentially expressed in various tissues of the four different ploidy level fish;2) PP-2A functions are highly regulated in different tissues;3) Within the same tissue of the four types of fish,PP-2A was localized in the same types of cells;and 4) the presence of strong PP-2Ac immunofluorescence signal in certain nuclei of neurons in both allotetraploid and triploid brains but not in those of the diploid parents indicates that the PP-2Ac may be used as a biochemical marker to distinguish the different types of fish.Overall,our results provide important information for the further exploration of PP-2A functions in lower vertebrates.