COMPARATIVE TRANSCRIPTOME ANALYSIS DURING THE SEVEN DEVELOPMENT STAGES OF CHANNEL CATFISH Ictalurus punctatus AND TRA CATFISH Pangasianodon hypophthalmus REVEALS NOVEL INSIGHTS FOR THE TERRESTRIAL ADAPTION

Channel catfish (Ictalurus punctatus ) and tra catfish (Pangasianodon hypophthalmus)  both belong to Siluriformes,  one of the most taxonomically diverse orders in all vertebrates. The two species are the largest aquaculture commodities in the United States (channel catfish) and in Vietnam (tra catfish).  Channel catfish does not possess an air breathing organ (ABO) and thus cannot survive in anoxic water . Tra catfish are facultative air breathers and use a modified swimbladder to breathe air when they encounter anoxic water,  which is  a highly advantageous survival mechanism. These two species present a unique research opportunity to study the mechanism of air breathing in teleost fish . In this study, we conducted RNA sequencing with whole fish samples for channel catfish at 2, 5, 6, 7, 8, 9, 10 dpf  (days post fertilization )  and tra catfish at 2, 4, 6, 8, 9, 10, 11 dpf. T he timepoints were selected based on the developmental stage and a previous study rather than absolute age, and have been verified using d evelopmental correspondences and heterochrony analysis.

I n this study, we identified differentially expressed

genes during development within each species. The expression profiles had considerable variation with  the development of channel catfish

and tra catfish (Fig. 1, 2).

Comparative transcriptome analysis revealed that a set of 193 genes were  present in the tra catfish but absent in the channel catfish. Expression profile analysis and subsequent time series analyses were performed to  further narrow the list of  key genes. Histology experiment s  were also conducted to substantiate the results , which indicated that the swimbladder started to develop at 6 dpf in tra catfish . Cluster  5  (Fig. 3) was concordant with our h istological experiments and contained 17 key genes potentially important for the air breathing ability in tra catfish.  This study provides a model for  studying  evolutionary genetics during the transition from life in the ocean to terrestrial living and a novel insight for the evolution of vertebrates.