Larval Development of Silver Sea Bream ( Sparus sarba ): Ontogeny of RNA-DNA Ratio, GH, IGF-I, and Na + -K + -ATPase

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Abstract
To ascertain some of the important biochemical and molecular events that take place during early larval development of silver sea bream (Sparus sarba), we undertook a study of changes in the morphology as well as the ontogeny of the RNA-DNA ratio, growth hormone (GH), insulin-like growth factor I (IGF-I) messenger RNA abundance, Na+-K+-ATPase subunit mRNA abundance, and Na+-K+-ATPase enzyme activity. Larvae samples were collected at 1 to 46 days posthatch (dph). At 7 dph the yolk sac was fully absorbed, and from 28 dph onward larvae underwent rapid developmental changes to the juvenile stage. The RNA-DNA ratio was highest at 1 dph, decreased to low levels between 7 and 21 dph, then increased by 28 dph, and then again by 46 dph. The ontogenetic profiles of GH, IGF-I, and Na+-K+-ATPase α1 and β1 subunits were studied using reverse transcriptase polymerase chain reaction, coupled with radioisotope hybridization of immobilized DNA. Growth hormone abundance reached a constant and high level from 35 dph onward, whereas the IGF-I level reached a peak at 35 dph and then significantly decreased. Both Na+-K+-ATPase α1 and β1 subunit mRNAs increased up to 35 dph, however, at 46 dph the α1 subunit remained high whereas the β1 subunit decreased. Na+-K+-ATPase activity was low in 1-dph larvae but increased rapidly as development progressed. The importance of these findings is discussed within the context of larval development.

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