Search

The major yolk protein(MYP) can be found in both genders of sea urchins.Both the immature ovary and testis contain a similar amount of MYP(about 9% of wet mass),constituting about 80% of total protein.This presumably acts as a nutrient reserve during gametogenesis.To determine its main production site and the mechanisms for adapting to starvation in Strongylocentrotus intermedius,MYP mRNA expression levels in intestine,stomach,gonad and coelomocytes were analyzed using real time reverse-transcription polymerase chain reaction.Urchins were starved for 15 days and groups of five individuals were sampled at 5,10 and 15 days.MYP mRNA was detected in all tissues in nonstarved urchins and the amounts expressed between tissues differed widely.The content was greatest in the intestine,28.94-fold of that measured in coelomocytes and 39.26-fold of that measured in immature gonads.MYP expression in the stomach—1.83% of that in coelomocytes—was the lowest among the four tissues studied.MYP expression levels decreased sharply after starvation in coelomocytes to 1.29%,33.33% and 1.58% of the controls(nonstarved) after 5,10 and 15 days,respectively.MYP mRNA levels increased in the gonads during starvation,to 2.93-,4.07-and 6.80-fold that of the control group,respectively,but the extent of increase slowed with extended time of fasting.The expression rate during the first 5 days increased by 192.82% but this reduced to 38.98% and 67.01% in the second and third 5-day periods.MYP mRNA expression levels in the stomach increased to 25.81-,23.43-and 106.52-fold of controls at 5,10 and 15 days of starvation,respectively.By contrast,levels in the intestine declined continuously during starvation to 67.60%,50.87% and 33.33% of the control,respectively.Thus,MYP mRNA could be expressed in the intestine,stomach,gonads and coelomocytes in S.intermedius.The intestine was the main site of production,which could be explained from the lack of any large alimentary gland in sea urchins.During starvation,the MYP mRNA expression level decreased in coelomocytes and increased in the stomach,but these levels were still minor compared with the amounts in the intestine and gonads.Interestingly,although MYP synthesis was suppressed in the intestine during starvation,the MYP mRNA expression levels were not declined but increased in gonads during starvation.Thus,the gonads,followed the intestine,provide a second main site of MYP production during starvation.Before the initiation of gametogenesis in sea urchins,the gonads increase in size by accumulating nutrients such as proteins,lipids and carbohydrates in nutritive phagocytes that fill the gonadal lumina.Therefore,it appears that the MYP mRNA expression level does not decline during starvation but increase continuously to provide nutrition for gametogenesis in resisting transient food deprivation.