Translation of maternal mRNA during early oogenesis in a telotrophic-meroistic insect ( Dysdercus intermedius)

Abstract

The previtellogenic oocyte of Dysdercus intermedius is provided with maternal mRNAs synthesized by trophocytes and transported via the trophic cord towards the ooplasm in this trophocyte-oocyte syncytium. Maternal mRNA accumulation occurs in the absence of transcriptional activity in the oocyte nucleus suggesting that ooplasmic protein synthesis is regulated at the translational level. Trophocytes and oocytes were isolated from each other and separated from the surrounding somatic tissues (“skinning”)' The distribution of cytosolic proteins in the trophocyte-oocyte syncytium at 72 h of adult age was analysed in extracts of “skinned” trophocytes and oocytes by 2-D gel electrophoresis followed by silver staining: 78 acidic proteins are accumulated in the trophocyte compartment and 59 of these proteins also appear after silver staining the oocyte extracts. When protein synthesis was analysed in 2-D gels by fluorography, oocytes were found to synthesize a less complex set of proteins than trophocytes: no proteins characterized by a pI between 6.2 and 6.9 (“pI window”) were synthesized in “skinned” oocytes incubated separately in the 35S-methionine-containing in vitro medium. The 59 silver stained ooplasmic proteins have two sources: the trophocytes deliver 46% of ooplasmic proteins via the trophic cord and 32% can be attributed to ooplasmic protein synthesis based on maternal mRNAs. 87% of the latter proteins are more acidic than 6.2. Since the cytosolic pH (determined with proton-selective microelectrodes) averaged 7.4, all proteins separated in 2-D gels are negatively charged in vivo. The synthesis of basic proteins was determined by IEF slab gels in the native state. Fluorography revealed the synthesis of three basic proteins in trophocytes as well as in oocytes. Two of them are close to their iso-electric points (pI 7.3, 7.4) whereas the third (pI 8.3) is positively charged in vivo.