A MATERNAL RNA LOCALIZED IN THE YELLOW CRESCENT IS SEGREGATED TO THE LARVAL MUSCLE-CELLS DURING ASCIDIAN DEVELOPMENT

A cDNA library prepared from one-cell zygotes of the ascidian Styela c lava was screened with probes from isolated cellular fractions to iden tify clones encoding RNAs localized in the yellow crescent or myoplasm , a cytoskeletal domain with multiple developmental roles. The differe ntial screen yielded five overlapping cDNA (Styela clava yellow cresce nt or SeYC) clones encoding a 1.2-kb polyadenylated RNA (yellow cresce nt or YC RNA) which is present throughout embryonic development, In si tu hybridization confirmed that YC RNA is localized in the yellow cres cent. Antisense probes containing the 3' region of YC RNA hybridize wi th multiple maternal and zygotic RNAs, suggesting sequence homologies with other transcripts. YC RNA was first detected during oogenesis whe n transcripts accumulate in the perinuclear region of vitellogenic ooc ytes and are gradually translocated to the cortex. The YC transcripts are localized in the cortex of unfertilized eggs but after fertilizati on segregate with the myoplasm to the yellow crescent. During cleavage most YC transcripts enter the primary muscle cell lineage. YC RNA is also present in the secondary muscle cells. The YC transcripts are ret ained in the myoplasm of oocytes and eggs extracted with the non-ionic detergent Triton X-100, suggesting that they are associated with the cytoskeleton. The nucleotide sequence of the longest ScYC clone contai ns a short open reading frame (ORF). The YC ORF would encode a putativ e polypeptide of 49 amino acids, which shows no significant homology t o known proteins. Several features of the YC RNA, however, suggest tha t it functions as an RNA rather than as a protein coding molecule. We conclude that the myoplasm contains a novel maternal RNA which is asso ciated with the cytoskeleton and segregated to the muscle cells during ascidian embryogenesis. The YC RNA may be a new member of a growing f amily of noncoding RNAs that play important roles in growth and develo pment. (C) 1995 academic press, Inc.