Research interests
The localization of messenger RNA molecules is a widespread and important mechanism
for establishing asymmetry in eggs and somatic cells. In the fruit fly Drosophila,
the anterior-posterior axis of the egg and embryo are specified by the localization
of mRNA and protein molecules at the anterior and posterior tips of the egg.
At the anterior tip of the egg, the localized molecule with the most significant
role in development is bicoid mRNA. Bicoid mRNA is localized at
the anterior margin of the oocyte during oogenesis, and following fertilization,
the bicoid protein is confined to the anterior embryo, where it specifies embryonic
structures in a concentration-dependent manner.
A long-term interest of my lab has been to determine the role of the swallow gene in RNA localization. Females homozygous for swallow mutations fail to localize bicoid mRNA and another anterior mRNA, htsN4 (see below). Various pieces of information suggest that swallow serves an anchoring function, holding the mRNA molecules in position, but does not act to determine the position of bicoid or htsN4 mRNAs. We have determined the position of swallow protein by several methods: swallow protein is present at all positions at the oocyte cortex, with enrichment at
the anterior oocyte margin during stages when bicoid and htsN4 mRNA localization is most active. We believe that swallow protein acts to anchor bicoid and htsN4 mRNAs, and we are using various methods to determine what other proteins swallow is associated with and how it acts in RNA localization.
Drosophila and other higher Diptera are unusual among the insects in having two distinct embryonic organizing centers, an anterior-determining system (bicoid), and a posterior-determining system (nanos et al.). The posterior organizing center is evolutionarily ancient, but the anterior center appears to be a recent evolutionary innovation. Swallow and the remainder of the localization machinery must also be of recent origin, or co-opted from some other purpose(s). We are interested in understanding how the complex mechanism of RNA localization evolved in the Diptera. The swallow gene seems to have evolved at an extremely rapid rate and shows several other interesting evolutionary quirks which we are currently examining.
Selected publications
Stephenson, E.C. 2004. Localization of swallow-green fluorescent protein in Drosophila oogenesis and implications for the role of swallow in RNA localization. Genesis 39: 280-287.
Pokrywka, N.J., L. Meng, K. Debiec and E.C. Stephenson. 2004. Identification of hypomorphic and null alleles of swallow via molecular and phenotypic analyses. Dev. Genes Evol. 214: 185-192.
Meng, J. and E.C. Stephenson. 2002. Oocyte and embryonic cytoskeletal defects caused by mutations in the Drososphila swallow gene. Dev Genes Evol 212: 239-247.
Huang, Z., N.J. Pokrywka, J.H. Yoder and E.C. Stephenson (2000) Analysis of a swallow homologue from Drosophila pseudoobscura. Dev. Genes Evol. 210: 157-161.
Petrov, D.A., Y.-C. Chao, E.C. Stephenson and D.L. Hartl (1998) High rate of DNA loss in a Drosophila pseudogene. Molecular Biology and Evolution 15: 1562-1567.
Stephenson, E. C. 1995. Cytoskeletal mechanisms of RNA localization during Drosophila oogenesis, in Localized RNAs, H. Lipshitz, ed. R. G. Landes Company, Austin.
Pokrywka, N. J., and E. C. Stephenson. 1995. Microtubules are a general component of mRNA localization systems in Drosophila oocytes. Dev. Biol. 176: 363-370.
Pokrywka, N. J., and E. C. Stephenson. 1994. Localized RNAs are enriched in cytoskeletal extracts of Drosophila oocytes. Dev. Biol. 166: 210-219.
Pokrywka, N. J. and E. C. Stephenson. 1991. Microtubules mediate the localization of bicoid RNA during Drosophila oogenesis. Development 113: 55- 66.