Expr12324

In contrast to a previous report (Zhao and Emmons, 1995), LIN-32::GFP expression was not observed in the Rn ("ray precursor") cells. Instead, expression was first apparent in the anterior daughters of these cells, the Rn.a or "ray neuroblast" cells. In wild-type animals, these cells undergo two additional rounds of division to generate the four postmitotic ray cells. Unexpectedly, we found that LIN-32::GFP was asymmetrically distributed in these divisions. When Rn.a divides, LIN-32::GFP is initially detectable in both daughter cells. However, its expression is maintained only in the anterior daughter, Rn.aa, such that LIN-32::GFP becomes rapidly undetectable in its sister Rn.ap. This pattern is repeated in the division of Rn.aa: shortly after an initially similar distribution of LIN-32::GFP, Rn.aaa displays high LIN-32::GFP levels, whereas expression in its sister Rn.aap, fated to undergo programmed cell death, is weaker. Several hours later, LIN-32::GFP becomes undetectable in Rn.aaa itself, as this cell differentiates into an RnA neuron. Thus, LIN-32 expression becomes reiteratively restricted to the anterior daughter cell shortly after two successive cell divisions.