The primary sex determination signal of Caenorhabditis elegans

An X chromosome counting process determines sex in Caenorhabditis elegans. The dose of X chromosomes is translated into sexual fate by a set of X-link ed genes that together control the activity of the sex-determination and do sage-compensation switch gene, xol-1. The double dose of X elements in XX a nimals represses xol-1 expression, promoting the hermaphrodite fate, while the single dose of X elements in XO animals permits high xol-1 expression a t two levels, transcriptional and post-transcriptional. The two molecularly characterized elements include an RNA binding protein and a nuclear hormon e receptor homolog. Here we explore the roles of the two mechanisms of xol- 1 repression and further investigate how the combined dose of X signal elem ents ensures correct, sex-specific expression of xol-1. By studying the eff ects of increases and decreases in X signal element dose on male and hermap hrodite fate, we demonstrate that signal elements repress xol-1 cumulativel y, such that full repression of xol-1 in XX animals results from the combin ed effect of individual elements. Complete transformation from the hermaphr odite to the male fate requires a decrease in the dose of all four elements , from two copies to one. We show that both mechanisms of xol-1 repression are essential and act synergistically to keep xol-1 levels low in XX animal s. However, increasing repression by one mechanism can compensate for loss of the other, demonstrating that each mechanism can exert significant xol-1 repression on its own. Finally, we present evidence suggesting that xol-1 activity can be set at intermediate levels in response to an intermediate X signal.