In the C. elegans germline, GLP-1/Notch signaling and two nearly identical RNA binding proteins, FBF-1 and FBF-2, promote proliferation. Here, we show that the
fbf-1 and
fbf-2 genes are largely redundant for promoting mitosis but that they have opposite roles in fine-tuning the size of the mitotic region. The mitotic region is smaller than normal in
fbf-1 mutants but larger than normal in
fbf-2 mutants. Consistent with gene-specific roles,
fbf-2 expression is limited to the distal germline, while
fbf-1 expression is broader. The
fbf-2 gene, but apparently not
fbf-1, is controlled by GLP-1/Notch signaling, and the abundance of FBF-1 and FBF-2 proteins is limited by reciprocal 3''UTR repression. We propose that the divergent fbf genes and their regulatory subnetwork enable a precise control over size of the mitotic region. Therefore,
fbf-1 and
fbf-2 provide a paradigm for how recently duplicated genes can diverge to fine-tune patterning during animal development.