Moreover, Chinmo governs multiple continuous fates in MB as well as in adPN lineages. Despite these similarities, detailed mechanisms of Chinmo actions are apparently distinct. In the MB lineages, reducing Chinmo expression elicits systematical early-to-late MB temporal fate transformations, and ectopic Chinmo can specify early MB fates in late siblings ( Zhu et al., 2006). By contrast, a partial reduction in Chinmo sometimes conferred hybrid adPN fate showing features of both the prospective cell fate and the chinmo-null default fate, rather than exhibiting the morphologies reminiscent of the fates in between

(e.g., Figures 2F and 2G; 100% penetrance in all partial transformation samples). And ectopic Chinmo also failed to promote early fates in late-born adPNs, providing no evidence for dosage-dependent Chinmo-mediated fate determination in the adPN lineage check details ( Figures S3E and S3F). Therefore, both loss- and gain-of-function genetic mosaic studies suggest that Chinmo does not directly LDN-193189 cost determine any temporal

cell fate in adPN lineage, but rather it suppresses a later temporal fate in early siblings to allow further neuronal diversification ( Figure 4C, Chinmo part). Further, mechanism(s) must exist to restrict the activities of Chinmo to specific windows, because ectopic Chinmo exerted no detectable effect on adPNs within the rest of the lineage. It is also not clear whether and how Chinmo directly diversifies neuron fate. Unlike Kr that regulates temporal fate transition in the NB, Chinmo apparently acts in the offspring and potentially downstream of some NB transcriptional cascade to increase neuron diversity. This distinction is supported by the follwing: (1) postmitotic expression of transgenic Chinmo restored proper temporal cell fates

in chinmo mutant adPNs ( Figures S3A–S3D), arguing that Chinmo acts in newborn neurons to regulate adPN temporal identity; (2) deleting chinmo from NB through the entire tuclazepam lineage did not affect overall temporal fate transitions, as evidenced by no change in total cell count or length of the lineage ( Figure 2 and Figure 4B); and (3) ectopic expression of chinmo exerted no detectable effect on the NB temporal fate transitions ( Figures S4E and S4F). All these observations indicate that Chinmo acts in postmitotic neurons to refine temporal identity. Temporal patterning by the Kr-containing transcriptional cascade in the NB and via Chinmo in newborn neurons exemplifies a hierarchical mode of temporal cell-fate specification. Identifying additional genes controlling adPN temporal identity and determining their mechanisms of action by iterative use of our strategy will allow elucidation of developmental mechanisms specifying the great diversity of neuron types in the complex brain. Fly strains used in chinmo and Kr mutant twin-spot MARCM clonal analyses are listed in the Supplemental Experimental Procedures.