Genetic analyses enabled by the fourth chromosome resource project reveal unexpected mutant phenotypes and suggest new disease models.

To date, the Fourth Chromosome Resource Project (FCRP) has deposited over 850 stocks for the genetic analysis of its 79 protein-coding genes. Here, we employ those stocks to reveal unexpected phenotypes for multiple exemplar genes. Expression studies of the transcript and protein provide clues to an adult function for maverick, a gene that, despite prior efforts, has remained inscrutable. Loss-of-function studies reveal an adult brain phenotype for the well-studied signal transducer legless and the first phenotype for datilographo, a gene with no prior mutations. Marked clones with a new eyeless null allele in the larval brain elicited the first heterochronic phenotype in flies. Mutant clones of myoglianin encompassing the entire adult glial blood-brain barrier elicited overgrowth of the underlying optic lobes. Complete clones of one glial layer within the two-layer barrier were obtained for multiple genes and provide an opportunity to interrogate barrier crossing mechanisms for neurological therapeutics. In overexpression studies, rough eye phenotypes were generated by two human genes known to cause autosomal dominant neurological diseases. How YY1 and GRM1 haploinsufficiency leads to inherited cognitive impairment and spinocerebellar ataxia visible from birth is currently unknown. Fly eye phenotypes provide a tractable disease model for understanding their mechanism of action. Taken together, the ease with which mutant phenotypes were revealed suggests that a considerable amount of interesting biology remains to be uncovered on the fourth chromosome.