A semi-lethal, sterile allele of the smooth locus (2-91.5), sm3 , was discovered in an artificial selection line for low abdominal bristle number that had been started from a P-M dysgenic cross. The fitness effects and extremely low bristle number phenotype of the allele could not be separated by recombination from a P -element insertion at cytological location 56E, and precise excision of the P element at this site was associated with reversion to wild type. The smooth gene was cloned using the P -element insertion as a tag. The gene encodes a 2.6-kb transcript derived from 10 exons and covers a genomic region of at least 80 kb. The Drosophila smooth gene shares substantial sequence identity with a group of RNA binding proteins, with the closest relationship being to the human heterogeneous nuclear ribonucleoprotein L gene. The smooth gene is by definition an abdominal bristle number quantitative trait locus, but further work is required to discern whether naturally occurring allelic variation at this locus is a source of genetic variation for abdominal bristle number in natural populations.
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