Association of orthodenticle with natural variation for early embryonic patterning in Drosophila melanogaster

Although it is well established that cis-acting regulatory variation contributes to morphological evolution between species, few concrete examples of polymorphism affecting developmental patterning within species have been demonstrated. Early embryogenesis in Drosophila is initiated by a gradient of Bicoid morphogen activity that results in differential expression of multiple target genes. In a screen for genetic variation affecting this process, we surveyed 96 wild-type lines of Drosophila melanogaster for polymorphisms in binding sites within 16 Bicoid cis-regulatory response elements. One common polymorphism in the orthodenticle (otd) early head enhancer is associated with a complex series of indels/substitutions that define two distinct haplotypes. The middle region of this enhancer exhibits an unusual pattern of nucleotide diversity that does not easily fit into standard models of selection and demography. Population Gene Expression Maps, generated by extracting binary expression profiles from normalized embryo images, revealed a ventral reduction of otd transcript abundance in one of the haplotypes that was recapitulated in expression of transgenic constructs containing the two alleles. We thus demonstrate that even a process as robust as early developmental patterning is affected by standing genetic variation, intriguingly involving otd, whose morphogenetic function bicoid is thought to have displaced during dipteran evolution.