Developmental noise and evolution of a new stable bristle pattern in D. santomea. Virginie Orgogozo¹, Isabelle Nuez¹, Amir Yassin¹, Daniel Matute², David Stern³, Jean David¹. 1) Institut Jacques Monod, CNRS UMR7592, Paris, France; 2) University of Chicago, Chicago, USA; 3) Janelia Farm, Ashburn, USA.

   How new robust phenotypes evolve despite developmental noise remains unclear. Drosophila santomea is unique among the nine D. melanogaster subgroup species in that it has lost a pair of bristles surrounding the phallus. All the tested D. santomea flies displayed 0 bristles and the other species 2 bristles at various raising temperatures, indicating that both the ancestral and the evolved phenotypes are stable. Nevertheless, a few hybrids between D. santomea and D. yakuba display an unstable phenotype, with only one genitalia bristle (left side for some individuals, right for others, fluctuating asymmetry). To map the genomic region(s) responsible for this loss of genitalia bristles, we performed backcrosses in both directions between D. santomea and D. yakuba and we genotyped all individuals with the Multiplexed Shotgun Genotyping (MSG) method. The largest effect is found on the left tip of chromosome X, which includes the achaete-scute gene complex, a complex encoding transcriptions factors that activate bristle formation. At least two other genomic regions are involved, and each corresponding D. santomea allele increases the probability of genitalia bristle loss. Interestingly, introgression lines carrying the entire achaete-scute gene complex from D. santomea in a D. yakuba background show that the D. santomea introgressed part is not sufficient in itself to cause hypandrial bristle loss. However, this introgressed region does reduce bristle number in presence of other D. santomea alleles (in hybrids between introgression lines and D. santomea). Our data are consistent with a threshold model where D. santomea alleles decrease the value of a continuous variable. Evolution of a new stable phenotype appears to require a large-effect QTL to pass over the instability range.