A major facilitator superfamily protein participates in the reddish brown pigmentation in Bombyx mori

MucK is a member of the major facilitator super family (MFS) and is within a subgroup involved in the uptake of organic acids. Here, we provide the first evidence that mucK is required for normal body color pattern in insect larvae. In the cts mutant strain of Bombyx mori (Linnaeus), the larval anal plates, prothoracic tergum, and the head are reddish brown, as opposed to the white color found in the wild type. Positional cloning of the cts gene was carried out and it was mapped within 90 kb on the 16th chromosome. Two molecular markers, 182A4 and 242GA, were found co-segregated with the cts phenotype. Only one candidate gene in the region, BGIBMGA013242, which codes for a major facilitator super family protein named BmMucK, located between the two loci. Sequence analysis revealed that a ∼3 kb deletion in the genome resulting in a 126 bp deletion in the open reading frame (ORF) of BmmucK in the cts mutant. Fluorescence in situ hybridization (FISH) and quantitative reverse transcription PCR showed that BmmucK expression coincided with reddish brown pigmented regions in the cts mutant strain. BmmucK expression in the head and anal plates were markedly elevated at the molting stages. We performed RNA interference of the mucK homolog in Tribolium castaneum and observed brown coloration accompanied by lethality in ∼1/3 of the insects. The results suggest that MucK plays an important role in the pigmentation process of insect larvae.