A Single Gene Causes an Interspecific Difference in Pigmentation in Drosophila

Abstract

The genetic basis of species differences remains understudied. Studies in insects have contributed significantly to our understanding of morphological evolution. Pigmentation traits in particular have received a great deal of attention and several genes in the insect pigmentation pathway have been implicated in inter-and intraspecific differences. Nonetheless, much remains unknown about many of the genes in this pathway and their potential role in understudied taxa. Here we genetically analyze the puparium color difference between members of the virilis group of Drosophila. The puparium of Drosophila virilis is black, while those of D. americana, D. novamexicana, and D. lummei are brown. We used a series of backcross hybrid populations between D. americana and D. virilis to map the genomic interval responsible for the difference between this species pair. First, we show that the pupal case color difference is caused by a single Mendelizing factor, which we ultimately map to an 11-kb region on chromosome 5. The mapped interval includes only the first exon and regulatory region(s) of the dopamine N-acetyltransferase gene (Dat). This gene encodes an enzyme that is known to play a part in the insect pigmentation pathway. Second, we show that this gene is highly expressed at the onset of pupation in light brown taxa (D. americana and D. novamexicana) relative to D. virilis, but not in the dark brown D. lummei. Finally, we examine the role of Dat in adult pigmentation between D. americana (heavily melanized) and D. novamexicana (lightly melanized) and find no discernible effect of this gene in adults. Our results demonstrate that a single gene is entirely or almost entirely responsible for a morphological difference between species.KEYWORDS morphological evolution; pigmentation; genetics of species differences U NDERSTANDING the genetic basis of morphological differences between species is a central goal of evolutionary biology. A key question focuses on complexity: Do species differ by many genes of small effect or by a few genes of large effect (Haldane 1937)? The answer to this question has remained elusive; cases studied reveal examples of each scenario (Orr 2001). This is not surprising, however, as traits and species may differ in divergence rates and types of selection (or lack thereof). The question can best be stated as one of relative frequency: How often do major genes cause morphological differences between species (Orr 2001)?Insect pigmentation traits have received much attention from geneticists (Wittkopp et al. 2003a;True 2003;Wittkopp and Beldade 2009). These studies benefited from the fact that the pathway that determines insect cuticular pigment is both conserved and well understood (Wittkopp et al. 2002;Wittkopp et al. 2003a;Jeong et al. 2008;Williams et al. 2008;Werner et al. 2010). Several genes in the pigmentation pathway have been implicated in pigmentation differences between and within species, particularly in Drosophila (Wittkopp et al. 2003b;Takahashi et al. 2007;Jeong et al. 2008;T...

The Genetic Basis of Pigmentation Differences Within and Between Drosophila Species

“…For example, in the virilis group of Drosophila (Figure 3), D. virilis has a distinctly darker pupal case color than its closest relatives, D. americana , D. lummei, and D. novamexicana (Stalker 1942). The D. virilis pupal case appears almost completely black, whereas pupal cases in the other species are lighter shades of brown and tan (Ahmed-Braimah and Sweigart 2015). The virilis species group is amenable to genetic dissection of this trait because D. americana , D. novamexicana , and D. virilis all produce fertile hybrids when crossed with each other (Heikkinen 1992).…”

“…Early studies investigating the genetic basis of this difference in pupal color between D. virilis and D. americana suggested that it was was due to a large effect locus on chromosome 5 as well as other loci, possibly linked to chromosomes 2 and 3 (Stalker 1942). To identify the molecular basis of pupal color divergence between D. virilis and D. americana more precisely, Ahmed-Braimah and Sweigart (2015) analyzed a backcross population between these two species and scored more than 30,000 recombinant offspring for pupal case color. This experimental design allowed them to identify an ~11-kb sequence on chromosome 5 that contributes to the difference in pupal case color.…”

“…This experimental design allowed them to identify an ~11-kb sequence on chromosome 5 that contributes to the difference in pupal case color. This region contains the first exon and non-coding regions of the Dat gene (Ahmed-Braimah and Sweigart 2015). Dat , as described above and in Figure 1, is required for the conversion of dopamine to NADA, which is then polymerized into a colorless pigment.…”

“…Dat , as described above and in Figure 1, is required for the conversion of dopamine to NADA, which is then polymerized into a colorless pigment. Expression differences were observed for Dat at the onset of pupation between D. americana (high expression) and D. virilis (low expression) (Ahmed-Braimah and Sweigart 2015) that suggest reduced expression of Dat in D. virilis creates an excess of dopamine that allows production of more dark melanins and thus a much darker pupal case. Pupal expression of Dat in D. novamexicana , which has a lighter body color than D. americana but a similarly colored pupal case, was similar to that observed for D. americana (Ahmed-Braimah and Sweigart 2015).…”

“…Expression differences were observed for Dat at the onset of pupation between D. americana (high expression) and D. virilis (low expression) (Ahmed-Braimah and Sweigart 2015) that suggest reduced expression of Dat in D. virilis creates an excess of dopamine that allows production of more dark melanins and thus a much darker pupal case. Pupal expression of Dat in D. novamexicana , which has a lighter body color than D. americana but a similarly colored pupal case, was similar to that observed for D. americana (Ahmed-Braimah and Sweigart 2015). Genetic variation linked to Dat did not explain any of the difference in body color between D. americana and D. novamexicana , consistent with prior work identifying ebony and tan as the primary drivers of divergent body color between these two species (Wittkopp et al 2009).…”