Divergence of the yellow trans-regulatory network plays a significant role in pigmentation diversity between species. Richard W. Lusk, Cassandra D. Kirkland, Gizem Kalay, Patricia J. Wittkopp. University of Michigan, Ann Arbor, MI.

   Genes are expressed according to the interaction between their cis-regulatory sequences and the trans-regulatory network that interprets them. While there are now several examples linking cis-regulatory changes to morphological variation, we know much less about the contributions made by variation in the trans-regulatory network. In this work, we use the yellow pigmentation gene to investigate this variation on a large scale. yellow is required for the production of black pigment, and the expression pattern of yellow in pupae prefigures the pigmentation pattern of adults. Here, we examine the activity of twelve yellow regulatory regions, taken from six species, in transgenic D. melanogaster and D. virilis hosts, allowing us to separate the contributions to pigmentation diversity made by variation acting in cis and in trans to this gene. We find that D. virilis, which is largely unpatterned, nevertheless maintains a complex trans regulatory network upstream of yellow which is capable of specifying complex patterns of expression. Moreover, although the two species' networks share this complexity, they otherwise appeared to have diverged, with the two hosts specifying sometimes strikingly different expression patterns from identical cis-regulatory sequences. We use these differences to outline where changes in cis and trans have generated pigmentation diversity and where different sets of regulators appear to underlie similar pigmentation phenotypes, highlighting how divergence of the trans-regulatory network can shape the evolution of phenotypic diversity between species.