The genetic basis of divergent pigment patterns in juvenile threespine sticklebacks.

Animal pigment patterns are important for a range of functions, including camouflage and communication. Repeating pigment patterns, such as stripes, bars and spots have been of particular interest to developmental and theoretical biologists, but the genetic basis of natural variation in such patterns is largely unexplored. In this study, we identify a difference in a periodic pigment pattern among juvenile threespine sticklebacks (Gasterosteus aculeatus) from different environments. Freshwater sticklebacks exhibit prominent vertical bars that visually break up the body shape, but sticklebacks from marine populations do not. We hypothesize that these distinct pigment patterns are tuned to provide crypsis in different habitats. This phenotypic difference is widespread and appears in most of the freshwater populations that we sampled. We used quantitative trait locus (QTL) mapping in freshwater-marine F2 hybrids to elucidate the genetic architecture underlying divergence in this pigmentation pattern. We identified two QTL that were significantly associated with variation in barring. Interestingly, these QTL were associated with two distinct aspects of the pigment pattern: melanophore number and overall pigment level. We compared the QTL locations with positions of known pigment candidate genes in the stickleback genome. We also identified two major QTL for juvenile body size, providing new insights into the genetic basis of juvenile growth rates in natural populations. In summary, although there is a growing literature describing simple genetic bases for adaptive coloration differences, this study emphasizes that pigment patterns can also possess a more complex genetic architecture.

Distinct startle responses are associated with neuroanatomical differences in pufferfishes.

Despite the key function of the Mauthner cells (M-cells) in initiating escape responses and thereby promoting survival, there are multiple examples of M-cell loss across the teleost phylogeny. Only a few studies have directly considered the behavioral consequences of naturally occurring M-cell variation across species. We chose to examine this issue in pufferfishes, as previous research suggested that there might be variability in M-cell anatomy in this group of fish. We characterized the M-cell anatomy and fast-start responses of two pufferfish species, Tetraodon nigroviridis and Diodon holocanthus. T. nigroviridis showed robust fast-starts to both tactile and acoustic startling stimuli. These fast-starts occurred with a latency typical of M-cell initiation in other fish, and retrograde labeling of spinal-projection neurons revealed that T. nigroviridis does have M-cells. By contrast, D. holocanthus only rarely exhibited fast-start-like behavior, and these responses were at a substantially longer latency and were much less extensive than those of T. nigroviridis. Using three complementary anatomical techniques we were unable to identify obvious M-cell candidates in D. holocanthus. These results provide a clear correlation between M-cell presence or absence and dramatic differences in fast-start behavior. The rich diversity within the pufferfish clade should allow future studies investigating the factors that contribute to this correlated anatomical and behavioral variation.