Evolutionary Ecology, Sep 17, 2014
Aposematic organisms have warning signals advertising their unpalatability to predators, and beca... more Aposematic organisms have warning signals advertising their unpalatability to predators, and because signal efficiency is better in higher densities, positive frequency-dependent selection is expected to select against less common signals. The wood tiger moth (Parasemia plantaginis) occurs across the Holarctic and its conspicuous hindwings serve as warning signals to predators. It also has conspicuous black and white forewing patterns that could act as warning signals, or help to hide the moth by preventing predators from seeing the outline of the moth’s body (a strategy known as disruptive coloration). In Alaska, the predominant forewing pattern changes distinctly between the regions around Fairbanks and Anchorage, suggesting local predators may maintain differences if the pattern functions as a warning signal. Alternatively, restricted gene flow along with drift could be responsible. We placed artificial moths with both local dominant and foreign forewing patterns in each of the two regions to test if predators select against the foreign forewing types, which would suggest the warning signal function of forewing patters. We also manipulated the level of disruptiveness in the forewing patterns to see if disruptiveness works in concert with the warning signal. The locally dominant forewing type was better protected in Fairbanks, but not in Anchorage where morphs were attacked equally. Manipulating the level of disruptiveness in the forewing pattern did not influence predation. Population genetic analyses from specimens caught during fieldwork showed the existence of two populations and restricted gene flow. Our results suggest that positive frequency dependent selection may be partially responsible for maintaining local signal differences, although predators seem to avoid both forewing patterns in Anchorage. Restricted gene flow between the two populations could be attributed to a combination of selection against foreign morphs in Fairbanks and physical barriers, which both likely contribute to warning signal differences in Alaska.
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Papers by Robert Hegna
Location The Holarctic.
Methods We sequenced a fragment of the mitochondrial cytochrome c oxidase subunit I gene (COI) from 587 specimens. We also examined more current population structure by genotyping 569 specimens at 10 nuclear micro- satellite loci. Species distribution modelling for present conditions and the Last Glacial Maximum (LGM) was performed to help understand genetic structure. Geographical patterns in hindwing warning coloration were described from 1428 specimens and compared to the genetic analyses.
Results We found only two instances of genetic divergence that coincided with distinct, yet imperfect, shifts in male hindwing coloration in the Caucasus region and Japan. A shift in female hindwing colour did not appear to be asso- ciated with genetic structure. A change from sexual monomorphism to sexual dimorphism was also observed. Mitogenetic (mtDNA) structure does not show the influence of glacial refugia during the LGM. Climate shifts following the LGM appear to have isolated the red Caucasus populations and other southerly populations. Populations at opposite ends of the moth’s distribution showed high levels of differentiation in the microsatellite data analysis compared to the shallow mitogenetic structure, supporting a more recent divergence.
Main conclusions Parasemia plantaginis populations appeared to have been historically well connected, but current populations are much more differenti- ated. This raises the possibility that incipient speciation may be occurring in portions of the species’ distribution. Some changes in colour align to genetic differences, but others do not, which suggests a role for selective and non- selection based influences on warning signal variation.
Data also show predation difference based ib color and relate to to trade-offs in warning signal efficacy