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Large-scale mutation in the evolution of a gene complex for cryptic coloration

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Science  24 Jul 2020:
Vol. 369, Issue 6502, pp. 460-466
DOI: 10.1126/science.aaz4351

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How stick insects got their colors

Stick insects of the genus Timema show variations in color that are likely due to selection for camouflage on their plant hosts. Some species are only brown or only green, whereas others span from reddish-pink hues to green to brown. To identify the genetics underlying this variation, Villoutreix et al. sequenced most extant California Timema species and identified a deletion that correlated with green body coloration in some species. However, this deletion appeared to be limited to the Northern California clade, with more species in the Southern California clade retaining this locus. In these southern species, a nongreen body color is likely caused by mutation. Even among closely related species, adaptive evolution can converge on the same phenotype from different evolutionary processes.

Science, this issue p. 460

Abstract

The types of mutations affecting adaptation in the wild are only beginning to be understood. In particular, whether structural changes shape adaptation by suppressing recombination or by creating new mutations is unresolved. Here, we show that multiple linked but recombining loci underlie cryptic color morphs of Timema chumash stick insects. In a related species, these loci are found in a region of suppressed recombination, forming a supergene. However, in seven species of Timema, we found that a megabase-size “supermutation” has deleted color loci in green morphs. Moreover, we found that balancing selection likely contributes more to maintaining this mutation than does introgression. Our results show how suppressed recombination and large-scale mutation can help to package gene complexes into discrete units of diversity such as morphs, ecotypes, or species.

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