PHYLOGENETIC CONSTRAINTS IN KEY FUNCTIONAL TRAITS BEHIND SPECIES’ CLIMATE NICHES: PATTERNS OF DESICCATION AND COLD RESISTANCE ACROSS 95DROSOPHILASPECIES

Abstract

Species distributions are often constrained by climatic tolerances that are ultimately determined by evolutionary history and/or adaptive capacity, but these factors have rarely been partitioned. Here, we experimentally determined two key climatic niche traits (desiccation and cold resistance) for 92-95 Drosophila species and assessed their importance for geographic distributions, while controlling for acclimation, phylogeny, and spatial autocorrelation. Employing an array of phylogenetic analyses, we documented moderate-to-strong phylogenetic signal in both desiccation and cold resistance. Desiccation and cold resistance were clearly linked to species distributions because significant associations between traits and climatic variables persisted even after controlling for phylogeny. We used different methods to untangle whether phylogenetic signal reflected phylogenetically related species adapted to similar environments or alternatively phylogenetic inertia. For desiccation resistance, weak phylogenetic inertia was detected; ancestral trait reconstruction, however, revealed a deep divergence that could be traced back to the genus level. Despite drosophilids' high evolutionary potential related to short generation times and high population sizes, cold resistance was found to have a moderate-to-high level of phylogenetic inertia, suggesting that evolutionary responses are likely to be slow. Together these findings suggest species distributions are governed by evolutionarily conservative climate responses, with limited scope for rapid adaptive responses to future climate change. K E Y W O R D S :Ancestral trait reconstruction, evolutionary history, niche conservatism, phylogenetic signal, species distribution, stress resistance.

How consistent are the transcriptome changes associated with cold acclimation in two species of the Drosophila virilis group?

“…We also observed that many more genes were DE in D. virilis than D. montana. One potential reason for this may be that D. virilis is less cold tolerant than D. montana (Kellermann et al, 2012;Vesala et al, 2012b) and thus the cold acclimation treatment may be more stressful for D. virilis than D. montana, promoting a stronger cold acclimation response. In support of this we also note that the average fold change in genes DE in both species was also slightly larger in D. virilis than D. montana (Figure 1b).…”

“…These species belong to the D. virilis species group, and diverged from each other approximately 9-11 mya (Reis et al, 2008;Morales-Hojas et al, 2011). Both species show a high level of cold tolerance compared to other drosophilids, and D. montana is significantly more cold tolerant than D. virilis (Kellermann et al, 2012;Vesala et al, 2012b). These species have adapted to live in quite different environmental conditions (D. montana is found at high latitudes (30-70°N) and altitudes while D. virilis is a human commensal found at lower latitudes (south from 35°N) (Throckmorton, 1982)) but both exhibit a similar cold acclimation phenotype, whereby flies kept at sub-optimal temperatures show increased cold tolerance as measured by chill-coma recovery time (Vesala et al, 2012b).…”

Upper thermal limits of Drosophila are linked to species distributions and strongly constrained phylogenetically

“…Briefly, flies were reared at 20°C at a 12-h/12-h light/dark cycle and maintained on an oat-based medium (Leeds); some species, however, required the addition of Opuntia cactus and banana to medium. Experimental flies were controlled for larval density and age effects and prepared for trait measurements (20). For each species we examined 10 flies per sex unless otherwise stated.…”

“…For a more detailed explanation of stocks and experimental methods, see ref. 20. Briefly, flies were reared at 20°C at a 12-h/12-h light/dark cycle and maintained on an oat-based medium (Leeds); some species, however, required the addition of Opuntia cactus and banana to medium.…”

“…In the past phylogenetic analyses have mainly aimed to control for the effects of phylogeny (4, 15, 16); however, studies are now emerging that examine phylogenetic signal with the purpose of determining the role of adaptation vs. constraints (17)(18)(19)(20).…”