DOI: 10.5061/dryad.q573n5tkg
title: Environmental variation and biotic interactions limit adaptation at ecological margins: lessons from rainforest Drosophila and European butterflies

description: Models of local adaptation to spatially varying selection predict that
 maximum rates of evolution are determined by the interaction between
 increased adaptive potential owing to increased genetic variation, and the
 cost genetic variation brings by reducing population fitness. We discuss
 existing and new results from our laboratory assays and field transplants
 of rainforest Drosophila and UK butterflies along environmental gradients,
 which try to test these predictions in natural populations. Our data
 suggest that: (i) local adaptation along ecological gradients is not
 consistently observed in time and space, especially where biotic and
 abiotic interactions affect both gradient steepness and genetic variation
 in fitness; (ii) genetic variation in fitness observed in the laboratory
 is only sometimes visible to selection in the field, suggesting that
 demographic costs can remain high without increasing adaptive potential;
 and (iii) antagonistic interactions between species reduce local
 productivity, especially at ecological margins. Such antagonistic
 interactions steepen gradients and may increase the cost of adaptation by
 increasing its dimensionality. However, where biotic interactions do
 evolve, rapid range expansion can follow. Future research should test how
 the environmental sensitivity of genotypes determines their ecological
 exposure, and its effects on genetic variation in fitness, to predict the
 probability of evolutionary rescue at ecological margins. This article is
 part of the theme issue ‘Species’ ranges in the face of changing
 environments (Part II)’.
description: <strong>Crossing
 design</strong> In 2011, we collected female
 <em>D. birchii</em> from sites along three elevation gradients
 in north-east Queensland (Mt Lewis, Mt Edith and Paluma) and established
 isofemale lines, as described previously (Bridle<em> et
 al.</em> 2009; O'Brien<em> et al.</em> 2017). We
 established 7 – 10 lines per site, with 10, 7 and 6 sites respectively at
 Mt Lewis, Mt Edith and Paluma. We used a series of fully
 reciprocal diallel crosses to estimate additive genetic (co)variances of
 quantitative traits of <em>D. birchii</em>, and assess how
 this changes along elevation gradients. Crosses were between isofemale
 lines from within each site. For each site, crosses were set up between
 all possible line combinations, including reciprocals and within-line
 crosses (full diallel design). The total number of unique crossing
 combinations at each site was therefore
 <em>n</em><sup>2</sup>, where
 <em>n</em> = the number of isofemale lines from that site.
 Each crossing combination was replicated 2 - 4 times. While the number of
 lines in each diallel varied slightly between gradients, where possible it
 was kept constant between sites within a gradient, to avoid variation in
 the number of crosses affecting genetic variance estimates along
 gradients.  In
 the two generations prior to establishing crosses, isofemale lines were
 reared at low density by limiting the number and laying time of flies
 laying in vials. For each of five vials per isofemale line, five mated
 females were placed on 10ml standard potato media, supplemented with live
 yeast, and left to oviposit for 72 hrs. This typically resulted in 30 - 50
 offspring per vial (Paluma mean = 34.8, Danbulla mean = 53.0, Mt Lewis
 mean = 45.3), which is well below the maximum larval density that can be
 supported by this volume of media. Eclosing offspring were sexed over
 three days under CO<sub>2</sub> anaesthesia within 24 hours of
 emergence to ensure they were unmated. Sexes were then held separately in
 food vials at densities of ≤ 10 flies per vial for 5 – 7 days to allow
 them to recover from effects of CO<sub>2</sub>, and to ensure
 they were reproductively mature. Crosses were established by placing a
 single virgin female in a vial with a single virgin male. Mating pairs
 were left in food vials to mate and lay for 72 hrs. Offspring were sexed
 upon eclosion (± 12 hrs) over a four day period to obtain flies for use in
 trait assays.  <strong>Measurement of
 quantitative traits</strong> Flies emerging from each cross
 were screened for cold tolerance and wing size. Wing size was measured on
 male offspring, and cold tolerance was measured on female offspring. All
 offspring were virgin prior to screening. Approximately 3 - 4 offspring
 from each cross were screened for each trait, or as many flies as were
 available if the cross produced fewer than this. Each fly was only
 screened for a single trait. Flies were held at 19 °C on a 12 hr:12 hr
 light:dark cycle until they were assayed. Cold
 tolerance Cold tolerance was assayed on
 13-day-old females as the productivity of a female following a cold shock,
 since reproduction is often more sensitive than viability to environmental
 stress (Hoffmann 2010). Flies were placed individually in 40 ml vials
 containing 10 ml of standard potato food media and cold-shocked for 2
 hours at 0 °C, which put all flies into a cold-induced coma. After cold
 shock, flies were left to recover at 25 °C for one hour (long enough for
 the vast majority of flies to emerge from chill coma). A single virgin
 male from a mass bred stock population was then added to each vial. The
 flies were left to mate and lay for 48 hrs, and then removed. The total
 number of flies emerging from each vial was counted as a measure of
 productivity. Any females that did not produce any offspring were excluded
 from analysis of productivity because reproductive failure could occur for
 reasons other than cold stress. Wing
 size To measure wing
 size, the right wing of male flies was removed and mounted on a microscope
 slide. Wings were photographed at 40 X magnification using a digital
 camera attached to a Nikon microscope. Wing images were landmarked using
 tpsDig v 2.16 (F.J. Rohlf,
 http://life.bio.sunysb.edu/morph/) at the same 10 landmarks
 used by Griffiths<em> et al.</em> (2005). Wing size was
 measured as the wing centroid size (square root of the squared distance
 between each landmark and the centroid), obtained using MorphoJ
 (Klingenberg 2011). This is given in mm.
description: A detailed description of the contents of each data file is
 included in the README.txt file.