Typical gas chromatograms of headspace for the four cactus hosts.

<p>(<b>A–D</b>) Barrel, prickly pear, organ pipe and agria cactus headspaces, respectively, from fermented samples are shown. Peak numbers correspond to compounds identified in time course experiment presented in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0070027#pone.0070027.s001" target="_blank">Figure S1</a>. (<b>E</b>) Principal component (PC) analysis of the volatile samples from all four cacti. The eigenvectors for the PCs are provided in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0070027#pone.0070027.s004" target="_blank">Table S3</a>. The fifty percent density eclipses for the cacti are indicated with different line styles.</p

Behavioral responses to single compounds.

<p>Dose response indices (mean ± std error) of males (M) and females (F) for single compounds. Significant differences among <i>D. mojavensis</i> populations are denoted by different letters above the bars.</p

Divergence in Olfactory Host Plant Preference in <i>D. mojavensis</i> in Response to Cactus Host Use

<div><p>Divergence in host adaptive traits has been well studied from an ecological and evolutionary perspective, but identification of the proximate mechanisms underlying such divergence is less well understood. Behavioral preferences for host plants are often mediated by olfaction and shifts in preference may be accompanied by changes in the olfactory system. In this study, we examine the evolution of host plant preferences in cactophilic <i>Drosophila mojavensis</i> that feeds and breeds on different cacti throughout its range. We show divergence in electrophysiological responses and olfactory behavior among populations with host plant shifts. Specifically, significant divergence was observed in the Mojave Desert population that specializes on barrel cactus. Differences were observed in electrophysiological responses of the olfactory organs and in behavioral responses to barrel cactus volatiles. Together our results suggest that the peripheral nervous system has changed in response to different ecological environments and that these changes likely contribute to divergence among <i>D. mojavensis</i> populations.</p></div

Site fidelity.

<p><b>A:</b> Overview of all animals showing site fidelity and short-distance movements only from all three years of observations. <b>B:</b> Higher magnification of animal No. 1520 that remained within one home range during the plotted period (19 December 2010 to 09 January 2011); open symbols: daytime fixes, solid symbols: nighttime fixes. <b>C and C1:</b> higher magnifications of animal 1500 that occupied three home ranges between 5 December 2010 and 17 January 2011. In C1 this period is sectioned in three episodes. <b>D: </b><i>B. latro</i> uses hollow trees as daytime refuges. <b>E:</b> A tagged animal hiding in a rock crevice during the day. <b>F, G: </b><i>B. latro</i> are strongly attracted to freshly fallen Lister's palms. Cut Arenga: site of a freshly fallen <i>Arenga listeri</i> palm. Scale bars in A, B, C, C1: 100 m.</p

Changes in behavioral preferences with host plant fermentation stage.

<p>(<b>A</b>) <i>D. mojavensis</i> populations specialize on different host cacti across their range. Its sibling species, <i>D. arizonae</i> uses columnar cactus and <i>Opuntia</i> as hosts. (<b>B–E</b>) Two choice behavioral preferences of males (M) and females (F) of the Mojave, S. Catalina, mainland Sonoran, and Baja populations for their own respective host plants. Behavioral preferences for uninoculated (NI) host cactus in comparison to fermented host cactus and preferences for different stages of fermentation of a given host cactus are shown. Cactus tissues were fermented for one to nine weeks (W1– W9). For the mainland Sonoran population, comparisons between uninoculated and five week fermented organ pipe are shown as no significant difference between uninoculated and one week fermented organ pipe was found (data not shown). For panels <b>B–E</b>, behavioral preferences are shown as mean ± standard error and significance within a given sex and choice test is depicted by asterisks (*: <i>P</i><0.05; ** <i>P</i><0.01; ***: <i>P</i><0.001; ****: P<0.0001).</p

Displacement-induced search behavior.

<p><b>A:</b> higher magnification of animal No. 1731 that was translocated roughly parallel to the coast out of the migratory corridor towards the west (compare pale blue line in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0049809#pone-0049809-g005" target="_blank">Fig. 5D</a>) and monitored from 15 July to 9 September 2011. The animal's behavior is much different from that of animals with typical site fidelity (compare <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0049809#pone-0049809-g003" target="_blank">Fig. 3B, C</a>). <b>B–K:</b> Search behavior sectioned in ten migratory episodes (for dates, compare boxed legend) shows excursions in almost all compass directions. Scale bars: 50 m.</p

Displacement experiments and homing.

<p><b>A, B:</b> Homing course of animals that were picked up at the house symbols and translocated in Y-axis direction towards the flagged release sites in the south (A) or the north (B). For all animals, the date (see boxed legend) and position of the first and last GPS fixes are given in addition to the track. Note that animal No. 1717 (green in B) was translocated twice and both times homed back to its pick-up site on an identical path. <b>C:</b> Two animals that were picked up at the house symbols and translocated roughly parallel to the coast out of the migratory corridor towards the flagged release sites in the east. Whereas animal No. 1720 remained in the release area, No. 1729 migrated to the coast, but ca. 1.5 km east of the regular migratory corridor. It then reversed its path to migrate far north past the release site. <b>D:</b> Two animals, Nos. 1725 and 1731, translocated roughly parallel to the coast out of the migratory corridor towards the flagged release sites in the west did not home back to the pickup area (orange symbols). Two animals translocated from north to south (blue symbols) within the migratory corridor during the 2011 mission also remained within the release area. Scale bars: 200 m.</p

Circadian rhythm of locomotory behavior. Upper panel:

<p>Relative activity = percentage of mean distances between GPS-fixes per total number of GPS-fixes ± SD [percentage of m/fix]. Horizontal bars indicate times of daylight (white) and night (black). <b>Lower panel:</b> Daily fluctuations of GPS fixes of all attached tags of the 2008, 2010, and 2011 missions (colored solid lines) and a stationary test tag (black dashed line). Data suggest that many animals hide in refuges during daytime thus blocking GPS reception. During the 2008 and 2010 missions (wet season) migratory activity of the animals begins at dusk to reach a maximum between 20:00 and 22:00.</p

Data from three years of GPS telemetric experiments on Christmas Island.

*<p><i>N</i> = 42 (only 2010 and 2011).</p

Long-distance Y-axis migrations.

<p><b>A:</b> Overview of all animals showing long-distance movements within a migratory corridor that extends from the coastal terrace ca. 2 km inland (data pooled from all three missions). Color ranges define point density estimates in points per km² using only data of directed movements (points of stationary phases or undirected movements were removed) <b>B:</b> One of several animals exposed to ocean spray as observed in the upper openings of the blowholes at the coastal terrace (compare <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0049809#pone-0049809-g001" target="_blank">Fig. 1J</a>). <b>C:</b> example of migratory animal No. 621, monitored from 5 December 2008 to 12 January 2009, and displaying typical Y-axis migration (with the X-axis being the shoreline) during this period. C1 and C2 section this migration into the outbound and inbound episodes. Note that in- and outbound paths are identical (permitting for the GPS error of max. 20 m); open symbols: daytime fixes, solid symbols: nighttime fixes. <b>D–P:</b> higher magnifications of all animals showing long-distance movements. For every animal, position and date (see boxed legend) of the tag application is plotted as well as dates and positions of the first and last obtained GPS fixes. Scale bars: 100 m.</p