Drosophila embryogenesis scales uniformly across temperature in developmentally diverse species

Abstract

Temperature affects both the timing and outcome of animal development, but the detailed effects of temperature on the progress of early development have been poorly characterized. To determine the impact of temperature on the order and timing of events during Drosophila melanogaster embryogenesis, we used time-lapse imaging to track the progress of embryos from shortly after egg laying through hatching at seven precisely maintained temperatures between 17.5 °C and 32.5 °C. We employed a combination of automated and manual annotation to determine when 36 milestones occurred in each embryo. D. melanogaster embryogenesis takes [Formula: see text]33 hours at 17.5 °C, and accelerates with increasing temperature to a low of 16 hours at 27.5 °C, above which embryogenesis slows slightly. Remarkably, while the total time of embryogenesis varies over two fold, the relative timing of events from cellularization through hatching is constant across temperatures. To further explore the relationship between temperature and embryogenesis, we expanded our analysis to cover ten additional Drosophila species of varying climatic origins. Six of these species, like D. melanogaster, are of tropical origin, and embryogenesis time at different temperatures was similar for them all. D. mojavensis, a sub-tropical fly, develops slower than the tropical species at lower temperatures, while D. virilis, a temperate fly, exhibits slower development at all temperatures. The alpine sister species D. persimilis and D. pseudoobscura develop as rapidly as tropical flies at cooler temperatures, but exhibit diminished acceleration above 22.5 °C and have drastically slowed development by 30 °C. Despite ranging from 13 hours for D. erecta at 30 °C to 46 hours for D. virilis at 17.5 °C, the relative timing of events from cellularization through hatching is constant across all species and temperatures examined here, suggesting the existence of a previously unrecognized timer controlling the progress of embryogenesis that has been tuned by natural selection as each species diverges.

Figure 1. Geographic and climatic origin and phylogeny of analyzed Drosophila species.

(A) Ancestral ranges are shown for each species , ,. While D. melanogaster and D. simulans are now cosmopolitan and D. ananassae is expanding in the tropics (green), their presumed ancestral ranges are shown. D. virilis is holarctic (gray) and restricted from the tropics, with a poor understanding of its ancestral range. Other species are more or less found in their native ranges, covering a variety of climates. Sites of collection are noted by arrows. (B) The phylogeny of the sequenced Drosophila species. Many of the tropical species are closely related, though D. willistoni serves as a tropical out-group compared to the melanogaster and obscura groups. Branch lengths are based on evolutionary divergence times . (C) Range sizes vary considerably between the species.

Figure 2. Developmental landmarks used in study.

Many images of each stage (examples on the left) were averaged to generate composite images (lateral view on the right) for each of the developmental stages, of which 29 are shown.

Figure 3. Developmental time of D. melanogaster varies with temperature.

(A) Images of developing D. melanogaster embryos at each temperature are shown for a selection of stages to highlight the overall similarity of development. (B) The time individual animals reached the various time-points are shown, with each event being a different color. Time 0 is defined as the end of cellularization, when the membrane invagination reaches the yolk. Between 17.5°C and 27.5°C the total time of embryogenesis, measured as the mean time between cellularization and trachea fill, has a logarithmic relationship to temperature described by where T is temperature in °C (). (C) The developmental rate in D. melanogaster changes uniformly with temperature, not preferentially affecting any stage. Timing here is normalized between the end of cellularization and the filling of the trachea.

Figure 4. Drosophila species develop at different rates and respond to temperature in distinct ways.

(A) Images of developing embryos of each species are shown to scale. All species go through the same stages in the same order at all viable temperatures. (B) At 17.5°C all species show uniformly long developmental times, with D. virilis and D. mojavensis being significantly longer than other species. (C) At 22.5°C and (D) 27.5°C there is considerably more variation between species. While developmental times decrease with increasing temperature across all species, the effect is muted in the alpine species. (E) At 30°C, developmental rate has stopped accelerating and the alpine species are seeing considerable slow-down in development time.

Figure 5. Temperature dependent developmental rates are climate specific.

The time between the end of cellularization and trachea fill are shown for all species at a range of temperatures. The climatic groups – tropical (warm colors), alpine (blues), temperate (purple), and sub-tropical (green) – clearly stand out from one another to form four general trends.

Figure 6. Proportionality of developmental stages is not affected by non-heat-stress temperatures.

(A) Across species, development maintains the same proportionality. D. pseudoobscura stands out as not being co-linear at higher temperatures. Instead, the later part of its development is slowed and takes up a disproportionally long time. (B) Plotting proportionality across all species and all temperatures reveals the approximately normally distributed proportionality of all morphological stages.