Evolution of larval segment position across 12 Drosophila species

Abstract

Many developmental traits that are critical to the survival of the organism are also robust. These robust traits are resistant to phenotypic change in the face of variation. This presents a challenge to evolution. In this article, we asked whether and how a well-established robust trait, Drosophila segment patterning, changed over the evolutionary history of the genus. We compared segment position scaled to body length at the first-instar larval stage among 12 Drosophila species. We found that relative segment position has changed many times across the phylogeny. Changes were frequent, but primarily small in magnitude. Phylogenetic analysis demonstrated that rates of change in segment position are variable along the Drosophila phylogenetic tree, and that these changes can occur in short evolutionary timescales. Correlation between position shifts of segments decreased as the distance between two segments increased, suggesting local control of segment position. The posterior-most abdominal segment showed the highest magnitude of change on average, had the highest rate of evolution between species, and appeared to be evolving more independently as compared to the rest of the segments. This segment was exceptionally elongated in the cactophilic species in our dataset, raising questions as to whether this change may be adaptive.

Keywords: Drosophila; evolution; larval stage; robustness; segment patterning.

Figure 1

Example larval images and description of measurements. Dark field images of first‐instar larvae are shown for three species. Drosophila ananassae is presented as it is closest to the mean of all species for segment positions, while D. persimilis and D. mojavensis are exceptional, especially in the length of their most posterior segments. Red lines mark the anterior border and white lines mark the posterior border of denticle belts, which are rows of bristles on the ventral surface of the animal and are recognized by our image processing program. The anterior border of the denticle belt was used as a proxy for segment border. To measure the position of an abdominal segment (e.g. A1) relative to the body length (Y), the distance from the anterior border of the larvae to the anterior border of that segment (X) is divided by the total body size (Y). Region encompassing Head, T1, T2, T3 is referred to as “h+t” throughout the main text. T, thoracic segment; A, abdominal segment.

Figure 2

Relative segment position and body size is highly variable among 12 species of Drosophila. (A) 12 Drosophila species on an evolutionary tree (Russo et al. 1995; Obbard et al. 2012; Russo et al. 2013). Adult male species photos from Nicolas Gompel were downloaded from FlyBase. (B) Each black bar represents the mean position in percent larval length for each segment in each species. The color spread on the two sides of each black bar is 95% confidence interval. Each segment is represented by a different color. x‐axis is relative segment position in percent larval length. A, abdominal segment. (C) Each black bar represents the average first‐instar larval body length for each species. The gray shaded area around the black bars is 95% confidence interval. x‐axis is average body length in micrometers.

Figure 3

Coordinated direction of relative segment position changes in 12 Drosophila species. Neighboring segments tend to shift together in a particular direction, toward the anterior or posterior of the larva rather than shifting in opposite directions. Yellow indicates anterior, blue indicates posterior shift in relative position as compared to the mean position across all species for a given segment. The area of the circle is proportional to the size of the shift in relative segment position.

Figure 4

Phylogenetic analysis of segment evolution in 12 Drosophila species. (A) Phylogeny inferred from nuclear loci with relative divergence times. Branches of the tree are colored to indicate the overall rate of relative segment position evolution. Rates are variable across the phylogeny, with some big differences in rate observed even between closely related species. (B) Boxplots indicate the posterior distribution of the evolutionary rate of relative segment position. The bar indicates the posterior mean rate; the boxes and whiskers indicate the 50% and 95% posterior credible intervals, respectively. The rate is higher for the most posterior segments, especially A7 and A8. (C) Marginal posterior probability distribution of the evolutionary rates of relative segment position across branches. Most changes are small in magnitude, with occasional larger changes.

Figure 5

Correlations between segment positions, within, and between species. (A) Within a species (here, D. melanogaster is shown as an example), correlations between relative segment positions are highest for neighboring segments, and fall off as the physical distance between two segments increases. Colors are used to distinguish between segment pairs with different number of segments separating them. This suggests local control of segment positioning. For plots for the rest of the species see Fig. S10. (B) This plot is similar to that in part A, but instead shows averages of correlation coefficients between segment positions over all species. Similarly, correlations between relative segment positions are highest between neighboring segments, but fall as distance between segments increase. In red are all comparisons with segment A8. At any distance, correlations with this segment are lower (in black are all other comparisons not involving A8). The gray and red shaded areas around the black and red points, respectively, indicate their 95% confidence intervals. This demonstrates that changes in the position of A8 are less correlated, and thus, are more independent, than changes in the position of the rest of the segments, given its distance from any other segment. The unit of the x‐axes in panels A and B, for mean relative distance between segments, is percent larval length. (C) Between species, correlations between evolutionary rates for a pair of segments are lower with increasing distance between segment pairs. This plot represents a similar analysis to part B, but in the phylogenetic framework and correlates rates of evolution for each segment. Color indicates the first segment in the comparison, with the most‐blue indicating comparisons with segment 1, and the most yellow being comparisons with segment 7. Rates of evolution are highly correlated between neighboring segments, and correlations decrease with increasing distance between segments.