Melanic pigmentation and light preference within and between two Drosophila species

Abstract

Environmental adaptation and species divergence often involve suites of co-evolving traits. Pigmentation in insects presents a variable, adaptive, and well-characterized class of phenotypes for which correlations with multiple other traits have been demonstrated. In Drosophila, the pigmentation genes ebony and tan have pleiotropic effects on flies' response to light, creating the potential for correlated evolution of pigmentation and vision. Here, we investigate differences in light preference within and between two sister species, Drosophila americana and D. novamexicana, which differ in pigmentation in part because of evolution at ebony and tan and occupy environments that differ in many variables including solar radiation. We hypothesized that lighter pigmentation would be correlated with a greater preference for environmental light and tested this hypothesis using a habitat choice experiment. In a first set of experiments, using males of D. novamexicana line N14 and D. americana line A00, the light-bodied D. novamexicana was found slightly but significantly more often than D. americana in the light habitat. A second experiment, which included additional lines and females as well as males, failed to find any significant difference between D. novamexicana-N14 and D. americana-A00. Additionally, the other dark line of D. americana (A04) was found in the light habitat more often than the light-bodied D. novamexicana-N14, in contrast to our predictions. However, the lightest line of D. americana, A01, was found substantially and significantly more often in the light habitat than the two darker lines of D. americana, thus providing partial support for our hypothesis. Finally, across all four lines, females were found more often in the light habitat than their more darkly pigmented male counterparts. Additional replication is needed to corroborate these findings and evaluate conflicting results, with the consistent effect of sex within and between species providing an especially intriguing avenue for further research.

Keywords: Drosophila americana; Drosophila novamexicana; behavioral choice experiment; correlated traits; ebony; histamine; light preference; melanin; pigmentation; pleiotropy; tan; vision.

FIGURE 1

Drosophila americana and D. novamexicana differ in abdominal pigmentation, a trait influenced by the pleiotropic genes ebony and tan. (a) Female and male flies of D. americana (lines A04, A00, and A01) and D. novamexicana (line N14). Young adult flies of each taxon were collected and photographed in 2021, within a single 2‐hr period under constant lighting conditions. In each case, the lateral view (left) and the dorsal view (right) show the same individual. (b) The balance of ebony and tan expression helps determine cuticular pigmentation. (c) The same genes, ebony and tan, also participate in histamine recycling in the visual system. (b) and (c) are redrawn from Takahashi (2013)

FIGURE 2

Behavioral choice trials were conducted using “light” versus “dim” artificial habitats. (a) Experimental design for mixed‐species versus single‐taxon experiments. Each cage is divided into a light habitat (white background) and a dim habitat (gray background) and is initially populated with 5 flies of each taxon per side. Dark brown ovals, D. americana‐A00. Light brown ovals, D. novamexicana‐N14. Drawings not to scale. (b) Fly cage with 15 cm ruler for scale. The purple dish is filled with instant fly food and is matched with a corresponding food dish on the dark side of the cage

FIGURE 3

In mixed‐species trials of male flies, Drosophila americana line A00 is found less often in the “light” habitat than D. novamexicana line N14. The number of successful trials is shown above each data column. A mean value was calculated across the 6 days of each successful trial. Bars represent the range, boxes represent quartiles, and horizontal lines inside the boxes mark the median, for each set of mean values. White bars show the results from 12 p.m. data collection in 2017, 2018, and 2019 combined; D. novamexicana was found in the light significantly more often than D. americana (Z = 6.003; p < .001). The gray and dotted bars show only the 2019 data, collected at 12 p.m. and 4 p.m., respectively. Within each collection time, D. novamexicana was found in the light significantly more often than D. americana (12 p.m.: Z = 6.789; p < .001; 4 p.m.: Z = 8.199; p < .001), but there was also a significant effect of data collection time with more flies found in the light habitat at 4 p.m. (Z = 2.951; p < .01)

FIGURE 4

Cage temperature is consistent across habitats. Bars represent the range, boxes represent quartiles, and horizontal lines inside the boxes mark the median. Sample size is shown above each data column. Data were collected once per day, for 6 days, on each of two cages, in 2019. Temperature did not differ significantly between light habitat and dark habitat (paired t test: t = 0.848, df = 23, p = .405)

FIGURE 5

In single‐taxon, single‐sex trials, females are consistently found in the “light” habitat more often than males. Taxa are arranged along the X‐axis from darkest to lightest. Lines A04, A00, and A01 are D. americana; line N14 is D. novamexicana. The number of successful trials is shown above each data column. Data were collected across five different experiments in 2020, at 12 p.m. daily. A mean value was calculated across the 6 days of each successful trial. Bars represent the range, boxes represent quartiles, and horizontal lines inside the boxes mark the median, for each set of mean values. Males were found less often in the light than females (Z = −7.454, p < .001). Drosophila americana‐A04 and Drosophila americana‐A01 were more often in the light habitat than D. americana‐A00 (Z = 2.134, p < .05 and Z = 4.452, p < .001, respectively), while D. novamexicana‐N14 did not differ significantly from D. americana‐A00 (Z = −0.641, p > .05)