The impact of life stage and pigment source on the evolution of novel warning signal traits

Abstract

Our understanding of how novel warning color traits evolve in natural populations is largely based on studies of reproductive stages and organisms with endogenously produced pigmentation. In these systems, genetic drift is often required for novel alleles to overcome strong purifying selection stemming from frequency-dependent predation and positive assortative mating. Here, we integrate data from field surveys, predation experiments, population genomics, and phenotypic correlations to explain the origin and maintenance of geographic variation in a diet-based larval pigmentation trait in the redheaded pine sawfly (Neodiprion lecontei), a pine-feeding hymenopteran. Although our experiments confirm that N. lecontei larvae are indeed aposematic-and therefore likely to experience frequency-dependent predation-our genomic data do not support a historical demographic scenario that would have facilitated the spread of an initially deleterious allele via drift. Additionally, significantly elevated differentiation at a known color locus suggests that geographic variation in larval color is currently maintained by selection. Together, these data suggest that the novel white morph likely spread via selection. However, white body color does not enhance aposematic displays, nor is it correlated with enhanced chemical defense or immune function. Instead, the derived white-bodied morph is disproportionately abundant on a pine species with a reduced carotenoid content relative to other pine hosts, suggesting that bottom-up selection via host plants may have driven divergence among populations. Overall, our results suggest that life stage and pigment source can have a substantial impact on the evolution of novel warning signals, highlighting the need to investigate diverse aposematic taxa to develop a comprehensive understanding of color variation in nature.

Keywords: Aposematism; carotenoids; chemical defense; ecological genetics; host adaptation; polytypic coloration.

Figure 1

Appearance and distribution of white and yellow color morphs of Neodiprion lecontei larvae. Photographs depict representative yellow‐bodied (a) and white‐bodied (b) larvae (both photos by R. K. Bagley). (c) Approximate collecting locations and recorded color of 823 N. lecontei larval colonies collected between 2001–2004 and 2009–2016.

Figure 2

White and yellow Neodiprion lecontei larvae are aposematic and do not differ in signal efficacy. (a) Color contrasts against different host plants and host‐plant parts. For reference, JND values for prey/background combinations that are <1 are indistinguishable, values between <1 and 3 are hard to distinguish unless under optimal conditions, and values >5 are easy to tell apart under most conditions (Vorobyev and Osorio 1998). Dashed line shows the threshold value for JND = 5 above which objects should appear clearly conspicuous for blue tits (Cyanistes caeruleus). Changes in attack latency (seconds) (b) across three trials for three different types of pine sawfly in avoidance learning assays. Error bars are 95% confidence intervals.

Figure 3

Genome‐wide markers reveal no evidence of color‐associated population structure in Neodiprion lecontei, but differentiation at a single color locus is elevated between white‐bodied and yellow‐bodied populations. (a) Color, location, and sample size for 65 individuals and 29 locations included in the population structure analyses. Points are colored based on the color of all larvae collected at that location, with size of the point reflecting number of individuals (colonies) sampled. (b) Inferred ancestry for each individual sorted by the color of the colony from which it was collected under a model of K = 2 based on the program admixture (top) and adegenet (bottom). Colors are denoted by the “larval color” bar, with colors indicated as in (a). Within each color, individuals are sorted in geographical order. The “West” genetic cluster contains only yellow larvae, whereas the “East” genetic cluster contains yellow, white, and mixed‐color colonies. (c) Relationship between observed heterozygosity and latitude for “East” individuals with colony color indicated as in panel A. Heterozygosity declines with latitude, and the lowest‐heterozygosity individuals are from yellow‐bodied populations collected in the northernmost part of the range. (d) Relationship between observed heterozygosity and longitude for “East” individuals with colony color indicated as in panel A. Heterozygosity declines with longitude. (e) Per‐site differentiation (F _ST) between white‐bodied and yellow‐bodied populations of N. lecontei (“East” populations only) across seven linkage groups and remaining unplaced scaffolds. Shaded gray boxes denote the locations of six QTL associated with larval body color in a cross between white‐bodied and yellow‐bodied populations (Linnen et al. 2018). The red point and asterisk indicate the only significant F _ST outlier, which falls within two major‐effect QTL intervals.

Figure 4

Relationship between host use and larval body color in N. lecontei.  (a) Compared to Pinus echinata and P. virginiana, P. rigida had a significantly higher proportion of white‐bodied larval colonies. Error bars are Clopper‐Pearson 95% confidence intervals. (b) Compared to P. echinata and P. virginiana, P. rigida foliage had significantly lower carotenoid content. In all panels, letters indicate significant pairwise differences between colony color or host plants at P < 0.05.