Genomic linkage of male song and female acoustic preference QTL underlying a rapid species radiation Kerry L. Shaw1 and Sky C. Lesnick Department of Biology, University of Maryland, College Park, MD 20742 Edited by May R. Berenbaum, University of Illinois at Urbana-Champaign, Urbana, IL, and approved April 10, 2009 (received for review January 9, 2009) The genetic coupling hypothesis of signal-preference evolution, directional sexual selection when signal and preference distri- whereby the same genes control male signal and female prefer- butions are mismatched within a population (11). Two recent ence for that signal, was first inspired by the evolution of cricket studies, one in the olfactory realm (12) and the other in the visual acoustic communication nearly 50 years ago. To examine this realm (13), have suggested that physical linkage or pleiotropy hypothesis, we compared the genomic location of quantitative might bolster behavioral coupling of sexual signaling in the face trait loci (QTL) underlying male song and female acoustic prefer- of divergent evolution, an idea with a long history (1, 6) but little ence variation in the Hawaiian cricket genus Laupala. We docu- empirical support. Furthermore, recent modeling suggests that ment a QTL underlying female acoustic preference variation be- physical linkage can promote a genetic correlation between tween 2 closely related species (Laupala kohalensis and Laupala signal and preference and thereby enhance coevolution in sexual paranigra). This preference QTL colocalizes with a song QTL iden- signaling systems (14). tified previously, providing compelling evidence for a genomic In crickets (family Gryllidae), males sing with specialized struc- linkage of the genes underlying these traits. We show that both tures on the forewings to produce a calling song, and females song and preference QTL make small to moderate contributions to respond to the calling song of potential mates by walking toward the the behavioral difference between species, suggesting that diver- gence in mating behavior among Laupala species is due to the acoustic source. These conspicuous sexual communication behav- fixation of many genes of minor effect. The diversity of acoustic iors first inspired the genetic coupling hypothesis as a possible mechanism to explain widespread and coordinated evolution of signaling systems in crickets exemplifies the evolution of elaborate EVOLUTION male displays by sexual selection through female choice. Our data male signal and female preference (1). The distinctiveness of male reveal genetic conditions that would enable functional coordina- song (and female acoustic preference when it has been tested) (15) tion between song and acoustic preference divergence during has proven highly useful in resolving species-level relationships, and speciation, resulting in a behaviorally coupled mode of signal- thousands of species initiate courtship using such cues (e.g., see refs. preference evolution. Interestingly, Laupala exhibits one of the 16 and 17). Research on male song and female acoustic preference fastest rates of speciation in animals, concomitant with equally in some species has shown a genetic correlation between song and rapid evolution in sexual signaling behaviors. Genomic linkage preference (18), a pattern that could simply be because of assor- may facilitate rapid speciation by contributing to genetic correla- tative mating and sexual selection, or due to physical linkage of tions between sexual signaling behaviors that eventually cause these traits (or both). Behavioral coupling in parental species and sexual isolation between diverging populations. behavioral intermediacy of songs and preferences in first generation hybrids have yielded genetic insights (2) but cannot address the Laupala ͉ sexual isolation ͉ signal ͉ speciation question of physical linkage (3). However, in a segregating hybrid population (e.g., an F2 intercross or backcross) the role for physical he evolutionary and genetic mechanisms causing variation in linkage or pleiotropy as a potential cause of genetic correlations can Tsexual signaling systems have challenged biologists for decades be tested. (1–6). On the one hand, sexual communication between males and Species of the endemic Hawaiian cricket genus Laupala are females should be evolutionarily constrained given that signals must morphologically and ecologically cryptic, and they exhibit the match preferences to remain functional. If signals or preferences fastest rate of speciation known among invertebrates (19). We outside the natural distribution of variation cause their bearers to tested the hypothesis of genetic linkage between signal and suffer a mating disadvantage they should be under stabilizing sexual preference in this system, where both male calling songs and selection (3). This stabilizing selection likely explains the frequent female acoustic preferences have diverged repeatedly (17, 20– observation of behavioral coupling within species. On the other 22) and contribute to sexual isolation (15) among closely related hand, even the most closely related species are typically differen- species. Laupala species with distinct songs can be hybridized, tiated in components of the sexual signaling system (7–8), demon- enabling studies of interspecific genetic architecture (23). Here, strating frequent divergence in mating signals and responses. Be- we employ a quantitative trait locus (QTL) approach to map cause sexual signaling systems are nearly always distinct among female acoustic preference loci that differ between the closely species, and their differences can cause sexual isolation among related Laupala paranigra and Laupala kohalensis, and we species, the evolution of sexual signaling is likely a powerful force combine these results with previous QTL work on male calling driving speciation (9, 10). Despite a long history of interest in speciation and sexual selection, the evolutionary processes and the song differences between these same species (23). We demon- genetic conditions by which sexual signals diverge among species remain poorly understood. Author contributions: K.L.S. designed research; K.L.S. and S.C.L. performed research; K.L.S. Theory suggests several mechanisms can cause the coevolu- contributed new reagents/analytic tools; K.L.S. and S.C.L. analyzed data; and K.L.S. and tion of signals and preferences, and much empirical evidence S.C.L. wrote the paper. supports a role for mate choice in divergent evolution of sexual The authors declare no conflict of interest. signaling (11). Sexual selection models often assume free re- This article is a PNAS Direct Submission. combination between signal and preference loci, yet predict that 1To whom correspondence should be sent at the present address: Department of Neuro- a genetic correlation between signal and preference behaviors biology and Behavior, Cornell University, Ithaca, NY 14853. E-mail: [email protected]. will arise within a population because of assortative mating. Such This article contains supporting information online at www.pnas.org/cgi/content/full/ genetic correlations can accentuate the evolutionary effects of 0900229106/DCSupplemental. www.pnas.org͞cgi͞doi͞10.1073͞pnas.0900229106 PNAS ͉ June 16, 2009 ͉ vol. 106 ͉ no. 24 ͉ 9737–9742 Downloaded by guest on September 29, 2021 maps, respectively). Based on segregation patterns from 181 F2 Kauai males and 47 F2 females, 8 linkage groups for the L. paranigra Oahu Maui map (Lp) (Fig. 2A) and 8 linkage groups for the L. kohalensis map (Lk) (Fig. 2B) were estimated (23), equal to the number A 30 L. kohalensis predicted by cytological analysis (24). Most AFLP markers are 25 Hawaii dominant, but inclusion of several codominant AFLP markers 20 allowed alignment of the Lp and Lk maps (Fig. S1) (23). Map size estimates were 613.1 and 476.6 cM, and average marker spacings 15 L. paranigra were 7.5 and 5.8 cM for Lp and Lk, respectively (23). 10 F1 hybrid QTL Analysis. 5 male song We identified a single acoustic preference QTL on Number of males Linkage Group 1 in both Lp and Lk maps (Fig. 2), based on those 0 females whose phenotypic and genotypic data were available 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 (n ϭ 26). QTL analyses for preference and song were performed B 14 by using interval mapping (IM) and composite interval mapping 12 F2 hybrid (CIM) with 5% experiment-wide error thresholds (Fig. 2) (25). female preference The logarithm of odds (LOD) peak on Lp6b exceeded the 10% 10 threshold and was used as a covariate in the Lp CIM analysis 8 (Fig. 2A). The orthologous region to Lp6b has not yet been 6 identified in Lk (Fig. S1) (23), thus preference analysis of Lk was 4 only performed with IM (Fig. 2B). The estimated map positions 2 (location of maximum LOD score; Table 1) for the preference Number of females 0 QTL on Lp1 and Lk1 coincide with estimated map positions for 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 song QTL from previous analyses (23). The marker closest to the estimated QTL (PcaacA5B12) is identical in all analyses and the C 60 1.5 LOD support intervals (26) coincide between song and F2 hybrid 50 preference in both Lp and Lk (Fig. 2). male song The additive effect of the preference QTL was significantly 40 different from 0 (t test, P Ͻ 0.05), and no dominance deviation of males 30 was detected. In the Lp and Lk analyses, the estimated effect 20 sizes for the preference QTL were 0.45 and 0.44 pps, corre- sponding to 15% and 14.6% of the pulse rate difference between 10 Number L. paranigra and L. kohalensis, and explaining 47.1% and 47.7% 0 of the F2 preference variance, respectively (Table 1). 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Pulses per second Discussion Fig. 1. Geographic range and male pulse rate variation of L. paranigra, L. Sexual signals and their responses are conspicuous, often ornate, kohalensis, and F1 hybrids (A) (23); F2 hybrid female pulse rate preference features of animal behavior.