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Color polymorphism often is associated with alternative reproductive strategies and may reﬂect different adaptive optima that coexist within populations. The equilibrium among morph frequen- cies is maintained by the occurrence of opposite selective pressures (disruptive vs. stabilizing), which promote polymorphism while preserving gene ﬂow. Sexual selection may contribute on both sides, particularly when morphs do not mate randomly. Reptiles offer a good model, notably lizards. Nevertheless, previous studies on mate choice in polymorphic lizards have generated con- trasting results, with some studies suggesting that female morphs might tune their preference de- pending on environmental/social conditions such as crowding. We experimentally manipulated the number of individuals a female common wall lizard Podarcis muralis perceives around her, to test if females of different morphs (white or yellow) tune their choice for white and yellow males in order to maximize the probability that hatchlings follow the strategy best adapted to the population density. Results showed that crowding experienced by females did not affect mate choice, arguing against a ﬂexible choice strategy by females. However, white females signiﬁcantly associated with white males, whereas yellow females did not signiﬁcantly associate with yellow males. Thus, sex- ual selection could contribute to the maintenance of color polymorphism in this species by a mix of assortative and non-assortative mating strategies, which could maintain the equilibrium between gene divergence and gene ﬂow among morphs. Key words: ﬂexible female choice, population density, color polymorphism, sexual selection. Color polymorphisms often associate with alternative reproductive selection), which preserves gene flow among morphs (Sinervo and strategies, which involve specific trade-offs among behavioral, mor- Svensson 2002). There is substantial evidence that natural selection phological, physiological, and life history characteristics (Sinervo maintains color morphs, notably negative frequency-dependent se- and Lively 1996; Svensson et al. 2001; Sacchi et al. 2007a, 2009), lection (reviewed in Gray and McKinnon 2007). and represent different adaptive optima that may coexist within Sexual selection may also promote heritable color polymorph- populations (Calsbeek et al. 2010). The evolutionary stable ism, particularly when individuals of alternative morphs do not co-occurrence of these alternative optima within a single species is mate randomly (Wellenreuther et al. 2014). In particular, a strong generally regarded as the result of the equilibrium between opposite sexual selection pressure may reinforce the divergence among alter- selective pressures: disruptive selection (i.e., correlational selection), native morphs, promoting morph maintenance (Corl et al. 2010; which promotes the association between life history traits and Pe ´ rez i de Lanuza et al. 2017), and a variation in the intensity of sex- coloration, and the stabilizing selection (e.g., frequency-dependent ual selection could drive inter-population differences in color morph V C The Author (2017). Published by Oxford University Press. 449 This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact email@example.com Downloaded from https://academic.oup.com/cz/article-abstract/64/4/449/4139748 by Ed 'DeepDyve' Gillespie user on 22 August 2018 450 Current Zoology, 2018, Vol. 64, No. 4 frequencies (McLean and Stuart-Fox 2014). Furthermore, the inten- sity of sexual selection could differ in males and females resulting in different morph composition between sexes (Pe ´ rez i de Lanuza et al. 2017). However, assortative mating alters the genotype frequencies without favoring one allele with respect to another one, and conse- quently it cannot maintain color polymorphism alone in the absence of other micro-evolutionary forces. In particular, positive assortative mating requires the simultaneous presence of heterozygote advan- tage or some other form of selection (such as the negative frequency- dependent selection) to maintain the polymorphism. On the contrary, disassortative mating (i.e., negative assortative mating) can by itself prevent the loss of rare phenotypes, maintaining morphs within populations (Wellenreuther et al. 2014). Figure 1. (A) The 2-choice arena used to test the female preference for white Lizards offer a good model to investigate the evolution and and yellow males. P(N): neutral area for the female; P(A) and P(B): preference maintenance of color polymorphism, because polymorphic systems compartments for male A or male B, respectively; the dashed lines represent are quite common and some underlying mechanisms have been al- the transparent plexiglas dividers separating male compartments from the fe- ready hypothesized and tested (Thompson and Moore 1991; male preference compartments. (B) An example of the trajectory of a female Thompson et al. 1993; Sinervo and Lively 1996; Zamudio and within the arena during the entire experiment produced by idTracker. Sinervo 2000; Sinervo et al. 2001; Sinervo and Zamudio 2001; Symbols as in panel A. Huyghe et al. 2007; Sacchi et al. 2007b; Runemark et al. 2010; Galeotti et al. 2013). Some studies have explored the contribution of sex linkage (Sinervo et al. 2001, Rankin et al. 2016) - white females sexual selection on color polymorphism maintenance, specifically mating consistently with white males will always produce a white off- that of female preference, but with contrasting results (Alonzo and spring, which best performs solely in high-density contexts. By con- Sinervo 2001; Healey et al. 2008; Lattanzio and Miles 2014; Sacchi trast a “flexible choice strategy” (sensu Alonzo and Sinervo 2001) et al. 2015). For example, female side-blotched lizards Uta stans- would be a more profitable solution for females in response to fluctu- buriana choose males, but do not mate following a strictly assorta- ations in population density. In this case, mating with a white male tive pattern and adopt a flexible mate choice rule according to will increase the proportion of white individuals among offspring, and population density (Alonzo and Sinervo 2001). By contrast, females mating with a yellow male will increase that of yellow ones. Hence, of the Australian painted dragon Ctenophorus pictus do not discrim- females may use local cues to project future conditions their offspring inate between single males of different head color, but preferentially will experience and, consequently, tune their preference from one associate with polymorphic as opposed to monomorphic male morph to the other. Therefore, in this article, we experimentally dyads, possibly to increase the likelihood of mating with different manipulated the population density in order to test whether the repro- males (Healey et al. 2008). Furthermore, female morphs of the tree ductive partitioning strategies of white and yellow females were tuned lizard Urosaurus ornatus actually discriminate male partners, but according to male morph in order to improve the proportion of hatch- mate preference does not relate with the color of male dewlap, but lings expressing the morph adapted more closely to the demographic rather with the secretions of their femoral pores (Lattanzio and conditions of the population. Our specific predictions are: 1) white fe- Miles 2014). males should prefer white males when reared in high-density condi- The common wall lizard Podarcis muralis is an European lacer- tion; 2) yellow females should select yellow males when reared under tid lizard showing a striking color polymorphism in both sexes, with low population density. 6 distinct morphs, including 3 pure (i.e., white, yellow, and red) and 3 intermediate phenotypes (white-red, yellow-red, and white- yellow, Figure 1 in Sacchi et al. (2013) shows a picture of male and Materials and Methods female morphs). Morphs occur within the same population, but their relative frequencies are highly variable among populations Subjects and housing conditions (Sacchi et al. 2007a, 2007b). Field observations of morph’s mating Between February and March 2015, we captured sexually mature behavior suggest that assortative mating between color morphs lizards (SVL> 50 mm, Sacchi et al. 2012) by noosing in 4 sites in the should actually occur (Pere ´ z i de Lanuza et al. 2013). However, ex- surrounding of Pavia (Lombardy, Northern Italy), which were at perimental tests failed to demonstrate that females really choose least 5 km apart each other. We collected only not-mated females as males according to their morph in a color assortative way (Sacchi determined by the lack of male’s bite signs on their belly (typically et al. 2015), despite the fact that females might be able to detect being present after copulation, Bauwens and Verheyen 1985; Sacchi morph by only smelling male scents (Pellitteri-Rosa et al. 2014). et al. 2015). Each individual was measured by a digital caliper for Female morphs display different reproductive partitioning of re- SVL, weighed, and transferred to the laboratory within 2 h from productive output, with yellow-throated females producing many capture. Overall, we housed 46 females (23 for each morph) and 31 small eggs and white-throated females producing few large eggs. This males (16 yellow and 15 white). We housed females in opaque plas- pattern has been assimilated to K/r strategies, in which yellow females tic cages (60 50 50 cm), provided with a paper sheet as substra- have been described as r-strategists, while white females as K-strat- tum and two 4-hole bricks covered by a tile as shelter. We set 2 egists (Sinervo and Zamudio 2001; Galeotti et al. 2013). In this scen- different density treatments: 1) in the “high-density” treatment, we ario, a fixed preference for a given morph would not be the best randomly assigned 24 females (12 for each morph) to 4 cages with 6 choice for females if population density changes over time. Given that individuals in each one (3 for each morph); 2) in the “low-density” color morphs in lizards apparently follow a simple Mendelian inherit- treatment, we randomly assigned 22 females (11 for each morph) to ance–that is, 3 alleles on 1 single locus or 2 alleles on 2 loci, without 11 cages with 2 individuals each (1 for each morph). We housed Downloaded from https://academic.oup.com/cz/article-abstract/64/4/449/4139748 by Ed 'DeepDyve' Gillespie user on 22 August 2018 Sacchi et al. Assortative mating in a polymorphic lizard 451 males individually in plastic cages (20 30 20 cm), provided with were interpreted as a measure of interest in the male and, thus, an a paper sheet as substratum and 2 small tiles as basking site and evaluation of the female choice. People who analyzed videos (M.B. shelter. Cages were maintained at ambient temperature in a range and A.J.C.) were “blind” with respect to both female treatment (i.e., that promotes lizard activity (15–30 C) and under natural photo- low vs. high density) and the morph of males kept in the 2 prefer- period (daylight). We maintained and fed all lizards for at least ence compartments. In order to check whether idTracker was giving 1 week before trials started and released them in their capture sites accurate results, we manually analyzed 6 randomly selected record- at the end of the experiment. Before release, we weighed the females ings, and we compared the estimates between the 2 methods. again and found that all of them maintained their original body The correlation was extremely high (Pearson’s correlation coeffi- mass (change: 0.10 g, paired-sample t-test; t ¼1.19, P ¼ 0.24). cient, r ¼ 0.988), confirming that the estimates of the times spent 43 p by females within the choice compartment obtained by idTracker were fully reliable. Experimental design We formed 44 dyads of males by randomly choosing 1 male of each Statistical analyses color morph. Mean SVL of males was 62.36 1.1 mm (range 50.0– As we simultaneously collected 2 measures (1 for each male morph) 69.6 mm) and did not differ between morphs (2-sample t-test: for each trial, we used a multivariate analysis of covariance t ¼ 0.43, P ¼ 0.66). The mean size difference between males within (MANCOVA) to compare the total time spent in the choice com- dyad was 9.0% of SVL (range: 0.1–29.6%). Mean SVL of females partments between female morphs. In this analysis, the matrix of the was 60.86 0.6 mm (range 53.8–69.1 mm) and did not differ among 2 measures was the dependent variable, whereas the female morph, morphs (2-sample t-test: t ¼ 0.38, P ¼ 0.70). We randomly as- the female body size (SVL), the density treatment, and the difference signed each female a dyad, and we experimentally tested the female in size between males were the predictors. The female morph dens- preference for males using a 2-choice arena. The arena was modified ity treatment interaction was also added to check for different pat- after LeBas and Marshall (2000) and Bajer et al. (2010) and was tern of choice by females. Finally, the arena was included among composed of a rectangular compartment (20 40 cm side) con- predictors to control for the 2 experimental sets. Multivariate homo- nected to 2 20 20 cm compartments (Figure 1). The rectangular geneity of variance was achieved, so raw data were used in the ana- compartment was regarded as neutral area for the female, while the lyses. Finally, we used univariate analyses of covariance (ANCOVA) 2 annexed compartments were defined as the preference ones as post hoc test to the MANCOVA, in order to disentangling the (Figure 1A). The 2 preference compartments were separated by a groups responsible for significant multivariate effects. We ran 2 sep- transparent plexiglas divider from the male compartments arate ANCOVAs, both including the same predictors as in the (20 20 cm, Figure 1). The temperature was kept constant using a MANCOVA. The dependent variables were, respectively, the time 45-W heat mat at 33 C. We placed each lizard in a thermostated spent in the choice compartment associated with the white (first terrarium (20 30 20 cm, 33 C) for 10 min before starting trial, model) and yellow (second model) males. We performed all the ana- then we gently moved individuals from pre-heating cages into the lyses using R version 3.1.0 (R Core Team 2016), and otherwise arena: males were assigned randomly among preference compart- stated, reported values represent mean and standard errors. ments, while females were placed in the center of the rectangular compartment within an opaque small box 5 min before the trial started. The box was then removed and female behavior was Results observed and videotaped with a SONY Super Night Vision Camera (M020-s53-001) connected by a 20-m isolated wire to a laptop PC Contrary to any expectation, the density treatment did not affect fe- located in a room adjacent to the one housing the arena. We male choice. Indeed, the time females spent in each of the 2 choice observed females continuatively for 1 h, and trials were carried out compartments did not significantly differ according to density treat- using 2 identical arenas. After each trial, we carefully washed the ment or the density treatment morph interaction (Table 1). arena with detergent in order to remove any chemical stimuli left by Similarly, females spent the same amount of time in the 2 choice com- lizards from the previous trial. A trial was invalidated if the female partments regardless of body size and the arena used for the trial had not approached any of the preference compartments at least (Table 1). However, we found a significant main effect of the female once, and consequently we excluded 2 recordings (1 for each morph (Pillai’s trace ¼ 0.241, P¼ 0.0069). The first post hoc morph), and the sample size for the analysis included 44 females (11 ANCOVA showed that white females spent on average more time females for each morph in each treatment). than yellow ones with the white male (contrast between white and yel- low females: b ¼ 14.56 4.2 min, t¼ 3.42, P ¼ 0.0015, Figure 2), whereas the second post hoc ANCOVA did not find any difference be- Female preference variables tween female morphs for the time spent in association with the yellow Video recordings of female behavior were analyzed automatically male (contrast between white and yellow females: b¼5.16 3.6 min, using the software idTracker (Pe ´ rez-Escudero et al. 2014), which t¼1.44, P ¼ 0.16, Figure 2). Consequently, white females spent more automatically tracks the movements in 2D of the target lizard and time in the white male side than in the yellow male one irrespective of supplies its position in each frame in terms of horizontal and vertical the treatment (2-sample t-test: t ¼ 3.906, P¼ 0.00033, Figure 2), coordinates (expressed in pixels). Since the resolution of recordings whereas yellow females did not show any difference (2-sample t-test: was 25 frames/s, the position of the target female was obtained with t ¼ 0.807, P¼ 0.42, Figure 2). the same frequency, supplying, therefore, a set of 90,000 locations for each lizard. Those locations were used to build the female trajec- tory within the arena during the entire experiment (Figure 1B). By Discussion counting the number of locations in each compartment of the arena and converting it in seconds (1 frame ¼ 1/25 s), we obtained the total In this article, we looked if female morphs of the common wall liz- time spent by each female in the 2 choice compartments. These times ards associate with male morphs in an assortative way. Since yellow Downloaded from https://academic.oup.com/cz/article-abstract/64/4/449/4139748 by Ed 'DeepDyve' Gillespie user on 22 August 2018 452 Current Zoology, 2018, Vol. 64, No. 4 a 3-choice experimental arena, we did not detect any association be- tween male and female morphs (Sacchi et al. 2015). This second ex- periment showed a positive assortative mating, but solely in white morphs, conflicting with our previous observations. However, the conflict is in appearance only. The experiment we did in 2015 was designed to check the occurrence of assortative mating in all female morphs and was likely not powerful enough to detect assortative mating in 1 morph only, when compared with the other 2. Conversely, this new experiment compared only 2 morphs (those whose breeding strategy are better known), thus providing a more stringent tool in highlighting assortative mating. The 2 experiments combined make some kind of morph-specific assortative mating pos- sible in common wall lizards: white females prefer white males, Figure 2. Mean time spent by white and yellow females in the 2-choice whereas the other 2 morphs do not associate with their correspond- compartments depending on the 2 density treatments. Bars represent stand- ing colors or they select male traits that are not strictly related with ard errors. male breeding strategy (Perez i de Lanuza et al. 2014). From an evo- lutionary point of view, a complex scenario emerges. Sexual selec- Table 1. Statistics of the MANCOVAs used to compare the tion could be actually working in P. muralis (at least in 1 morph), responses of females to male color morphs but it still remains to be determined whether this is enough to main- tain the color polymorphism in this species. Positive assortative mat- Variables Pillai’s trace df P ing cannot maintain polymorphism alone (Wellenreuther et al. Morph 0.241 1 0.0069 2014). Indeed, assortative mating in white morph should promote Density 0.057 1 0.34 divergence (until to speciation) if not counterbalanced by gene flow Morph Density 0.052 1 0.38 among morphs (Rosenblum et al. 2004; Rosenblum 2006). Given Female SVL 0.008 1 0.86 the lack of choice in yellow females, a disassortative mating in red Arena 0.066 1 0.29 females with a preference for white males could be enough to ensure Difference in SVL between males 0.072 1 0.26 gene flow between morphs and, thus, maintain polymorphism. Gene flow could be even maintained either in the absence of choice (ran- dom mating) with respect to male morphs, if yellow and red females and white females adopt r/K reproductive strategy (sensu Alonzo (at least 1 of them) would select males based on male quality (e.g., and Sinervo 2001; Galeotti et al. 2013), we tested if female mate traits that are costly to produce thus revealing “good genes” of preference changed depending on lizard density, in order to maxi- male, Zahavi 1975, 1977) rather than on male strategy. mize the probability that hatchlings will express the morph adapted In conclusion, our results support the hypothesis that sexual se- more closely to the demographic conditions of the population. lection might play a relevant role in maintenance of the color poly- Experimental manipulation of density did not give significant re- morphism in the common wall lizard, but new controlled sults, since it did not have any effect on mate preference by white experiments specifically designed to decoupling the effects of female and yellow females. Even if density was manipulated in laboratory mate choice on the fitness of both male and female morphs are and for short time, these results suggested that white and yellow fe- needed to enhance our understanding of how sexual selection oper- males do not really tune mate preference according to the popula- ates on the strategies adopted by morphs. tion density. In the common wall lizard, alternative breeding strategies also occur in male morphs (Sacchi et al. 2007a; Calsbeek et al. 2010; Scali et al. 2013), and male and female strategies could couple in Acknowledgments order to maximize the fitness of each player (Sinervo and Zamudio We thank Stefania Cantoni for the help during video recording. Captures and 2001). Since the cycles by which male and female morphs maximize lizard housing were carried out in accordance with European Habitat their fitness are not necessarily synchronous (Sinervo and Zamudio Directive (92/43/CEE) and Italian law for the use of animals in scientiﬁc re- 2001), females could tune their choice in a much more complex way search (Aut. Prot. 0011511/PNM). than accordingly only to their own strategy. For example, females might also take into account the strategy adopted by males in order to maximize fitness for their offspring. For example, in side-blotched lizards orange females are r-strategists, and territorial orange males Funding are a convenient choice at low density since hatchlings will share Financial support was provided by the Italian Ministero dell’Istruzione, increased aggression and will better perform in defending territories dell’Universita ` e della Ricerca – FAR2012. and resources. 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Current Zoology – Oxford University Press
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