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Cricket Song in Sympatry: Species Specificity of Song Without Reproductive Character Displacement in Gryllus Rubens

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Cricket Song in Sympatry: Species Specificity of Song Without Reproductive Character Displacement in Gryllus Rubens BEHAVIOR Cricket Song in Sympatry: Species Specificity of Song without Reproductive Character Displacement in Gryllus rubens 1 AMANDA S. IZZO AND DAVID A. GRAY Department of Biology, California State University, Northridge, CA 91330Ð8303 Ann. Entomol. Soc. Am. 97(4): 831Ð837 (2004) ABSTRACT Previous work with the cryptic sister species pair of Þeld crickets, Gryllus texensis Cade & Otte and Gryllus rubens Scudder, has implicated sexual selection in the speciation process. That study examined reproductive character displacement (RCD) in male song and female preference for song in G. texensis. No evidence of RCD was found. Here, we provide a similar analysis of RCD in G. rubens song and examine the songs of both species from areas of sympatry in an effort to document the species-speciÞcity of song in sympatry and to look for individuals with songs indicative of F1 hybrid status. We 1) Þnd no evidence for RCD in G. rubens song, 2) demonstrate the distinctness of song in sympatry, and 3) document the rarity of songs typical of F1 hybrids. KEY WORDS speciation, reinforcement, crickets MATING SIGNALS ARE PARTICULARLY interesting because Gray and Cade 2000, Shaw and Herlihy 2000). In this they mediate behavioral interactions that affect gene context, we have examined the pulse rates in the male ßow both within and among groups of organisms. calling song of the Þeld cricket Gryllus rubens Scudder Sexual signals may be both species-speciÞc identiÞers and its broadly sympatric cryptic sister species Gryllus and subject to sexual selection (Ryan 1990, Ryan and texensis Cade & Otte. Previous work with these species Rand 1993). During speciation events, sympatric di- has shown that they are each othersÕ closest relative vergence in mating traits may follow allopatric diver- (Huang et al. 2000) and are capable of producing gence in morphology, ecology, or genetic architecture fertile hybrids in the laboratory (Smith and Cade 1987, and so reinforce the species differences (Dobzhansky Cade and Tyshenko 1990). The species co-occur in the 1937). Such a process is expected to produce a pattern southern United States gulf region from western Flor- of reproductive character displacement, in which sex- ida to eastern Texas. Obvious hybrids in the Þeld have ual signals are more distinct in sympatry than in allo- not been found, although before the study described patry. At equilibrium, species-speciÞc identifying here, only a small portion of the geographic range in traits may be subject primarily to stabilizing selection sympatry had been systematically studied, namely, the and so may show relatively little intraspeciÞc varia- far western portion of mainland Florida (Walker 1998, tion, whereas sexually selected traits are often among 2000). Males of both species produce calling song with the most variable phenotypic features of organisms long trains of pulses called trills. G. rubens tends to (static and dynamic traits sensu Gerhardt 1991). Ex- produce longer trills than G. texensis, but the two amining levels of intraspeciÞc variation in mating sig- species differ most conspicuously in the pulse rate of nals, species differences in signals, and the pattern of the calling song. G. rubens from Gainesville, FL, pro- geographic variation in mating signals has thus been a duces trills with pulse rates averaging Ϸ56 pulses per productive area of inquiry at the interface of behav- second at 25ЊC, whereas G. texensis from Austin, TX, ioral and evolutionary research (Butlin and Ritchie trill at a much faster rate, typically 80 pulses per sec- 1994, Gerhardt 1994, Ritchie and Gleason 1995, Wells ond at 25ЊC (Walker 1998, 2000; Gray and Cade 2000). and Henry 1998, Henry et al. 1999, Ritchie et al. 1999, Laboratory produced hybrids call with intermediate Tregenza et al. 2000, Tregenza 2002). pulse rates averaging 67 Ϯ 5 pulses per second (n ϭ 27 Because of the conspicuous male calling song used males; unpublished data). Males also differ in the qui- to attract females for mating, crickets have been ex- eter close-range courtship song used just before mat- ceptionally productive model systems for investigat- ing (Fitzpatrick and Gray 2001). Most females can be ing the role of behavioral isolation in speciation (Hill identiÞed based on relative ovipositor length (Gray et et al. 1972, Walker 1974, Harrison 1985, Doherty and al. 2001), but other than song no means of distinguish- Storz 1992, Doherty and Howard 1996, Veech et al. ing males is currently known. A previous study (Gray 1996, Gregory et al. 1998, Roff et al. 1999, Shaw 1999, and Cade 2000) examined reproductive character dis- placement in pulse rates of G. texensis song and found 1 E-mail: [email protected]. no evidence for displacement in either male song or 0013-8746/04/0831Ð0837$04.00/0 ᭧ 2004 Entomological Society of America 832 ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA Vol. 97, no. 4 female response to song. Too few G. rubens were opportunities. That hybrid offspring would produce caught for the previous study to include an analysis of signals unattractive to parental populations is sug- character displacement in G. rubens song. Cryptic gested by female preference function studies with species pairs may reveal character displacement in Ephippiger (Ritchie 1996) and Laupala (Shaw 2000), only one species. For example, Marshall and Cooley and with these two species of crickets (Gray and Cade (2000) documented character displacement in pitch 2000). and female preference for pitch in one species of periodical cicada, Magicicada neotredecim (Marshall & Materials and Methods Cooley), but no change in Magicicada tredecim (Walsh & Riley). Here, we present the results of a Field crickets were collected in summer and fall of follow-up study designed to address reproductive 1999 and 2002 from throughout the southeastern character displacement in G. rubens song and further United States (Table 1; Fig. 1). Collection localities examine the species speciÞcity of song across a much were designated as allopatric or sympatric based on broader area of sympatry than studied previously. Be- distribution maps from T. J. Walker and T. E. Moore cause G. rubens has a slower pulse rate than G. texensis, (2003). Singing Insects of North America (http:// reproductive character displacement would predict buzz.ifas.uß.edu/479mc.gif and http://buzz.ifas. that sympatric G. rubens should have a slower pulse uß.edu/482mc.gif), plus Þrsthand knowledge. We are rate than allopatric G. rubens. uncertain whether Decatur, AL, should be considered It is important to note that a pattern of reproductive as allopatric (G. rubens) or sympatric; we have treated character displacement (RCD) is possible even in the it as sympatric in analyses. Because only one cricket absence of hybrid inviability (Higgie et al. 2000). In was recorded from Decatur, we are conÞdent that this discussions of RCD, it has been generally considered uncertainty in no way affects our conclusions. that it is hybrid inviability that selects for more distinct Crickets were collected at lights at night and by mating signals in sympatry (Howard 1993). Some au- searching during the day. The 1999 collections were thors have interpreted this to mean that without hy- conducted in the fall (SeptemberÐmid-October) and brid inviability there is no a priori reason to predict a consisted of wild-caught females whose offspring pattern of RCD. However, where it has been shown were laboratory reared in sibling groups at 28 Ϯ 1ЊC that mating success is dependent upon production of with a photoperiod of 13:11 (L:D) h. They were pro- a species-typical mating signal, we may infer that hy- vided with dry cat food and water in cotton-plugged brids with intermediate mating signals would have low vials (Gray and Cade 2000). From one to four male Þtness even if they are completely viable and fertile. offspring per wild-caught female were recorded using That is, sexual selection can favor more distinct mating a Sony WM-D3 Professional Walkman, and the songs signals in sympatry (i.e., produce a pattern of RCD) were digitized at 22.05 kHz (202 males from 107 wild- due to selection on parents to avoid interspeciÞc mat- caught females, mean Ϯ SD, 1.9 Ϯ 0.65 males per ings that produce hybrid offspring with unattractive female). To avoid pseudo-replication of data, the off- intermediate mating signals and thus limited mating spring of each wild-caught female cricket were treated ;(shaded area) and G. rubens (///// shaded areaگگگگگ) Fig. 1. Approximate geographic distributions of G. texensis sympatric areas are cross-hatched. Collection locality key: one Minden, LA; two Van Buren, AR; three North Little Rock, AR; four Dumas, AR; Þve Tallulah, LA; six Greenwood, MS; seven Starkville, MS; eight Tuscaloosa, AL; nine Decatur, LA; 10 Carrollton, GA; 11 Tifton, GA; 12 Columbia, SC; 13 Florence, SC; 14 Palatka, FL; 15 Gainesville, FL; 16 Orlando, FL; 17 Intercession City, FL; 18 Hwy 60 ϫ Hwy 27, FL; 19Zolfo Springs, FL; 20 Perry, FL; 21 Marianna, FL; 22 Milton, FL; 23 Pensacola, FL; 24 Mobile, AL; 25 De Soto National Forest, MS; 26 WhiteÕs Kitchen, LA; 27 Alexandria, LA; 28 De Ridder, LA; 29Sulfur, LA; 30 Kisatchee National Forest, LA; 31 Hankamer, TX; and 32 Bastrop, TX. July 2004 IZZO AND GRAY:PULSE RATES IN CRICKET SONG 833 Table 1. Collection locality information and sample sizes Latitude, Locality Population type N G. rubens N G. texensis Year longitude Alexandria, LA 31Њ 18Ј N SYM 7 1999 91Њ 45Ј W Bastrop, TX 30Њ 06Ј N SYM 9 1999 97Њ 19Ј W Carrollton, GA 33Њ 34Ј N SYM 3 1999 85Њ 04Ј W Columbia,
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