Spermatophore displacement and male fertilization success in the ground beetle Carabus insulicola

Yasuoki Takami takami@terra 0 1 2 0 The Author 2007. Published by Oxford University Press on behalf of the International Society for Behavioral Ecology. All rights reserved. For permissions , please 1 zool.kyoto-u.ac.jp. Received 5 July 2006; revised 13 February 2007; accepted 16 February 2007 2 Department of Zoology, Graduate School of Science, Kyoto University , Kitashirakawa-Oiwake, Sakyo, Kyoto 606-8502, Japan Sperm displacement is an effective male mating tactic of sperm competition to ensure paternity. However, the effectiveness of sperm displacement may depend on defensive mating tactics employed by rival males and also may be affected by females. To test this hypothesis, I conducted double-mating experiments with microsatellite paternity analyses using the ground beetle Carabus insulicola. First, I examined how often spermatophore displacement by the second male occurs. Spermatophore deposition success of the second male was high (85%) as was that of the first male (100%), in which 80% of the second males displaced the first male's spermatophore, suggesting that spermatophore displacement is a highly effective offensive tactic of sperm competition in C. insulicola. Second, I examined the effects of postcopulatory guarding by the first male and remating interval and sperm utilization pattern of the female on fertilization success of the second male. P2 values were moderate on average (0.30-0.57) and negatively correlated with postcopulatory guarding duration of the first male. Sperm use by the female was temporally nonrandom, such that sperm of the second male tended to be used first, although there was no significant effect of female-remating interval. These results suggest that the effectiveness of spermatophore displacement depends on postcopulatory guarding by the first male and the sperm utilization pattern of the female. These findings provide empirical evidence for the conflicting nature of selection for male offensive and defensive tactics in sperm competition and significant contribution of females therein. Key words: genitalia, microsatellite, P2, sexual selection, sperm competition. [Behav Ecol 18:628-634 (2007)] - Spromiscuous species and results in various morphological, perm competition is a strong agent of sexual selection in physiological, and behavioral adaptations (Parker 1970; Smith 1984; Birkhead and Mller 1998; Simmons 2001). Male mating strategies under sperm competition are expected to be favored by 2 opposing selective forces: selection favoring offensive tactics for preempting stored rival sperm and counterselection favoring defensive, anti-preempting tactics (Parker 1970, 1984). The former tactics include takeover and subsequent sperm displacement, and the latter include physical blocking of the female genitalia using a mating plug, prolonged copulation, pre- and postcopulatory guarding, takeover avoidance behavior, and induction of nonreceptivity in females (Parker 1984; Simmons 2001). Sperm competition theory predicts that selection for offensive and defensive tactics in male mating strategies is in conflict and that the only evolutionary stable strategy in this conflict is one in which males invest in both tactics (Parker 1984, 1998). This means that if males acquire a strong offensive tactic, such as sperm displacement, selection will in turn favor defensive tactics, in this case to prevent the sperm displacement. The conflicting nature of selection for male mating tactics in sperm competition suggests that the effectiveness of one tactic may depend on other tactics employed by rivals. Additionally, the effectiveness of male mating tactics may also be affected by aspects of female behavior (Eberhard 1996): such as female mating frequency and sperm utilization pattern. Thus, the effectiveness of male mating tactics should be evaluated in relation to those possible factors. However, such evaluation has been insufficiently performed within a single species. Sperm displacement is a common offensive tactic in sperm competition and involves various mechanisms (Birkhead and Mller 1998; Simmons 2001). Morphological adaptations for sperm displacement have been reported in male genitalia, which have evolved as a special tool for removing rival sperm from the female sperm-storage organ. Waage (1979) described how the male genitalia of the damselfly Calopteryx maculata were used to remove sperm stored from within the bursa copulatrix and spermatheca of the mated female. This mechanism is widespread in damselflies (Waage 1986). Although such morphological and behavioral adaptations are expected to be very beneficial in sperm competition, they have been found in limited groups of organisms (Waage 1986; Yokoi 1990; Haubruge et al. 1999; Kamimura 2000, 2005; Wada et al. 2005). Alternatively, male genitalia are used to stimulate the female genital tract to release stored sperm (von Helversen D and von Helversen O 1991; Eberhard 1996 for review; Co rdoba-Aguilar 1999). In other cases without apparent morphological adaptations of the male genitalia, ejaculates per se are used to flush or push out the sperm of rivals (Gack and Peschke 1994; Hosken and Ward 2000). Prolonged copulation and pre- and postcopulatory guarding are typical tactics to monopolize a female from rivals and are favored under increased risk of sperm competition (Parker 1974; Yamamura 1986; Yamamura and Tsuji 1989). Mate guarding is well documented in Odonata (damselflies and dragonflies) and other groups of insects, which allows the male to prevent females from remating with other males and to ensure his fertilization success (reviewed in Simmons 2001, p. 149168). Additionally, the ejaculate has defensive functions in the context of sperm competition other than fertilization, such as producing mating plugs or releasing pheromones for reducing receptivity in females (Simmons 2001, p. 9396). Carabus insulicola is a flightless ground beetle (Coleoptera, Carabidae) endemic to Japan. The genitalia of male C. insulicola are equipped with a strongly sclerotized, hook-like copulatory piece on the dorsal wall of the membranous endophallus (Figure 1A; Ishikawa 1987). The female genitalia consist of a membranous bursa copulatrix, a dorsal lobe, a tube-like spermatheca, a sclerotized vaginal apophysis, and a membranous pocket called the vaginal appendix (Figure 1B; Ishikawa 1987; Yahiro 1998). The copulatory piece is inserted into the vaginal appendix during insemination (Figure 1C; Ishikawa 1987; Takami 2002). Copulation lasts roughly 80100 min and is followed by about 60 min of postcopulatory guarding (Takami 2002). The male attaches a spermatophore to the innermost part of the bursa copulatrix. The spermatophore of C. insulicola covers the openings of the spermatheca, oviduct, and vaginal appendix. Consequently, a second male that attempts immediate remating could be prevented from inserting his copulatory piece into the vaginal appendix. However, based on the genital mechanics (Takam (...truncated)