Adaptive Significance of Egg Size Plasticity in Response To

Popul Ecol (2006) 48:159–166 DOI 10.1007/s10144-006-0253-4 ORIGINAL ARTICLE Tomokazu Seko Æ Fusao Nakasuji Adaptive significance of egg size plasticity in response to temperature in the migrant skipper, Parnara guttata guttata (Lepidoptera: Hesperiidae) Received: 29 July 2005 / Accepted: 16 December 2005 / Published online: 22 February 2006 Ó The Society of Population Ecology and Springer-Verlag Tokyo 2006 Abstract Females of the migrant skipper, Parnara gut- Keywords Natural selection Æ Environmental cue Æ tata guttata, that are reared under lower temperatures Reproductive allocation Æ Leaf toughness Æ Fitness Æ lay smaller eggs. The adaptive significance of egg size Trade-off Æ Parnara guttata guttata plasticity in response to temperature is unknown in this species. We suggest, based on the following experimental results, that P. g. guttata uses temperature as an indirect cue to predict the host condition (leaf toughness) of the Introduction next generation. First, larvae were reared under the typical conditions of temperature and photoperiod Egg size is subject to selection because it has substantial experienced during the immature stages in the first, fitness effects on progeny. Larvae hatched from larger second, and overwintering (third) generations (LD 16:8 eggs have higher resistance to environmental stresses at 25°C, LD 14:10 at 25°C and LD 14:10 at 20°C). Fe- such as larval competition, starvation, desiccation, and males reared under LD14:10 at 20°C produced more, low temperature (Fox and Czesak 2000). However, there smaller eggs than those reared under LD14:10 and is often a trade-off between egg size and fecundity (Roff LD16:8 at 25°C. Secondly, survival rates of first instar 1992; Cummins 1986; Braby 1994; Sinervo and Licht larvae derived from females reared under the three 1991; Heath et al. 1999). Life-history theory predicts photoperiod/temperature treatments were measured on that organisms inhabiting environments with relatively young soft rice leaves (‘‘soft’’), or tough, old rice leaves harsh conditions for the growth and survival of their (‘‘tough’’). Survival rates of hatchlings reared on soft offspring should produce fewer and larger offspring and tough leaves did not differ when females were reared (Sibly and Calow 1985). under LD16:8 and LD14:10 at 25°C. However, hatch- Temperature is a particularly important and wide- ling survival was significantly higher on soft than on spread mediator of phenotypic variation in ectothermic tough leaves when females were reared under LD14:10 animals, resulting in predictable plastic changes in egg at 20°C. Thirdly, we found that egg size plasticity in and body size (Fischer et al. 2003a). Egg size in ecto- response to temperature in P. g. guttata may be a therms commonly increases under colder conditions threshold response. Temperatures below 20°C experi- (Ernsting and Isaaks 1997; Azevedo et al. 1996; enced during the immature stages may be effective for Yampolsky and Scheiner 1996; Blanckenhorn 2000; production of smaller and more eggs in the overwin- Fischer et al. 2003a). Much of this variation is probably tering generation of P. g. guttata. attributable to egg size plasticity. However, the adaptive significance of this response to temperature has not been investigated in most study systems (Azevedo et al. 1996; Blanckenhorn 2000; Fox and Czesak 2000). The migrant skipper, Parnara guttata guttata, typically has three generations per year in Japan and T. Seko (&) National Agricultural Research Center for Western Region, shows a large variation in egg size among generations 6-12-1 Nishifukatsu, Fukuyama 721-8514, Japan (Nakasuji and Kimura 1984). Nakasuji et al. (1986) and E-mail: sekot@affrc.go.jp Nakasuji and Ishii (1988) hypothesized that this varia- Tel.: +81-84-9234100 tion was the result of adaptation to the seasonal changes Fax: +81-84-9235219 in leaf toughness of host plants. Nakasuji and Kimura F. Nakasuji (1984) suggested that females use photoperiod as an Okayama University, Okayama, Japan indirect cue to determine the egg size of each generation 160 since this species lays smaller eggs under LD 16:8, but 1988; Ishii and Hidaka 1979). Overwintering individuals larger eggs under LD 14:10 at the same thermal condi- emerge from late May to early June, and are dark-col- tion. Nakasuji and Nakano (1990) compared the size of ored with a small body (overwintering and/or third eggs laid by females reared under LD 16:8, 14:10 and generation). Kidokoro (1992) suggested the possibility 12:12, corresponding to the photoperiod conditions of a return migration by adults of the overwintering during larval development of the three generations. generation from southern to northern Japan, but no However, the order of egg size among the three pho- clear evidence of spring migration has been found as yet. toperiods in the laboratory did not correspond with that P. g. guttata changes its habitat seasonally. The adults of of egg size in the field. On the other hand, the thermal the overwintering and first generations lay their eggs on environment experienced by larvae during development grasses in wet lowland areas, and the adults of the sec- has an effect on egg size in this species (Hareyama et al. ond generation lay their eggs in dry upland areas. The 1991). Females that developed under lower temperatures main host plant of the first two generations is the rice laid smaller eggs, in contrast to the common response to plant, Oryza sativa (Nakasuji 1982). The overwintering temperature in ectotherms. The adaptive significance of generation larvae feed primarily on Imperata cylindrical this response has remained unknown in P. g. guttata as (cogon grass), Miscanthus sinensis (eulalia), and Festuca well as in other species that show egg size plasticity in ovinna (sheeps fescue). Female adults of the overwin- response to temperature (but see Fischer et al. 2003b). tering generation and those of the first generation lay In the present study, three experiments were per- smaller eggs on grasses with soft leaves. These grasses formed to verify the possibility that the egg size plasticity grow in wet lowlands. On the other hand, females of the of P. g. guttata in response to temperature is related to a second generation lay larger eggs on grasses with tough seasonal adaptation, such as photoperiod. First, we leaves. Larvae hatched from these eggs have to feed on reared larvae under the combinations of temperature and tough leaves in dry uplands because grasses in wet photoperiod experienced by larvae in each of three gen- lowlands are unavailable in winter seasons (Nakasuji erations in the field. To elucidate the effect of tempera- 1982, 1987). Nakasuji and Kimura (1984) reported that ture on the reproductive allocation patterns, we larvae hatched from smaller eggs in the overwintering compared the size and number of eggs laid by females and first generations were not able to survive on tough among treatments (experiment 1). Predictions developed leaves in dry uplands. They also determined that egg size in previous work indicate that the reproduction of fewer is a function of the photoperiod under which the larvae but larger eggs is favored when host plant leaves are were raised, although as Roff (2002) suggests, the pho- tough. Therefore, survival rates of first instar larvae un- toperiod effect alone is not great enough to produce der each treatment were compared on both soft and naturally occurring eggs of such a size. tough rice leaves. A total fitness measure comprised of survival and fecundity was also compared among photoperiod/temperature treatments and among leaves of Study population and experimental design different degrees of toughness (experiment 2). Thirdly, to verify whether egg size plasticity in response to temper- Stock culture ature in P. g. guttata is a gradual or threshold response, the size of eggs laid by females was compared among the The stock culture of P. g. guttata used in this study different temperature treatments (experiment 3). originated from adults caught in Nara (western Japan; 34°31¢N, 135°44¢E) in July 2003. The stock culture was established using the method of Nakasuji and Honda Materials and methods (1979). Females were allowed to lay eggs in a plastic cage (25·35·35 cm3) with 1-mm mesh netting. The hatched Life history of P. g. guttata larvae were reared on 2-week-old rice seedlings that had been planted in plastic pots (10 cm in diameter and 8 cm Parnara guttata guttata inhabits the central and western in height) at 25°C under a photoperiod of LD 16:8. The regions of Japan as the northern limit of its distribution. emerged adults were fed on a 10% honey solution, The adults in the central regions migrate in a south- which was supplied in artificial vinyl flowers (a plastic western direction during late August and September tube attached by a wire) (Nakasuji and Honda 1979), every year (Hiura 1973). P. g. guttata adults have mor- and they were allowed to mate and to lay eggs on rice phological traits that differ according to the season in seedlings. The stock culture of the fifth generation from which they emerge (Ishii and Hidaka 1979). Adults the field was used in this study. emerging in July under long day lengths are light-colored and have a small body (first generation). Adults Experiment 1: difference in the reproductive allocation emerging from late August to September under short patterns between size and number of eggs among three day lengths are dark-colored with a large body (second environmental conditions First instar larvae were generation). Day lengths shorter than 12.5 h induce randomly selected from the stock culture, and reared at diapause in larvae derived from female adults of the a density of 40 individuals per plastic container second generation in western Japan (Nakasuji and Ishii (25·17·8cm3) with rice seedlings under the following 161 three environmental conditions: 25°C under LD 16:8, tip of soft or tough leaves.

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