Phenotypic plasticity in the range-margin population of the lycaenid butterfly Zizeeria maha

Abstract

Background: Many butterfly species have been experiencing the northward range expansion and physiological adaptation, probably due to climate warming. Here, we document an extraordinary field case of a species of lycaenid butterfly, Zizeeria maha, for which plastic phenotypes of wing color-patterns were revealed at the population level in the course of range expansion. Furthermore, we examined whether this outbreak of phenotypic changes was able to be reproduced in a laboratory.

Results: In the recently expanded northern range margins of this species, more than 10% of the Z. maha population exhibited characteristic color-pattern modifications on the ventral wings for three years. We physiologically reproduced similar phenotypes by an artificial cold-shock treatment of a normal southern population, and furthermore, we genetically reproduced a similar phenotype after selective breeding of a normal population for ten generations, demonstrating that the cold-shock-induced phenotype was heritable and partially assimilated genetically in the breeding line. Similar genetic process might have occurred in the previous and recent range-margin populations as well. Relatively minor modifications expressed in the tenth generation of the breeding line together with other data suggest a role of founder effect in this field case.

Conclusions: Our results support the notion that the outbreak of the modified phenotypes in the recent range-margin population was primed by the revelation of plastic phenotypes in response to temperature stress and by the subsequent genetic process in the previous range-margin population, followed by migration and temporal establishment of genetically unstable founders in the recent range margins. This case presents not only an evolutionary role of phenotypic plasticity in the field but also a novel evolutionary aspect of range expansion at the species level.

Figure 1

Field work on Z. maha in Fukaura, Aomori Prefecture, Japan. (A) Range expansion of Z. maha to the north in Honshu Mainland, Japan. Inset shows the locations of Fukaura, Hiratsuka, and Tokyo in the Japanese Islands. (B) Temperature dynamics of Fukaura from 1994 to 2005. Highest (shown in red), average (black), and lowest (blue) temperature records for a given year are indicated. The highest temperature in Fukaura had steadily increased year by year since 1996 until 2000. In 2001, the highest and lowest temperature suddenly dropped. (C) Estimated percentages of the color-pattern modified individuals among the total population of Z. maha in Fukaura (line graph). Relative population size is indicated as bar graph. Asterisks indicate a possible psychological bias. That is, the 2002 data (indicated by double asterisks) may be an overestimate, and the 2003 data (a single asterisk) may be an underestimate. Similarly, for 2004 and 2005, the modification occurrence is connected with broken lines (instead of continuous lines) and the relative population size is indicated by white bars (instead of yellow bars) to indicate the incompleteness of data records for these years. (D) Daily fluctuations of average temperature and the daily occurrence of the modified individuals in 2003. Red portion in a bar indicates the number of modified individuals that were caught, which are mostly from the late August to the early October when temperature is relatively high. Arrows indicate days when the field work was carried out without any Z. maha individual confirmed.

Figure 2

Modified individuals obtained from the Fukaura area. Various degrees of modifications were observed, but spot elongation (both inward and outward elongation) and spot reduction (or disappearance) were key features. (A-E) Individuals of the inward-type modifications. (F-J) Individuals of the reduction-type modifications. (K-O) Individuals of the outward-type modifications. (P-R) Individuals of asymmetrical modifications. (S) Modification-type profiles of Z. maha caught in the Fukaura area in 2002, 2003, and 2004. The outward type increased from 2002 to 2004 especially in male, but the reduction type seems to be almost constant in male.

Figure 3

Cold-shock experiment. (A) Induction rate (IR) and failure rate (FR) of the Hiratsuka (HIR) and Fukaura (FUK) populations of Z. maha treated for 15 days at 4°C. Higher IRs in females and in the Hiratsuka population are observed. FR is also higher in the Hiratsuka population. The IR difference in male between the Hiratsuka and Fukaura population is conspicuous. (B) Dose-dependent responses of the IR (pink, orange, and blue lines) and FR (black line) of the Hiratsuka population. No modification was able to be clearly detected in 0-d and 5-d cold-shock treatments. ANCOVA indicated statistical difference in sexes (p < 0.001). (C-L) Various wing color-patterns of the cold-shock treated individuals for 10 days (C-F), 15 days (G-J), and 20 days (K, L). (M) Modification-type profiles of the cold-shock-treated Z. maha individuals. Result of the Fukaura individuals (FUK) for the 15 day treatment is also shown, but all other data were obtained from the Hiratsuka individuals (HIR). For the color assignment, see Fig. 2S. Note the step-wise increase of the outward type and decrease of the inward type, and almost constant proportion of the reduction type in male. (N) Principal component analysis, indicating relationships among the four modification types and between sexes.

Figure 4

Selective breeding experiment for the outward type. (A) Modification-type profiles for generations (from the first generation [G1] to the tenth generation [G10]) after the cold-shock treatment. The fourth generation was treated differently, producing the smaller number of the modified individuals. The inward type was not produced. The outward type steadily increases until the fifth generation (G5). (B) The outward induction rate (IR) and reduction IR after the cold-shock treatment in males and females for generations. Females show higher outward IR than males do throughout generations. The outward IR appears to be in a plateau state after the fifth generation (G5). Males show higher reduction IR. (C) Modification rate (MR) for generations without immediate cold-shock treatment to pupae. As in the outward IR, females show higher MR than males do throughout generations. MR increases after the fifth generation (G5). (D) Failure rate (FR) after the cold-shock treatment for generations. FR of the selective line is less than 30% in G6 and in the following generations, indicating that the line is well maintained. The field-caught samples indicated by the red line were not genetically manipulated. Their parent females were caught freshly from the field to collect virgin adults for mating. (E-G) Modified individuals of the tenth generation (G10) in response to the cold-shock treatment. Both fore- and hindwings are severely modified as the outward type in many individuals. (H-J) Modified individuals of the tenth generation (G10) without immediate cold-shock treatment to pupae. Modifications are not severe but clearly observed.