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Journal of the Lepidopterists' Society 36(3), 1982, 174-177
REDUNDANCY IN PIERID POLYPHENISMS: PUPAL
CHILLING INDUCES VERNAL PHENOTYPE IN
PIERIS OCCIDENTALIS (PIERIDAE)
Arthur M. Shapiro
Department of Zoology, University of California, Davis, California 95616
ABSTRACT. Chilling young pupae of Sierra Nevada Pieris occidentalis induces the vernal-alpine "calyce" phenotype as effectively as rearing on short days. Temperatures of 6°C or less sustained for 10 days or more appear equally efficacious. This redundancy in phenotypic-induction mechanisms parallels that found in various other butterflies, and is underlain by apparent genetic variation in sensitivity within populations.
The Western white, Pieris occidentalis Reakirt, has two seasonal phenotypes once considered separate species (Edwards, 1876). Shapiro (1973) demonstrated that rearing on a short day (10L:14D) without chilling induced the heavily-marked phenotype "calyce" in Boreal Ridge, California stock, and that temperatures of 10°C were ineffective in doing so under long-day (14L:10D) conditions. Later, Shapiro (1978) demonstrated that redundancy existed in the phenotypic-induction systems of various multivoltine Pieridae, including the closely-related species P. protodice Boisduval and LeConte. This discovery prompted a re-evaluation of thermal influences in pheno-typic determination in P. occidentalis; specifically, chilling of long-day pupae at lower temperatures than used heretofore.
Ova were obtained from a single wild female collected in Donner Pass, of the Sierra Nevada, Nevada County, California (2100 m), 7 August 1980. This locality is about 4.5 km from Boreal Ridge and at similar elevation. Rearing was done under our standard conditions (Shapiro, 1975) with continuous light at 25°C on Lepidium virginicum L. var. pubescens (Greene) Thellung (Cruciferae), a natural host at Donner Pass. Allocation of pupae to treatments was randomized to obviate effects of the sequence of oviposition on offspring quality. Control pupae were held at the rearing conditions. Experimental pupae were refrigerated as close to eight hours after pupation as possible (effectively ±1 h) and held in one of four regimes: 6°C for 10 days; 5°C for 14 days; 2°C for 14 days; 10 days at 6°C followed by 2 days at 25°C followed by 7 days at 2°C. After chilling, the pupae were returned to 25°C and allowed to develop and eclose. Mortality was negligible (4/154). Adults were classified into three phenotypic grades based on the degree of melanization of the ventral hindwing; standards are shown in Fig. 1 and correspond to those used by Shapiro
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FlG. 1. Phenotypic standards for Pieris occidentalism ventral surfaces, males at left. Top row, light; middle, intermediate; bottom, dark. All bred ex Donner Pass.
(1973). The results are given in Table 1. They were analyzed as follows: a log-linear model (Fienberg, 1977) was fitted to the 5x3 contingency table for pooled sexes. Using the large-sample distribution of the natural logarithms of the frequencies, it was possible to express the subsequent tests in terms of linear contrasts. When the hypothesis that the treatments have no effect on the phenotypic distribution was tested (pooled treatments vs. controls), the resulting x2 statistic was
176
Journal of the Lepidopterists' Society
Table 1. Phenotype distributions in a brood of 150 Pieris occidentalis from one Donner Pass, CA, female, reared on 24L/25°C, subjected to five different pupal temperature regimes.
|
Males |
Females |
3oth sexes |
|||||||
|
Treatments: |
Dark |
Intermediate |
Light |
Dark |
Intermediate |
Light |
Dark |
Intermediate |
Light |
|
A. Control (unchilled) |
4 |
12 |
15 |
1 |
10 |
21 |
5 |
22 |
36 |
|
B. 10 days at 6°C |
2 |
11 |
4 |
4 |
5 |
1 |
6 |
16 |
5 |
|
C. 14daysat5°C |
4 |
6 |
0 |
7 |
6 |
1 |
11 |
12 |
1 |
|
D. 14 days at 2°C |
3 |
6 |
0 |
4 |
6 |
0 |
7 |
12 |
0 |
|
E. 10daysat6°C, then 2 days at 25°C, then 7 days at 2°C |
5 |
3 |
2 |
3 |
3 |
1 |
8 |
6 |
3 |
|
2 chilled |
14 |
26 |
6 |
18 |
20 |
3 |
32 |
46 |
9 |
Statistical comparisons (pooled sexes):
A x B A X C; A x D A X E: B x E C x D
X23 = 11.09 (0.01 < P < 0.025);
x\ = 18.25 (P < 0.005);
X23 = 13.88 P < 0.005 ;
X23 = 13.64 (P < 0.005);
X23= 3.12 (0.KP);
X23= 0.59 (0.KP).
33.89 (df = 8, P < 0.005). Comparisons between the control and the individual treatments were all significant (Table 1), while two sample comparisons were not. I therefore concluded that the phenotype of P. occidentalis is inducible by pupal temperature exposure but that it behaves as a threshold phenomenon with the switch occurring at some temperature between 10°C and 6°C.
Though further refinement of the effects is likely using larger samples and perhaps analyzing the sexes separately, it is evident that P. occidentalis adheres to the pattern of P. napi L., P. protodice, and Colias eurytheme Bdv. in having built-in redundancy in its pheno-typic-induction system, with short days irreversibly determining dark phenotype but, with long day-light phenotype decisions reversible by subsequent chilling. Also, the response is not all-or-none; some seemingly inappropriate phenotypes are produced in most groups, and the distribution of this phenomenon exceeds the variance in age at refrigeration. Thus differences in sensitivity to chilling, as to pho-toperiod, may be genetically determined. Extreme "calyce" phenotypes (Shapiro, 1978) probably represent the combination of larval short-day exposure and post-diapause pupal chilling. Since my 1973 paper I have examined many populations of this complex from the Rocky Mountains, the Sierra Nevada, and Alaska, all of which retain both "calyce" and estival occidentalis phenotypes.
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Acknowledgments
I thank Mike Miller for statistical advice, Marc Minno for technical assistance, and the Department of Zoology for support of field and lab studies in the Nearctic Pierinae.
Literature Cited
EDWARDS, W. H. 1876. Catalogue of the diurnal Lepidoptera of America north of
Mexico. Trans. Amer. Entomol. Soc. 6:1-68. FiENBERG, S. E. 1977. The analysis of cross-classified categorical data. M.I.T. Press,
Cambridge, Mass. 151 pp. Shapiro, A. M. 1973. Photoperiodic control of seasonal polyphenism in Pieris occi-
dentalis Reakirt (Lepidoptera: Pieridae). Wasmann J. Biol. 31:291-299.
--------- 1975. Photoperiodic control of development and phenotype in a subarctic
population of Pieris occidentalis (Lepidoptera: Pieridae). Canad. Entomol. 107: 775-779.
--------- 1976. The biological status of Nearctic taxa in the Pieris protodice-occidentalis
group (Pieridae). J. Lepid. Soc. 30:289-300.
--------- 1978. The evolutionary significance of redundancy and variability in pheno-
typic-induction mechanisms of Pierid butterflies (Lepidoptera). Psyche 85:275-283.