First evidence of pupal summer diapause in Pieris brassicae L.: the evolution of local adaptedness
Introduction
The Large White (Pieris brassicae L.) is widespread throughout the Palaearctic (Fig. 1). Unlike most of its relatives its eggs are laid in batches and the gregarious larvae may seriously damage cruciferous crops (cauliflower, cabbage, rape, etc.). For many decades P. brassicae has served as a standard experimental animal in studies concerning the physiology and ecology of diapause and the chemical and biological control (Danilevskii, 1965, Feltwell, 1982, Danks, 1987, Zaslavski, 1988).
All larval instars respond to daylength and temperature as environmental cues which trigger a facultative winter diapause in the pupal stage (Spieth, 1985, Spieth, 1995, Spieth and Sauer, 1991). The switching time (Taylor, 1986) above which diapause is averted at a given temperature is an innate characteristic of the individual. A local population consists of several individuals with different such switching times thus forming a photoperiodic response (PPR), which is characteristic of the population and can be interpreted as an adaptation to local climatic conditions (Danks, 1987).
Local populations experience very different climates and there is clinal variation in life history pattern. The so-called critical daylength which induces direct development in 50 per cent of the population decreases from north to south and the variation of individual switching times around the populations mean increases. This has been interpreted as an adaptation to variation in duration and temporal predictability of the vegetation period across a range of latitudes (Spieth, 1985).
The genetic basis of the individual switching times allows natural selection to shape their distribution within each population, giving each population their characteristic photoperiodic response.
From north to south there is also an increase in influence of temperature on the PPR, with higher temperatures shortening the critical daylength of a population. This modifying effect ensures a phenotypically plastic adaptation to local climates, e.g. a local decrease in temperature due to altitude (Bradshaw and Holzapfel, 1990). Pieris brassicae is capable of long distance migration (Baker, 1978, Baker, 1984), and many migrating individuals might therefore encounter climates to which they are not adapted. Temperature dependence of PPR and intermediate PPRs in crosses between differently adapted populations (Danilevskii, 1965, Spieth, 1985) may also increase the chance of survival of the offspring.
At least some populations appear to have a genetically fixed preference for direction of migration which changes according to the mode of development (Spieth and Kaschuba-Holtgrave, 1996). Spring generations are thus enabled to migrate in northerly directions while the autumn generations return south (Baker, 1978, Baker, 1984).
Up till now, it was thought that clinal variation in triggering one of two developmental pathways—nondiapause or winter diapause—and different propensities for performing return migrations explain the ecological flexibility of the Large White from the Arctic Circle to Mediterranean and subtropical climates. We were therefore surprised to find a population of Pieris brassicae near the western limit of distribution in southern Spain which exhibits a pronounced summer diapause as a third strategy of development. This is particularly intriguing since there is no evidence for summer diapause from other areas with similar climatic conditions (Gardiner, 1978, Ghosh, 1914, Klein, 1932, Maslennikova, 1959).
The following questions concerning the newly discovered summer diapause in P. brassicae are addressed in this study:
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what consequences does it have for the ecology of the species?
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when and where has it evolved?
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how is it inherited?
Section snippets
Origin of the Spanish population and rearing conditions
The Spanish population of Pieris brassicae was collected at the end of April 1993 and end of March 1994 in San Enrique de Guadiaro near the village Sotogrande in southern Spain (36.32°N/5.31°W; see Fig. 1). In 1994 a laboratory strain (strain S) was established from more than 50 egg batches collected from two cauliflower fields. The rearing in the laboratory took place in temperature-controlled cabinets (accuracy ±0,5°C) and simulated photoperiod (see below) under approximately constant
Photoperiodic response of Pieris brassicae in southern Spain
The photoperiodic response of strain S was experimentally determined between 10 h 30 min and 15 h (18 h at 24°C) of light under a 24 h light/dark cycle, temperatures ranged from 17°C to 24°C (Fig. 2, Fig. 3). Some additional data were collected at 15°C and 12°C (see Fig. 3).
Winter diapause as a reaction to short daylengths is well known in all populations of P. brassicae tested so far (Fig. 1). Diapause as a response to long daylengths, however, represents a previously unknown summer diapause
Properties of the photoperiodic response of Pieris brassicae in southern Spain
The population from southern Spain is the first for which summer diapause in Pieris brassicae has been indicated. The unambiguous distinction between summer and winter diapause in P. brassicae on the basis of the duration of the pupal stage is not possible. However, unlike the situation in other lepidopteran species such as Mamestra brassicae (Kimura and Masaki, 1992, Sauer and Grüner, 1988) an unequivocal distinction between summer and winter diapause can be made in Pieris brassicae when the
Acknowledgments
Several local farmers deserve thanks for the support and permission to collect on their fields. The Zambrana family provided accomodation and Dina Heitland helped collecting during the 1994 season. J.B. Mallet and two anonymous reviewers provided valuable comments on earlier versions and helped to greatly improve the quality of this manuscript.
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