ADAPTIVE SIGNIFICANCE OF PIGMENT POLYMORPHISMS IN COLIAS BUTTERFLIES, II. THERMOREGULATION AND PHOTOPERIODICALLY CONTROLLED MELANIN VARIATION IN Colias eurytheme

Abstract

Abstract.-The butterfly Colias eurytheme requires body temperatures above 30'C for flight. When cold, it orients its exposed wing undersides to present maximum surface area to sunlight; when too warm, it orients for minimum exposure. Dark-winged color forms heat faster in sunlight than light ones. The seasonal color polymorphism of Colias appears to have been evolved to maximize solar heating in cold seasons and minimize overheating in warm seasons.Temperate zone organisms experience great seasonal variation in many environmental parameters. An organism limited in occurrence by one (or more) of these might adapt to this condition in several ways. It might restrict especially sensitive parts of its life cycle to favorable seasons or suspend activity altogether in unfavorable seasons. It might simply evolve broader tolerence to the limiting forces, or it might retain narrow tolerances but evolve some means of changing the limits of these from season to season. The latter might be done by establishment of a genetic polymorphism in the population, so that one morph would be maximally adapted to each season, and frequencies would cycle seasonally, as in certain Drosophila chromosomal polymorphisms.1 Alternatively, all individuals might show the same phenotype at one time but alter it from season to season to increase adaptation to each. Bearers of such a seasonal shift mechanism would not be selectively disfavored in some seasons as would, for example, a warmadapted genetic-morph in a cool season.Such a seasonal shift is exhibited by the common "sulfur" butterflies, Colias eurytheme Boisduval and C. philodice Latreille. Summer broods of these closely related species2 display clear, unclouded orange or yellow color, due to pteridine pigments,3 on the hindwing undersides. Spring and fall broods show an intense darkening of this area due to replacement of the pteridines by black melanin. Ae,4 after a suggestion by Remington,' showed that the day length to which the immature stages are exposed controls the adult pigment. Long photoperiods cause larvae to produce summer-form adults, whereas sibling larvae reared under a short photoperiod produce dark spring-fall forms.In the normal adult Colias' resting position, the wings are folded over the back, exposing the undersides; thus, thorax and abdomen are closely covered by that part of the hindwing which is most darkened in the short-day form. This suggested that the dark form might absorb more solar energy than its light counterpart and thus be better adapted to the cool climate of spring and fall. Development of thermistor probes so small as to monitor Colias' temperatures without impeding their normal physical activity6 has made possible the test of this suggestion. 767

How do phenology, plasticity, and evolution determine the fitness consequences of climate change for montane butterflies?

“…For example in many insects, thermal conditions for generations emerging in the early spring can be drastically different from those for generations emerging in midsummer, and plastic responses to developmental environments can lead to different phenotypes in different seasonal generations (Brakefield & Larsen, 1984; Shapiro, 1976). Developmental plasticity is central to adaptation to seasonal environments in both tropical and temperate systems (Brakefield, 1987; Tauber & Tauber, 1976; Watt, 1969). However, the extent to which developmental plasticity and evolution are adaptive and influence fitness in variable, seasonal environments is poorly understood.…”

Viability Selection on Seasonally Polyphenic Traits: Wing Melanin Pattern in Western White Butterflies

“…Between late March and early August there are 4-5 adult generations each year at this site. males also sometimes use a lateral basking posture (Watt 1968) when they first emerge from roosting sites in the morning (D. Wiernasz unpubl. data); in this posture, increased melanin in the posterior regions of the ventral hind wing (VHW) increases body temperature (Watt 1968;Kingsolver 1987).…”

“…males also sometimes use a lateral basking posture (Watt 1968) when they first emerge from roosting sites in the morning (D. Wiernasz unpubl. data); in this posture, increased melanin in the posterior regions of the ventral hind wing (VHW) increases body temperature (Watt 1968;Kingsolver 1987). When P. occidentalis experiences body temperatures above 40-42°C, it uses a heat-avoidance posture that reduces solar radiative heating; in this posture, increased melanin in the posterior regions of the ventral hindwing (VHW) increases body temperature (Kingsolver 1987 …”

“…Many aspects of melanin pattern and wing size exhibit striking seasonal polyphenism in this species (Shapiro 1976;Kingsolver and Wiernasz 1991a). Analyses of the functions of melanin pattern show that some aspects of this polyphenic response are adaptive for thermally varying seasonal environments (Watt 1969;Kingsolver 1987;Kingsolver and Wiernasz 1991a). The goal of this study is to use mark-release-recapture studies to estimate selection via adult survival on polyphenic wing traits during several seasons.…”

The Evolution of Wing Color in Colias Butterflies: Heritability, Sex Linkage, and Population Divergence

“…This has been demonstrated under laboratory conditions in many insects, including beetles (Brakefield and Willmer 1985;Stewart and Dixon 1989;de Jong et al 1996;Rhamhalinghan 1999), bees (Pereboom and Biesmeijer 2003), butterflies (Watt 1968(Watt , 1969, and grasshoppers (Forsman 1997). However, it has also been suggested that the difference in body temperature between dark and light individuals would be too small to be of any ecological importance, or that other effects, such as predator avoidance, would swamp any thermoregulatory advantage (Digby 1955;Willmer and Unwin 1981).…”