Allele Frequency Shifts in Response to Climate Change and Physiological Consequences of Allozyme Variation in a Montane Insect

Abstract

Rapid changes in climate may impose strong selective pressures on organisms. Evolutionary responses to climate change have been observed in natural populations, yet no example has been documented for a metabolic enzyme locus. Furthermore, few studies have linked physiological responses to stress with allozyme genotypic variation. We quantified changes in allele frequency between 1988 and 1996 at three allozyme loci (isocitrate dehydrogenase, Idh; phosphoglucose isomerase, Pgi; and phosphoglucomutase, Pgm) for the leaf beetle Chrysomela aeneicollis in the Bishop Creek region of the Sierra Nevada of California (2900-3300 m). Beetles often experience high daytime (> 32 degrees C) and extremely low nighttime (< -5 degrees C) temperatures during summer. Bishop Creek weather station data indicated that conditions were unusually dry before 1988, and that conditions were cool and wet during the years preceding the 1996 collection. We found directional changes in allele frequency at Pgi (11% increase in the Pgi-1 allele), but not at Idh or Pgm. We also found that physiological response to thermal extremes depended on Pgi genotype. Pgi 1-1 individuals induced expression of a 70-kD heat shock protein (HSP) at lower temperatures than 1-4 or 4-4 individuals, and 1-1 individuals expressed higher levels of HSP70 after laboratory exposure to temperatures routinely experienced in nature. Survival after nighttime laboratory exposure to subzero temperatures depended on gender, previous exposure to cold, and Pgi genotype. Females expressed higher levels of HSP70 than males after exposure to heat, and recovery by female Pgi 1-1 homozygotes after exposure to cold (-5 degrees C) was significantly better than 1-4 or 4-4 genotypes. These data suggest that the cooler climate of the mid-1990s may have caused an increase in frequency of the Pgi-1 allele, due to a more robust physiological response to cold by Pgi 1-1 and 1-4 genotypes.

Phosphoglucose isomerase (Pgi) performance and fitness effects among Arthropods and its potential role as an adaptive marker in conservation genetics

“…In this species, PGI allele frequencies are found to vary along a thermal cline that follows latitude and altitude (Dahlhoff and Rank 2000;Rank and Dahlhoff 2002). Specific alleles are associated with cold vs. warm habitats and differ in both their biochemical performance and thermal stability (Dahlhoff and Rank 2000), suggestive of strong direction selection along the observed clines rather than widespread within population balancing selection.…”

“…An intriguing aspect of the work on C. aeneicollis is the correlation of heat shock protein (Hsp70) expression with different PGI genotypes (Dahlhoff and Rank 2000;Rank and Dahlhoff 2002). Detailed studies of heat vs. cold stress on adult fecundity, metabolic rate, running speed, and survival, as well as larval growth rate and survival, have found that PGI genotypes and Hsp70 interact with thermal exposure duration and temp, thermal history, and sex (Dahlhoff et al 2008;Dahlhoff and Rank 2000;McMillan et al 2005;Neargarder et al 2003;Rank et al 2007;Rank and Dahlhoff 2002).…”

“…Detailed studies of heat vs. cold stress on adult fecundity, metabolic rate, running speed, and survival, as well as larval growth rate and survival, have found that PGI genotypes and Hsp70 interact with thermal exposure duration and temp, thermal history, and sex (Dahlhoff et al 2008;Dahlhoff and Rank 2000;McMillan et al 2005;Neargarder et al 2003;Rank et al 2007;Rank and Dahlhoff 2002). Hsp70 expression appears to buffer thermal tolerance and performance among PGI genotypes, with higher expression in the less thermally stable genotypes, thereby facilitating the maintenance of alleles across the thermally variable habitat of C. aeneicollis ).…”

“…As a result, Dahloff and Range have developed a research system that can explore the ways in which genetic variation interacts with climate change, providing a powerful study system that can connect population level genetic variation with anthropogenic impacts. Indeed analysis of long term climate data suggests an effect of local climate on repeated directional changes in PGI allele frequencies (Rank and Dahlhoff 2002).…”

“…First, both cold and heat stress appear to play an important selective role in the wild (McMillan et al 2005;Rank et al 2007;Rank and Dahlhoff 2002). Second, although the literature suggests there should be a cost to Hsp70 expression to buffer low stability of some PGI genotypes, there does not appear to be a cost to Hsp70 expression on fecundity (Dahlhoff et al 2008), but rather there may be more of a tradeoff between general thermal tolerance vs. energy allocation (Neargarder et al 2003).…”