Components of fitness and the PGI polymorphism in the freshwater isopod Asellus aquaticus (L). 1. Fecundity selection

Mitonuclear mismatch alters performance and reproductive success in naturally introgressed populations of a montane leaf beetle

Coordination between nuclear and mitochondrial genomes is critical to metabolic processes underlying animals' ability to adapt to local environments, yet consequences of mitonuclear interactions have rarely been investigated in populations where individuals with divergent mitochondrial and nuclear genomes naturally interbreed. Genetic variation in the leaf beetle Chrysomela aeneicollis was assessed along a latitudinal thermal gradient in California's Sierra Nevada. Variation at mitochondrial cytochrome oxidase II (COII) and the nuclear gene phosphoglucose isomerase (PGI) shows concordance and was significantly greater along a 65 km transect than 10 other loci. STRUCTURE analyses using neutral loci identified a southern and northern subpopulation, which interbreed in the central drainage Bishop Creek. COII and PGI were used as indicators of mitochondrial and nuclear genetic variation in field and laboratory experiments conducted on beetles from this admixed population. Fecundity, larval development rate, running speed and male mating frequency were higher for beetles with geographically "matched" than "mismatched" mitonuclear genotypes. Effects of mitonuclear mismatch were largest for individuals with northern nuclear genotypes possessing southern mitochondria and were most pronounced after heat treatment or at high elevation. These findings suggest that mitonuclear incompatibility diminishes performance and reproductive success in nature, effects that could intensify at environmental extremes.

Allele frequency shifts in response to climate change and physiological consequences of allozyme variation in a montane insect

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.