Inferring the past and present connectivity across the range of a North American leaf beetle: combining ecological niche modeling and a geographically explicit model of coalescence.

The leaf beetle Chrysomela aeneicollis occurs across Western North America, either at high elevation or in small, isolated populations along the coast, and thus has a highly fragmented distribution. DNA sequence data (three loci) were collected from five regions across the species range. Population connectivity was examined using traditional ecological niche modeling, which suggested that gene flow could occur among regions now and in the past. We developed geographically explicit coalescence models of sequence evolution that incorporated a two-dimensional representation of the hypothesized ranges suggested by the niche-modeling estimates. We simulated sequence data according to these models and compared them to observed sequences to identify most probable scenarios regarding the migration history of C. aeneicollis. Our results disagreed with initial niche-modeling estimates by clearly rejecting recent connectivity among regions, and were instead most consistent with a long period of range fragmentation, extending well beyond the last glacial maximum. This application of geographically explicit models of coalescence has highlighted some limitations of the use of climatic variables for predicting the present and past range of a species and has explained aspects of the Pleistocene evolutionary history of a cold-adapted organism in Western North America.

Effects of temperature on physiology and reproductive success of a montane leaf beetle: implications for persistence of native populations enduring climate change.

Understanding how climate change impacts natural systems requires investigations of the effects of environmental variation on vulnerable species and documentation of how populations respond to change. The willow beetle Chrysomela aeneicollis is ideal for such studies. It lives in California's Sierra Nevada on the southern edge of its worldwide range. Beetles experience elevated air temperatures during summertime egg laying and larval development. Exposure to these temperatures causes physiological stress, which may reduce reproductive success and endanger populations. The glycolytic enzyme phosphoglucose isomerase (PGI) is a marker of temperature adaptation in C. aeneicollis. PGI allele frequency varies across a latitudinal gradient: allele 1 is common in Rock Creek (RC), which is cooler and to the north, and allele 4 is common in Big Pine Creek (BPC), which is warmer and to the south. In populations that are intermediate in geography and climate (e.g., Bishop Creek [BC]), PGI-4 frequency increases from north to south such that alleles 1 and 4 are in relatively equal frequency in southern BC. Over the past decade, Sierra Nevada beetle populations have colonized high elevations and have become extinct at lower elevations where they were once common. In BC, the magnitude of PGI allele frequency fluctuations among life-history stages is related to maximal air temperature, with the frequency of PGI-4 increasing after the hottest part of summer. To identify mechanisms that may cause shifts in PGI allele frequency, we measured metabolic rate and fecundity for beetles collected at BC. Metabolic rate of males and females was measured at 20 degrees and 36 degrees C using flow-through respirometry. To measure laboratory fecundity, mating pairs were acclimated for 4 h each afternoon at a control temperature (20 degrees C) or at mildly elevated temperatures (26 degrees or 32 degrees C) and number of eggs laid was counted daily for 24 d, after which tissue levels of 70-kD heat shock proteins (Hsp70) were determined. Previous studies had demonstrated differences in Hsp70 expression among PGI genotypes at these temperatures. To measure field fecundity, mating pairs from BC were transplanted to similar elevations in BPC, BC, and RC and were monitored in situ for 24 d. Metabolic rate was higher for PGI 4-4 genotypes than for PGI 1-4 or PGI 1-1 individuals at 36 degrees C but not at 20 degrees C. In contrast, laboratory fecundity was greatest for females possessing PGI-1, independent of acclimation temperature. At the end of the laboratory fecundity experiment, Hsp70 expression was positively related to fecundity, suggesting minimal reproductive cost of upregulation of heat shock proteins in response to mild heat stress. In the field, fecundity was highest for PGI 1-1 and PGI 1-4 individuals in RC and PGI 4-4 individuals in BPC and was similar for all genotypes in BC. Thus, fecundity in nature was greatest for the genotypes that were most common in each area. Taken together, data reported here suggest that hot, dry summers in the Sierra Nevada may result in an increase in frequency of the PGI-4 allele and shifts to higher elevations for C. aeneicollis populations.