Limits to the southern border of Drosophila serrata: Cold resistance, heritable variation, and trade-offs

There are a number of evolutionary hypotheses about why species distributio ns are limited, but very little empirical information to test them. We pres ent data examining whether the southern distribution of Drosophila serrata is limited by cold responses. Species comparisons were undertaken for cold resistance, development time, and viability at 15 degrees C and 25 degrees C for D. serrata and other species with a more southerly distribution (D. m elanogaster, D. simulans, and D. immigrans). Relative to the other species, D. serrata had a long development time at both temperatures and a low leve l of cold resistance. Using isofemale lines collected in different seasons, central and marginal populations were compared for cold resistance, as wel l as development time and viability at 14 degrees C. The border population had a relatively higher resistance to cold shock in postwinter collections, but there was no population differentiation for prewinter collections or f or the other traits. The presence of variation among isofemale lines within the border populations suggests that genetic variation as measured in the laboratory is unlikely to limit range expansion. Population cages were used in the field to determine if D. serrata persisted over winter at borders. Although all cages yielded adult offspring at northern sites, only a few pr oduced offspring at or just south of the border. In contrast, all cages wit h D. simulans produced adult offspring, suggesting that climatic factors li mited D. serrata numbers. Offspring from surviving adults showed a phenotyp ic trade-off between fecundity and cold resistance. Comparisons of the cold resistance of field males and females with their laboratory-reared offspri ng provided evidence for heritable variation in field-reared flies. Overall , the results suggest that cold stress is important in limiting the souther n distribution of D serrata but it seems unlikely that a lack of genetic va riation restricts range expansion.