Chromosome Studies in Wild Populations of DROSOPHILA MELANOGASTER. II. Relationship of Inversion Frequencies to Latitude, Season, Wing-Loading and Flight Activity.

In the midwestern and eastern U.S. populations of Drosophila melanogaster, the Standard gene arrangements show higher frequencies in the north than in the south. In a Missouri population, and to a lesser extent in a south Texas population, the frequencies of Standard chromosomes regularly rise during the cold season and drop during the warm season, thus paralleling the north-south frequency differences. In the Missouri population in 1976 and 1978, wild males were tested for their ability to fly to bait at different ambient temperatures. In both years, males flying in nature in the temperature range of 13 degrees to 15 degrees showed significantly higher frequencies of Standard chromosomes than did those flying in the 16 degrees to 28 degrees range. Wild males flying at 13 degrees to 15 degrees also have different thorax/wing proportions and significantly lower wing-loading indices than do those flying at 16 degrees to 28 degrees . Moreover, wild flies homozygous Standard in 2R and/or 3R have significantly lower wing-loading indices than flies carrying inversions in these arms. Thus, wild flies with high frequencies of Standard chromosomes are karyotypically northern, are selectively favored during the cold season, have a relatively low wing-load and are most capable of flying at critically low ambient temperatures.-In summary, in Missouri, presence or absence of the common cosmopolitan inversions is an important factor in low temperature adaptation, and at least part of the adaptive mechanism involves control of thorax/wing proportions and thus control of wing-loading.