COLONIZATION OF AMERICA BY DROSOPHILA SUBOBSCURA: ANALYSIS OF THE O5 INVERSIONS FROM EUROPE AND AMERICA AND THEIR IMPLICATIONS FOR THE COLONIZING PROCESS.

Abstract

The American colonization by the Palearctic species D. subobscura has been studied at the biogeographical, ecological and genetic levels (Bmcic et al., 1981; Prevosti et al., 1983, 1985, 1988, 1990; Beckenbach and Prevosti, 1986; Latorre et al., 1986; Ayala et al., 1989; Rozas and Aguade, 1991). In the present study we will focus on the variability of lethal genes and their allelism. The first studies at this level carried out in American populations of D. subobscura (Gilroy, California and Puerto Montt, Chile) have shown a strong association between a lethal gene and the 05 chromosomal inversion (Prevosti et al., 1989; Mestres et al., 1990). All 05 inversions so far analyzed in America are associated with the same lethal gene. Therefore, this lethal gene probably arose in the original Palearctic population and only one 05 inversion carrying this lethal gene reached America. Thus, the populations from North and South America would derive from the same original colonizing sample. Assuming that not all the European 05 inversions are associated with this lethal gene, the finding of this association in one European population would suggest that it was the population from which the colonization probably started. If the population of origin were detected, it would be possible to estimate the number of colonizers. We have analyzed samples of D. subobscura along a latitudinal cline in Europe to find 05 inversions and study their viabilities. Those that proved to be lethal were used to analyze the lethal allelism with the 05 inversions from America (both, from Gilroy and from Puerto Montt). Furthermore, an additional Chilean population (Santiago de Chile, located 900 km north of Puerto Montt) was analyzed to establish whether there was interpopulational allelism for other lethal genes in addition to that associated with the 05 inversion both in North and South America. Finally, the cytological pairing between the 05 inversions from Europe and from America was analyzed to determine their similarity at this level of observation.