A. Prevosti

THE COLONIZATION OF DROSOPHILA SUBOBSCURA IN CHILE. II. CLINES IN THE CHROMOSOMAL ARRANGEMENTS

Drosophila subobscura is a Palearctic species that was first detected in the New World in Puerto Montt (Chile) in February 1978. Since that time, it has spread over a broad area and increased in population density. The South American populations exhibit a high level of chromosomal polymorphism: 20 different arrangements exist, distributed among five chromosomes. Chromosomal arrangement heterozygosity varies from 0.55 to 0.61 in the nine populations examined. Incipient clines in the frequencies of the arrangements are appearing; these clines follow the same latitudinal direction as in the Old World. Wing length significantly decreases with latitude, as it does in Europe. The colonization of South America by D. subobscura appears to be a major natural experiment with outcomes that duplicate the distributional patterns—in chromosomal polymorphism and in wing length—observed in the Old World, thereby strongly supporting the adaptive significance of these patterns. The data show a very rapid effect of natural selection promoting genetic differentiation among natural populations.

Colonization of Drosophila subobscura in Chile I. First population and cytogenetic studies

In February 1978, in Puerto Montt (Chile) the palearctic species Drosophila subobscura was detected. The expansion of the species in this country has been very rapid, and now it is found over a distance of at least 2000 km North-South. The newly established populations are very flourishing and show a high degree of inversion chromosomal polymorphism. On the basis of the chromosomal arrangements present, an hypothesis can be formulated about the origin and characteristics of the founder group. Possibly, the founders came from Eastern or South Eastern Spain and formed a group of 10 or more individuals. The colonization of D. subobscura in Chile seems to follow the model of expansion of a cosmopolitan species, passively transported by man rather than the more active expansion of colonizers less tied to human activity, envisaged in Carsons models of colonization processes. The high level of chromosomal polymorphism observed in the populations of D. subobscura in Chile, coincides with this interpretation.

CLINES OF CHROMOSOMAL ARRANGEMENTS OF DROSOPHILA SUBOBSCURA IN SOUTH AMERICA EVOLVE CLOSER TO OLD WORLD PATTERNS

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COLONIZATION OF AMERICA BY DROSOPHILA SUBOBSCURA: LETHAL GENES AND THE PROBLEM OF THE O5 INVERSION

In this work, the process of colonization of North and South America by the species Drosophila subobscura has been studied by analyzing the variability of lethal genes. The genetic structures of a Palearctic natural central population (Bordils, Spain) and a colonizer population from America (Gilroy, California) have been compared. The frequencies of lethal chromosomes and their allelism are 29.007% and 0.0069 in the first population and 14.414% and 0.0526 in the American population. A founder effect is detected after the computation of some population parameters (Ne, h, he and the lethal load). Furthermore, the allelism of lethal chromosomes has revealed a strong association between a lethal gene and the O5 inversion both in Gilroy and in the population of Puerto Montt (Chile). The interpopulation allelism shows that the O5 arrangement from the USA and Chile is the same, confirming that the colonizing processes of North and South America are correlated. The O5 arrangement can also be useful as a genetic marker to trace the origin of the colonization. The frequency of the O5 arrangement in the original population of the colonization could be used to estimate the number of colonizers. This population is still unknown, but taking the extreme values of the frequency of the O5 inversion in natural Palearctic populations (1–15%), the number of colonizers could vary between 9 and 149 individuals.

An experimental study of evolution in progress: Clines for quantitative traits in colonizing and Palearctic populations ofDrosophila

SummaryDrosophila subobscura has recently colonized the American continent and is an excellent model for studying evolution in action. Previous analyses have shown that these colonizing populations have significant latitudinal clines for the frequencies of some chromosomal arrangements that parallel those clines found in the Old World. These results strongly suggest that this polymorphism is adaptive. In the present study, significant latitudinal clines for continuous morphometric variables (flies are larger in the north) have been detected in Old World populations ofD. subobscura. The adaptive nature of these clines is reinforced by the fact that parallel latitudinal clines for body size have also been detected inDrosophila obscura, a closely related sympatric species, as well as previously in otherDrosophila. On the other hand, no significant latitudinal clines for continuous morphometric traits, not even when using an overall size index, have been detected in colonizing populations ofD. subobscura. This is a rather surprising result given the number of generations that have elapsed since the species was detected in America and given that significant clines in chromosomal inversions are already established. Thus, the adaptive response of quantitative morphometric variables is not as rapid as that found for chromosomal inversions. Nevertheless, canonical correlation analysis suggests that significant latitudinal clines for body size might soon be detected in the American continent. The results obtained here are included in a projected time series with the aim of documenting size evolution in action.

Chromosomal inversion polymorphism in the southewestern range ofDrosophila subobscura distribution area

The level of chromosomal inversion polymorphism is much higher in three populations from South Morocco than in the nearby Canary and Madeira Islands. The composition of the chromosomal arrangements is also strikingly different in both areas. The similarity among the populations from the islands excludes drift or the founder principle as an explanation of these differences. A variety of reasons support the conclusion that the polymorphism in the islands is a relict polymorphism, corresponding to a less evolved stage than the one present in the mainland. The populations from South Morocco have a relatively low index of free recombination, like others from the southern range of the distribution ofD. subobscura. This contrasts with the high values found in the northern range. The populations from the North are relatively recently established ones, whereas those from the South are longer established and have become marginal only in relatively recent times. This difference in evolutionary history is sufficient to explain the differences in the index of free recombination.

Temporal changes of chromosomal polymorphism in natural populations of Drosophila subobscura

The behaviour of the chromosomal polymorphism of D. subobscura was analyzed in relation to temporal changes, daily, seasonal and annual. Firstly, chromosome analyses were carried out over a year in a natural population. Samples were taken at monthly intervals in Tibidabo, a locality close to Barcelona. In all the months except January, the number of individuals captured was enough to carry out a chromosome analysis of that population. The A, E and O chromosomes showed a great uniformity in the frequencies of gene arrangement over the year. However, significant changes occur in the frequencies of J and U chromosomes. The J1 and U1+2+8 arrangements showed a similar tendency, with two maxima, in June and February, and a minimum in September. These changes seem to be unrelated to the cyclical climatic changes. A chromosome analysis of the subsamples captured during the day, in the May sample, was done. In no case was the change in chromosome frequencies statistically significant. The behaviour of the Ust arrangement must be noted, the frequency of which decreased through the day. Finally, the data for the Tibidabo population were compared with samples captured in nearby localities over a period of 21 years. Significant differences were found in the frequencies of all the chromosomes, with the exception of J. The most differentiated population was the last sampled, from 1982. In this population the st arrangements tended to decrease in the A, E and O chromosomes and A2, E1+2+9+12 and O3+4+7 increase, respectively.

Colonization of America by Drosophila subobscura : analysis of the O5 inversions from Europe and America and their implications for the colonizing process

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.

High ethanol tolerance in two distantly related Drosophila species: A probable case of recent convergent adaptation

Abstract 1. D. lebanonensis was found to be more abundant in wine cellars of Spain than outdoors. 2. This suggested that the species could have developed a high alcohol tolerance in a similar way to D. melanogaster . 3. Laboratory analysis confirmed this hypothesis and, surprisingly, showed that D. lebanonensis was much more tolerant to alcohol and more able to use it as as food than is D. melanogaster . 4. Environmental alcohol seems to be the selective factor which determined this convergent adaptation. 5. However the difference between the two species could also be explained by assuming that the main target of natural selection was not the same in the two species: ethanol detoxification in D. melanogaster and metabolic utilization in D. lebanonensis .

Colonization and Establishment of the Paleartic Species Drosophila Subobscura in North and South America

Until 1978 Drosophila subobscura was a Palearctic species distributed all over Europe (except in Central and Northern Scandinavia), the Macaronesian Islands, North Africa and some parts of Western Asia. In most of this area it is a common species with rather dense populations. This species was detected for the first time in Chile in February 1978, in Puerto Montt in the South of the country (Brncic et al. 1981). Subsequently it has spread very quickly in Chile and in 1981 was present from La Serena (29°55′ LS) to Punta Arenas (53°10′ LS) (Budnik and Brncic 1982). In November 1981 we did find the species in large numbers in San Carlos de Bariloche (Argentina), east of the Andes (Prevosti et al. 1983). Later on, in November 1986, it was collected east of the Andes in Argentina from San Juan (31°33′ LS) to Esquel (42°55′ LS) (Table 1). A small and isolated collection by Lopez (1985) in Mar del Plata on the Atlantic Coast, about 400 km south of Buenos Aires, is the only finding of the species in other parts of Argentina. The same author (personal communication) did not find the species in other parts of the Buenos Aires province.