Enzymatic and quantitative variation in European and African populations of Drosophila simulans

Abstract

Allozyme polymorphism at 15 loci of D. simulans was studied in 7 natural populations from Europe, North and tropical Africa. Morphological traits were studied in nine European and eleven Afrotropical strains.Within a population, the biochemical polymorphisms of Drosophila simulans and its sibling Drosophila melanogaster are not very different, although D. simulans has a lower heterozygosity. Between-populations genetic differentiation is however much lower in D. simulans than in D. melanogaster. Several loci of D. simulans do exhibit latitudinal trends but these are relatively weak.For morphological traits, both species show an increase of size with latitude, but geographic variation is again less pronounced in D. simulans. Both species are native to tropical Africa and have colonised the rest of the world. During this process, D. simulans has undergone much less geographic differentiation than has D. melanogaster, so that the ecological success of the two species is not correlated with similarities in their genetic properties. INTflODUCTIONAmong about 2500 species known in the drosophilid family (Wheeler, 1981), only 21 have a subcosmopolitan status, in that they exist in at least three non-adjacent biogeographic realms (David and Tsacas, 1981). All these species occupy their present geographic range as a result of involuntary transport by man. Among the 21 species, the sibling D. melanogaster and D.simulans are especially interesting since they are the only two which possess large populations in both tropical and temperate conditions (David and Tsacas, 1981). Many studies of the ecology, ecophysiology and genetics of these domestic species have attempted to understand their population dynamics, ecology and other reasons of which may underlie their demographic success (e.g., Parsons, 1975; l980a;Parsons and Stanley, 1981).Populations of D. melanogaster, are genetically highly differentiated, for chromosomal polymorphism (Ashburner and Lemeunier, 1976;

Mitochondrial DNA variability in Drosophila simulans: quasi absence of polymorphism within each of the three cytoplasmic races

“…simulans have shown almost as high a heterozygosity as that of D. melanogaster (Hyytia et al, 1985; Choudhary and Cariou, personal communication). The low mitochondrial variability within each race, associated with a normal heterozygosity of nuclear genes, is an argument in favour of founder event (see Wilson et a!., 1985).…”

“…The recency of such an expansion is attested to by its mitochondrial monomorphism over large areas and by the great genetic similarities of the nuclear genomes of remote populations (Hyytia et a!., 1985;Choudhary and Singh, 1987).…”

“…D. melanogaster shows great geographic variation and such "racial" differentiation may have allowed the colonization of places with very different climates. D. simulans is less studied but, for several traits, such as chromosomal polymorphism (Ashburner and Lemeunier, 1976), allozyme frequencies (Hyytia et a!., 1985; thoracic trident pigmentation (Capy eta!., 1988) and alcohol tolerance (David and Bocquet, 1975) appears to be genetically much less variable between populations.…”

Molecules versus morphology: the detection of selection acting on morphological characters along a cline in Drosophila melanogaster

“…The present study did not observe the same monotonic dines that have been previously reported. This is not surprising when one considers the sampling schemes used in other studies where flies were collected: at irregular intervals (Stalker & Carson, 1947;Sokoloff, 1965), from too few locations (Hyytia Ct al., 1985), without regards for longitude or altitude (Stalker & Carson, 1947;Sokoloff, 1965), and at different times of the year (Stalker & Carson, 1947;Sokoloff, 1965;Hyytia et a!., 1985;Watada et a!., 1986). Any or all of these factors could have added variation which would tend to obscure a nonnionotonic trend (Stalker & Carson, 1948, 1949.…”

Thoracic trident pigmentation in natural populations of Drosophila simulans: a comparison with D. melanogaster

“…Moreover, when two closely related species, like D. melanogaster and D. simulans, which have similar geographic distributions and population dynamics (David and Tsacas, 1981), exhibit parallel genetical trends, the argument for adaptation becomes still stronger (Tantawy and Mallah, 1961;Parsons, 1983;Anderson and Oakeshott, 1984;Hyytia et a!., 1985;Lemeunier et a!., 1986).…”