Contrasting population genetic structures using allozymes and the inversion polymorphism in Drosophila buzzatii

The cactophilic Drosophila buzzatii (buzzatii complex ± repleta group, Ruiz & Wasserman, 1993) is native to South America and has attained a subcosmopolitan distribution by following passively its native host plants of the genus Opuntia. Studies in natural populations have shown that second chromosome polymorphic arrangements affect life history traits and morphometric traits associated with ®tness providing evidence of the adaptive role of the inversion polymorphism (Ruiz et al., 1986; Hasson et al., 1991, 1992; Ruiz et al., 1991; Barbadilla et al., 1994; Norry et al., 1995; Fanara et al., 1996, 1997; Norry et al., 1997a,b; RodrõÂguez et al., 1999; FernaÂndez Iriarte & Hasson, 2000). Clinal variation over large geographical areas can be taken as indirect evidence of strong selective forces (Endler, 1977). Previous work on D. buzzatti has shown that inversion frequencies are correlated with latitudinal gradients in different continents (Knibb & Barker, 1988; Hasson et al., 1995). Moreover, in South America population structure is coincident with phytogeographical regions, suggesting that karyotype ®tnesses vary in different areas (Ruiz, 1982; Hasson et al., 1995). Studies of allozyme variation in colonized populations from Australia and Spain have suggested a signi®cant role of natural selection shaping allele frequency distributions for several loci, particularly Esterase-1 (Est-1), Esterase-2 (Est-2) and Aldehyde-oxidase (Aldox) (Barker & East, 1980; Barker et al., 1986; Quezada-DõÂaz, 1993). However, these loci are tightly linked to the chromosomal segments rearranged by inversions (Quezada-DõÂaz, 1993; Schafer et al., 1993; BetraÂn et al., 1995). Thus, ®tness differences among arrangements may be causally