Academic journal article Genetics

Genomics of Natural Populations: How Differentially Expressed Genes Shape the Evolution of Chromosomal Inversions in Drosophila Pseudoobscura

Academic journal article Genetics

Genomics of Natural Populations: How Differentially Expressed Genes Shape the Evolution of Chromosomal Inversions in Drosophila Pseudoobscura

Article excerpt

MODIFIERS reducing rates of recombination are likely to play an important role in evolution when a species is adapting to a heterogenous environment (Otto and Barton 2001). When a species encounters a heterogeneous environment, positive modifiers of recombination may initially be favored to generate many different gene combinations as a way to best adapt to the novel habitat. The problem then becomes how to preserve these adaptive combinations once good solutions are found. Negative modifiers of recombination will be favored if they capture adaptive combinations of genes in linkage disequilibrium that provide higher fitness in the new niche (Feldman 1972; Charlesworth and Charlesworth 1973; Charlesworth 1974). Inversion mutations are a type of negative recombination modifier that could allow an organism to hold associations between beneficial combinations of genes together.

Inversionsare aclassof structuralmutationwhere theorder of genes within a chromosomal segment is reversed. Not only do chromosomal inversions modify gene order, but they suppressrecombinationinheterokaryotypes (Dobzhanskyand Epling 1944; Levine and Levine 1954; Levine 1956), and have been shown to change levels of expression of genes that flank the breakpoints (Puig et al. 2004). Since their initial discovery nearly a century ago in Drosophila melanogaster, inversions have been implicated in such evolutionary processes as adaptation, disease susceptibility, sex chromosome evolution, and speciation (Sturtevant 1921; Hartl 1975; Lahn and Page 1999; Noor et al. 2001; Stefansson et al. 2005; Kirkpatrick and Barton 2006; Evans et al. 2007). Numerous fixed inversion differences have been detected among homologous chromosomes from different species, including members of the Drosophila genus and between humans and chimpanzees (Carson and Yoon 1982; Wasserman 1982; Feuk et al. 2005; Bhutkar et al. 2008; Kirkpatrick 2010). Furthermore, chromosomal inversions are also polymorphic within species, including humans (Sperlich and Pfriem 1986; Andolfatto et al. 2001; Bansal et al. 2007). Despite their ubiquity in nature, the evolutionary factors influencing the origin, spread, and maintenance of inversions in populations are not well understood.

The third chromosome (Muller element C) (Schaeffer et al. 2008) of D. pseudoobscura offers an ideal system to test hypotheses regarding the evolutionary forces responsible for the establishment and maintenance of inversions in natural populations. This chromosome is polymorphic for .30 naturally occurring gene arrangements that were generated by a series of overlapping inversion mutations (Dobzhansky and Queal 1938; Dobzhansky and Sturtevant 1938; Dobzhansky 1944). Circumstantial evidence has suggested that inversions are adaptive. The relative frequencies of arrangements in D. pseudoobscura populations form a stable east-to-west cline across the American Southwest (Dobzhansky 1944; Anderson et al. 1991; Schaeffer and Miller 1992) where the frequency shifts coincide with major climatic and geographic differences that fall into one of six ecological niches (Lobeck 1950; Schaeffer 2008). Niches are found to be either polymorphic or monomorphic for gene arrangements (Figure 1). For example in niche 1, three gene arrangements dominate the westernmost region of California. In niche 4, however, one chromosome type is near fixation in the Four Corners region of the Colorado Plateau. In the eastern niche 6, two arrangements are frequent in western Texas. The cline has been stable since the 1940s despite sufficient gene flow to homogenize frequencies among populations (Anderson et al. 1991; Schaeffer and Miller 1992). Previous studies have provided evidence of extensive migration and low genetic differentiation across the genome, with the exception of the inverted regions (Riley et al. 1989; Kovacevic and Schaeffer 2000; Schaeffer et al. 2003). Furthermore, population structure is not observed on Muller C within arrangements (Schaeffer et al. …

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