Abstract
Fixed chromosomal inversions can reduce gene flow and promote speciation in two ways: by suppressing recombination and by carrying locally favoured alleles at multiple loci. However, it is unknown whether favoured mutations slowly accumulate on older inversions or if young inversions spread because they capture pre-existing adaptive quantitative trait loci (QTLs). By genetic mapping, chromosome painting and genome sequencing, we have identified a major inversion controlling ecologically important traits in Boechera stricta. The inversion arose since the last glaciation and subsequently reached local high frequency in a hybrid speciation zone. Furthermore, the inversion shows signs of positive directional selection. To test whether the inversion could have captured existing, linked QTLs, we crossed standard, collinear haplotypes from the hybrid zone and found multiple linked phenology QTLs within the inversion region. These findings provide the first direct evidence that linked, locally adapted QTLs may be captured by young inversions during incipient speciation.
Highlights
Fixed chromosomal inversions can reduce gene flow and promote speciation in two ways: by suppressing recombination and by carrying locally favoured alleles at multiple loci
Do inversions drift to higher frequency, and acquire new advantageous mutations after they are common? Or are multiple linked, advantageous alleles captured in a new inversion, allowing them to spread together? Analysis of younger inversions may elucidate the evolutionary forces controlling the initial spread of chromosome inversions, which influence their role in adaptation and speciation[4,8]
We showed that B. stricta has two ecologically differentiated subspecies (‘East’ and ‘West’)[15,16], which form a contact zone across >200 km in the northern Rocky Mountains
Summary
Fixed chromosomal inversions can reduce gene flow and promote speciation in two ways: by suppressing recombination and by carrying locally favoured alleles at multiple loci. To test for phenotypic effects of the inversion or transcription changes near the breakpoints, we compared closely related inverted and standard haplotypes from the inversion zone, using a sympatric West-inv ×West-std F2 cross (Fig. 3b, left).
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