One fundamental signature of reinforcement is elevated prezygotic reproductive isolation between related species in sympatry relative to allopatry. However, this alone is inadequate evidence for reinforcement, as traits conferring reproductive isolation can occur as a by-product of other forces. We conducted crosses between Silene latifolia and S. diclinis, two closely related dioecious flowering plant species. Crosses with S. latifolia mothers from sympatry exhibited lower seed set than mothers from five allopatric populations when S. diclinis was the father. However, two other allopatric populations also exhibited low seed set. A significant interaction between style length and sire species revealed that seed set declined as style length increased when interspecific, but not intraspecific, fathers where used. Moreover, by varying the distance pollen tubes had to traverse, we found interspecific pollen placement close to the ovary resulted in seed set in both long- and short-styled S. latifolia mothers. Our results reveal that the long styles of S. latifolia in sympatry with S. diclinis contribute to the prevention of hybrid formation. We argue that forces other than reinforcing selection are likely to be responsible for the differences in style length seen in sympatry.
We utilized available Drosophila simulans molecular population genomic data to characterize sequence polymorphism in noncoding regulatory regions and their corresponding transcribed target genes or interacting transcription factors. We highlight two properties of regulatory evolution. First, we find that sequence divergence between D. melanogaster and D. simulans is greater at regulatory sites than expected from levels of intraspecific polymorphism, suggestive of positive selection. Second, we compared variation in cis-regulatory regions to that of the transcription factors and transcribed target genes with which they are associated. We find a correlation in polymorphism between cis-regulatory regions and the transcription factors which they bind, but not between cis-regulatory regions and the physically neighboring target genes which they directly regulate. This result indicates that the evolution of cis-regulatory elements is constrained by the interactions among transcription factors and regulatory regions. More broadly, these findings support the general importance of noncoding regulatory DNA in evolution.
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