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Introgression and genomic differentiation in sympatric, hybridizing Colias butterflies
Colias eurytheme and C. philodice are sister species with broad sympatry in North America. They hybridize frequently and likely share a significant portion of their genomes through introgression. Using AFLP markers, we conducted linkage analysis on two families of Colias butterflies. The paternal map from the backcross family is composed of 452 markers spanning 1114.6 cM distributed over 40 linkage groups. Statistical tests indicate that these AFLP markers tend to cluster over the map. This non-random marker distribution can account for the nonequivalence between the number of linkage groups and the haploid chromosome number. A similar pattern was found for the paternal map from the pure family. Our results also confirmed that female Colias do recombine, but at a very reduced rate, a situation known as heterochiasmy. While performing the genetic mapping analysis, we found a unique pattern of segregation distortion shown by X-linked AFLP markers present in the backcross family. The distorted segregation seems to provide the philodice phenotype a significant advantage, as philodice-specific loci are favorably transmitted into the offspring. We incorporated the effect of such segregation distortion into a simulation of the evolutionary dynamics of the two species. Although the distortion does confer C. philodice more evolutionary leverage, an unstable polymorphism still remains. With the map serving as the foundation, we investigate the genetic architecture of multiple wing pattern elements in Colias butterflies. QTL mapping analysis was conducted with the same two families used for linkage analysis. Our results reveal the polygenic nature of these quantitative wing characters. Remarkably, a majority of the traits measured are controlled by at least one X-linked factor, indicating large influence of the X-chromosome. Finally, we estimate the gene exchange rates throughout the genome and clarify the spatial distribution of introgressed vs. differentiated regions on the chromosomes. Our results indicate that C. eurytheme and C. philodice share a significant portion of their autosomal loci, but differ largely in the X-linked regions. We conclude that these butterflies comprise an example of an animal syngameon, and their species identities are better viewed as mosaic of stable, correlated polymorphisms in a dynamic, fluid gene pool.
Wang, Baiqing, "Introgression and genomic differentiation in sympatric, hybridizing Colias butterflies" (2005). Doctoral Dissertations Available from Proquest. AAI3179932.