Hybrid speciation refers to the establishment of novel hybrid genotypes that are reproductively isolated from their parental species and genetically stabilized. Most frequently, reproductive isolation is achieved via an increase in ploidy. However, in some instances new hybrid species arise and become reproductively isolated without a change in chromosome number, a process known as diploid or “homoploid” hybrid speciation. The annual sunflowers of the genus Helianthus provide a well-studied example of this latter mode of speciation. Here, I review this work, placing individual studies in their proper context. These include (1) computer simulations that describe the evolutionary conditions under which hybrid speciation is most likely; (2) molecular phylogenetic studies that document the origins of three hybrid sunflower species; (3) comparative genetic mapping studies that describe the karyotypic changes associated with hybrid speciation; (4) experimental re-creations of homoploid hybrid species that allow genotypic and phenotypic comparisons between synthetic and ancient hybrid lineages; (5) quantitative trait locus (QTL) studies that describe the genetic basis of phenotypic differences between the parental species and the mode of gene action underlying the generation of extreme phenotypes in hybrids; (6) phylogeographic studies that estimate the ages and number of origins of each hybrid species; (7) selection studies that measure the strength of selection on individual traits and QTLs in synthetic hybrids transplanted into hybrid habitats; and (8) candidate gene studies that search for correlations between candidate genes for ecological divergence and traits and QTLs shown to be under selection in the habitats of the hybrid species. Ongoing work includes searches for the molecular signature of selection during hybrid speciation, surveys of gene expression shifts associated with hybrid speciation, and experiments that evaluate the role of new hybrid gene combinations versus reproductive isolation in the ecological divergence of hybrid lineages.
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Vol. 93 • No. 1