Homoploid hybrid speciation in action

Homoploid hybrid speciation is the origin of a hybrid species without change in chromosome number. Although currently thought to be a rare form of speciation, especially relative to the more common allopolyploid hybrid speciation, it is feasible that many examples of homoploid hybrid species will be discovered in the future now that genetic resources are readily available for testing their occurrence. In this review, we focus on the speed of homoploid hybrid speciation, the importance of ecological and spatial isolation in the process, and the nature of genetic changes that occur in a new hybrid during its origin and establishment in the wild. With reference mainly to the extensive work carried out on homoploid hybrid species of Helianthus, and to our own work on the very recently originated diploid hybrid species Senecio squalidus, we review evidence showing: (1) that new fertile homoploid hybrid species can originate very quickly, although a longer period is likely to be required before the species becomes fully stabilized both genomically and phenotypically; (2) ecological divergence of the hybrid species from its parents is key to successful establishment, and that this can occur even in the absence of post­zygotic isolation caused by chromosomal and/or genetic sterility barriers; (3) transgressive changes in phenotypic traits and gene expression are of great importance in adapting homoploid hybrid species to habitats that are ecologically and spatially divergent from those of the parents; (4) adaptive differences distinguishing a homoploid hybrid species from its parental species are likely to be maintained in the face of parental gene flow, and evolve in concert across populations representing multiple origins of the species; (5) in the absence of parental gene flow, i.e., under conditions of geographical isolation, rapid genetic divergence of the hybrid species is likely to be enhanced due to the combined effects of founder events, genetic drift and selection.