Abstract
Hybridization and polyploidization are important processes for plant evolution. However, classification of hybrid or polyploid species has been notoriously difficult because of the complexity of processes and different evolutionary scenarios that do not fit with classical species concepts. Polyploid complexes are formed via combinations of allopolyploidy, autopolyploidy and homoploid hybridization with persisting sexual reproduction, resulting in many discrete lineages that have been classified as species. Polyploid complexes with facultative apomixis result in complicated net-work like clusters, or rarely in agamospecies. Various case studies illustrate the problems that apply to traditional species concepts to hybrids and polyploids. Conceptual progress can be made if lineage formation is accepted as an inevitable consequence of meiotic sex, which is established already in the first eukaryotes as a DNA restoration tool. The turnaround of the viewpoint that sex forms species as lineages helps to overcome traditional thinking of species as “units”. Lineage formation and self-sustainability is the prerequisite for speciation and can also be applied to hybrids and polyploids. Species delimitation is aided by the improved recognition of lineages via various novel -omics methods, by understanding meiosis functions, and by recognizing functional phenotypes by considering morphological-physiological-ecological adaptations.
Highlights
The evolution and diversification of flowering plants has been largely shaped by hybridity and polyploidy
Hybridization can be connected to polyploidy, i.e., the multiplication of chromosome sets in the nuclear genome, which results in a whole genome duplication (WGD) or multiplication
Homoploid hybrid speciation is constrained by a narrow window of genetic divergence of the parental species: it must be low enough to allow for some viability and fertility of the first-generation hybrids, but high enough to provide a reproductive barrier against the parents [51]
Summary
The evolution and diversification of flowering plants has been largely shaped by hybridity and polyploidy. This review will provide an overview of evolutionary processes related to hybridization and polyploidy Based on these premises, the applicability of current species concepts to hybrids and polyploids will be discussed. Homoploid hybrid speciation is constrained by a narrow window of genetic divergence of the parental species: it must be low enough to allow for some viability and fertility of the first-generation hybrids, but high enough to provide a reproductive barrier against the parents [51]. No general rule can be set on the best circumstances for homoploid hybrid speciation, it depends on the opportunities for hybrid formation and establishment [1] This situation makes it difficult to apply theoretical concepts and operational criteria (see under Sections 3 and 4 below)
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