Abstract
Hybridization is recognized as an important process in plant evolution, and this may be particularly true for island plants where several biotic and abiotic factors facilitate interspecific hybridization. Although rarely done, experimental studies could provide insights into the potential of natural hybridization to generate diversity when species come into contact in the dynamic island setting. The potential of hybridization to generate morphological variation was analysed within and among 12 families (inbred lines) of an F4 hybrid generation between two species of Tolpis endemic to the Canary Islands. Combinations of characters not seen in the parents were present in hybrids. Several floral and vegetative characters were transgressive relative to their parents. Morphometric studies of floral, vegetative and fruit characters revealed that several F4 families were phenotypically distinct from other families, and from their parents. The study demonstrates that morphologically distinct pollen-fertile lines, potentially worthy of taxonomic recognition if occurring in nature, can be generated in four generations. The ability of the hybrid lines to set self-seed would reduce gene flow among the lines, and among the hybrids and their parental species. Selfing would also facilitate the fixation of characters within each of the lines. Overall, the results show the considerable potential of hybridization for generating diversity and distinct phenotypes in island lineages.
Highlights
The prevalence and significance of hybridization in evolution have been debated over the past decades with zoologists tending to minimize its impact (e.g. Mayr 1942; Dobzhansky 1970) and botanists, with rare exceptions (Wagner 1970), seeing a more important role for hybridization (Anderson and Stebbins 1954; Stebbins 1959; Raven 1976; Arnold 2016)
Both transgressive traits and new character combinations can facilitate evolutionary change in plant lineages (Stebbins 1959; Rieseberg et al 2003a; Arnold et al 2012; Arnold 2016), including the establishment and evolution of independent homoploid lineages that may be recognized as species (Abbott et al 2010; Schumer et al 2014; Yakimowski and Rieseberg 2014)
Whether a particular F4 plant is homozygous for the T. coronopifolia or T. santosii allele will vary, but should be positively correlated within F4 families
Summary
The prevalence and significance of hybridization in evolution have been debated over the past decades with zoologists tending to minimize its impact (e.g. Mayr 1942; Dobzhansky 1970) and botanists, with rare exceptions (Wagner 1970), seeing a more important role for hybridization (Anderson and Stebbins 1954; Stebbins 1959; Raven 1976; Arnold 2016). Hybrids may display traits exceeding those found in their parents, i.e. transgressive traits (Rieseberg et al 1999, 2003b; Stelkens and Seehausen 2009; Yakimowski and Rieseberg 2014), as well as exhibit novel combinations of traits from the two parents (Rieseberg and Ellstrand 1993; García-Verdugo et al 2013) Both transgressive traits and new character combinations can facilitate evolutionary change in plant lineages (Stebbins 1959; Rieseberg et al 2003a; Arnold et al 2012; Arnold 2016), including the establishment and evolution of independent homoploid lineages that may be recognized as species (Abbott et al 2010; Schumer et al 2014; Yakimowski and Rieseberg 2014). Several factors make natural interspecific hybridization feasible in plant lineages on oceanic islands, most notably the combination of few intrinsic reproductive barriers with a dynamic ecological landscape. It has been estimated that 10 % of the Hawaiian flora has been involved in natural hybridization (Whitney et al 2010). Anderson and Stebbins (1954), in a classic paper on the impacts of hybridization on evolution, highlighted oceanic islands as places with changing environmental conditions that facilitate rapid bursts of hybridization
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