Recent, Allopatric, Homoploid Hybrid Speciation: The Origin of Senecio Squalidus (Asteraceae) in the British Isles From a Hybrid Zone on Mount Etna, Sicily

Abstract

Abstract. Homoploid hybrid speciation occurs through stabilization of a hybrid segregate (or segregates) isolated by premating and/or postmating barriers from parent taxa. Theory predicts that ecological and spatial isolation are of critical importance during homoploid hybrid speciation, and all confirmed homoploid hybrid species are ecologically isolated from their parents. Until recently, such species have been identified long after they originated, and consequently it has not been possible to determine the relative importance of spatial and ecological isolation during their origin. Here we present evidence for the recent origin (within the past 300 years) of a new homoploid hybrid species, Senecio squalidus (Asteraceae), in the British Isles, following long-distance dispersal of hybrid material from a hybrid zone between S. aethnensis and S. chrysanthemifolius on Mount Etna, Sicily, Italy. Historical records show that such hybrid material from Sicily was introduced to the Oxford Botanic Garden in Britain in the early part of the 18th century and that S. squalidus began to spread from there after approximately 90 years. A survey of randomly amplified polymorphic DNA/intersimple sequence repeats (RAPD/ISSR) marker variation demonstrated that S. squalidus is a diploid hybrid derivative of S. aethnensis and S. chrysanthemifolius that grow at high and low altitudes, respectively, on Mount Etna and that form a hybrid zone at intermediate altitudes. Senecio squalidus contained 11 of 13 RAPD/ISSR markers that were recorded at high frequency in S. chrysanthemifolius but were absent or occurred at low frequency in S. aethnensis, and 10 of 13 markers for which the reverse was true. Bayesian admixture analysis showed that all individuals of S. squalidus surveyed were of mixed ancestry with relatively high mean proportions of ancestry derived from both S. chrysanthemifolius and S. aethnensis (0.644 and 0.356, respectively). We argue that long-distance isolation of hybrid material from its parents on Mount Etna would have helped favor the origin and establishment of S. squalidus in the British Isles, regardless of whether the initial hybrid material introduced to Britain was preadapted to local conditions.

Hybrid vigor between native and introduced salamanders raises new challenges for conservation

“…Such recombinant homozygotes appear only in later generations, and they can breed true. True breeding recombinant genotypes can therefore establish new lineages with genomes that are mosaics of the two ancestral genomes (41)(42)(43)(44).…”

Altitudinal gradients, plant hybrid zones and evolutionary novelty

“…R. Soc. B 369: 20130346 Two closely related ragwort species, Senecio aethnensis and Senecio chrysanthemifolius (Asteraceae), occur at high (more than 2500 m) and low altitudes (less than 600 m), respectively, on Mount Etna and form a hybrid zone at intermediate altitudes [38,39]. Both species are diploid (2n ¼ 20), selfincompatible, short-lived herbaceous perennials that are easily distinguished from each other by differences in leaf shape, flower head (capitulum) size and fruit (achene) size.…”

“…This is not the case for S. squalidus, which originated in Britain following the introduction of plants from the hybrid zone between S. aethnensis and S. chrysanthemifolius on Mount Etna approximately 300 years ago [38,63,71,72]. Thus, in this case it is possible to compare the new homoploid hybrid species with parental and hybrid rstb.royalsocietypublishing.org Phil.…”

Changes to gene expression associated with hybrid speciation in plants: further insights from transcriptomic studies inSenecio

“…Senecio squalidus is the product of hybridization between two native Sicilian species, Senecio aethnensis (2nZ2xZ20) and Senecio chrysanthemifolius (2nZ2xZ20), which form a large hybrid zone between the edges of their ranges on Mt Etna. Hybrid material on Mt Etna is interfertile with both S. chrysanthemifolius and S. aethnensis, but its removal to Britain has allowed it to diverge sufficiently in allopatry to give rise to the homoploid species S. squalidus ( James & Abbott 2005). Since its escape from the Oxford Botanic Garden in the eighteenth-century, S. squalidus has established itself as a highly successful invasive species in the British Isles, being found as far north as Inverness in Scotland and as far west as Donegal in Ireland.…”