The Origin of Ecological Divergence in Helianthus Paradoxus (Asteraceae): Selection on Transgressive Characters in a Novel Hybrid Habitat

Abstract

Diploid hybrid speciation in plants is often accompanied by rapid ecological divergence between incipient neospecies and their parental taxa. One plausible means by which novel adaptation in hybrid lineages may arise is transgressive segregation, that is, the generation of extreme phenotypes that exceed those of the parental lines. Early generation (BC2) hybrids between two wild, annual sunflowers, Helianthus annuus and Helianthus petiolaris, were used to study directional selection on transgressive characters associated with the origin of Helianthus paradoxus, a diploid hybrid species adapted to extremely saline marshes. The BC2 plants descended from a single F1 hybrid backcrossed toward H. petiolaris. The strength of selection on candidate adaptive traits in the interspecific BC2 was measured in natural H. paradoxus salt marsh habitat. Positive directional selection was detected for leaf succulence and Ca uptake, two traits that are known to be important in salt stress response in plants. Strong negative directional selection operated on uptake of Na and correlated elements. A significant decrease in trait correlations over time was observed in the BC2 population for Na and Ca content, suggesting an adaptive role for increased Ca uptake coupled with increased net exclusion of Na from leaves. Patterns of directional selection in BC2 hybrids were concordant with character expression in the natural hybrid species, H. paradoxus, transplanted into the wild. Moreover, the necessary variation for generating the H. paradoxus phenotype existed only in the BC2 population, but not in samples of the two parental species, H. annuus and H. petiolaris. These results are consistent with the hypothesis that transgressive segregation of elemental uptake and leaf succulence contributed to the origin of salt adaptation in the diploid hybrid species H. paradoxus.

Evolution through genetically controlled allometry space

“…Even though A. majus and A. charidemi do not differ greatly in shape, variation in shape is captured in the allometric space because of the QTL vectors with complementary components along PC axes influencing shape (PC2 and PC3). Complementary effects of QTL (leading to transgression in progeny) have similarly been described for several other interspecific crosses (25)(26)(27)(28).…”

Hybridization in contact zone between temperate European pine species

“…Natural selection can cause the fixation of advantageous alleles (or chromosomal segments) in ecologically diverged hybrids (Beaumont and Balding 2004;Buerkle and Lexer 2008;Lexer et al 2003). Introgressed alleles may often have a positive fitness effect in their new genetic background and traits responsible for adaptation can be transferred between species (Martin et al 2006).…”

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

“…) The fact that hybrids may have higher fitness than their parents in certain habitats is now well documented (Arnold and Hodges 1995;Arnold 1997;Lexer et al 2003a). Moreover, it was recently demonstrated that transgressive segregation can be very important in generating hybrid genotypes that are adapted to novel habitats Lexer et al 2003b). Whether transgressive segregation has been important in the adaptation of S. squalidus to conditions in the British Isles awaits investigation.…”