Absence of pollinator-mediated premating barriers in mixed-ploidy populations of Gymnadenia conopsea s.l. (Orchidaceae)

Abstract

Polyploidy has played a key role in plant evolution and diversification. Despite this, the processes governing reproductive isolation among cytotypes growing in mixedploidy populations are still largely unknown. Theoretically, coexistence of diploid and polyploid individuals in sympatric populations is unlikely unless cytotypes are prezygotically isolated through assortative pollination. Here, we investigated the pre-mating barriers involved in the maintenance of three co-occurring cytotypes from the genus Gymnadenia (Orchidaceae): tetraploid and octoploid G. conopsea and tetraploid G. densiflora. We assessed differences in flowering phenology, floral morphology, and visual and olfactory cues, which could lead to assortative mating. Gas chromatography coupled with Electronic supplementary material The online version of this article (electroantennographic detection was used to identify scent compounds with physiological activity in the two main pollinators, Deilephila porcellus and Autographa gamma. The importance of olfactory cues was also assessed in the field by analysing the moths' responses to the olfactory display of the plants, and by following the pollinator's behaviour on artificial arrays. Our complex approach demonstrated that the coexistence of Gymnadenia cytotypes in mixed-ploidy populations was only partly explained by differences in floral phenology, as cytotypes with overlapping flowering (i.e., octoploid G. conopsea and tetraploid G. densiflora) might freely exchange pollen due to only 1 mm differences in spur lengths and the lack of assortative behaviour of pollinators. While floral colour among the cytotypes was similar, floral scent differed significantly. Though both pollinator species seemed to physiologically detect these differences, and the floral scent alone was sufficient to attract them, pollinators did not use this cue to discriminate the cytotypes in the field. The absence of premating barriers among cytotypes, except partial temporal segregation, suggests the existence of other mechanisms involved in the cytotypes' coexistence. The genetic differences in ITS sequences among cytotypes were used to discuss the cytotype's origin.

Breeding barriers at a diploid–hexaploid contact zone in Aster amellus

“…In both cases, pollinator behavior lead to assortative pollination and contributed to high rates of reproductive isolation. Similar to what was observed in Gymnadenia conopsea (Jersáková et al 2010), neither preference nor constancy was observed in A. amellus. Also as in G. conopsea, pollinator behavior mediated disassortative pollination between diploid and hexaploid A. amellus, and thus it is excluded as a breeding barrier to inter-cytotype hybridization.…”

“…Previous studies comparing phenological differences between diploids and polyploids have shown diverse patterns, from total flowering divergence (Petit et al 1997) to different degrees of segregation (Lumaret et al 1987;Bretagnolle and Thompson 1996;Husband and Sabara 2004) and nearly complete overlap (Jersáková et al 2010). Phenological shifts in polyploid complexes may result directly from genetic differences generated by the process of polyploidization (Stebbins 1950), from subsequent selection processes acting over flowering time variation (van Dijk and Bijlsma 1994;Nuismer and Cunningham 2005) and/or micro-environmental differences that influence plant growth (Lumaret et al 1987).…”

“…Despite the heterogeneity in pollinator assemblages among populations of A. amellus, the most common floral visitors revealed no preference for a specific cytotype and displayed random behavior in the mixed-ploidy arrays, leading to low levels of reproductive isolation (17%). Taxa segregation due to pollinator behavior has been extensively studied (see review in Grant 1994); however, only a few studies have addressed polyploid complexes (Kennedy et al 2006;Thompson and Merg 2008;Jersáková et al 2010). Strong differentiation in the visits to different cytotypes was observed in Chamerion angustifolium as a result of both floral preference and floral constancy (Kennedy et al 2006); strong differentiation was also observed in Heuchera grossulariifolia as a result of different sets of pollinators visiting each cytotype (Thompson and Merg 2008).…”

“…A diverse array of breeding barriers can mediate assortative mating in natural populations, including heterogeneity in the spatial distribution of cytotypes (van Dijk et al 1992;Husband and Schemske 1998;Kolař et al 2009) due to different microhabitat preferences (Felber-Girard et al 1996); temporal isolation due to shifts in flowering phenologies (Petit et al 1997;Marques et al 2007;Jersáková et al 2010); mechanical isolation due to differences in floral morphology (reviewed in Grant 1994); segregation resulting from divergent behaviors and preferences of pollinators (Grant 1994;Thompson et al 2004;Marques et al 2007; Thompson and Merg 2008); gametic isolation (Husband et al 2002;Mráz 2003;Brock 2004); and/or reduced fitness or sterility of the hybrids (Ramsey and Schemske 1998;Buggs and Pannell 2006;Prentis et al 2007). All together, such breeding barriers will act in concert and govern the levels of pollen flow between cytotypes, thereby determining the cytotype composition of the offspring and population.…”

Cytotype coexistence in the field cannot be explained by inter-cytotype hybridization alone: linking experiments and computer simulations in the sexual species Pilosella echioides (Asteraceae)

“…On the other hand, the coexistence of different cytotypes can be more or less stable thanks to either (i) reproductive isolation as a consequence of a diverse array of pre- and postzygotic inter-cytotype breeding barriers [22–26] or, alternatively, (ii) an absence of breeding barriers linked with interfertility between cytotypes [27, 28]. The most interesting are systems with free mating because they allow us to study intercytotype interactions and evolutionary dynamics of mixed-ploidy stands.…”