Evaluation of reproductive barriers and realisation of interspecific hybridisations depending on genetic distances between species in the genus Helleborus

Abstract

The genus Helleborus comprises 22 species, which are allocated to six sections. H. x hybridus and H. niger, which belong to different Helleborus sections, are economically important ornamentals. Several other species with minor impact exhibit interesting features, e.g. flower size, flower colour, foliage, scent and disease resistance, which should be introgressed into H. x hybridus or H. niger through interspecific hybridisation. The aims of this study were to investigate whether and which kind of hybridisation barriers occur in crosses between Helleborus species and if they differ in their manifestations, depending on the genetic distance of the respective partners. In order to obtain interspecific hybrids despite crossing barriers, a method to overcome these barriers should be developed. Crossing barriers in Helleborus were localised as predominantly post-zygotic according to in situ pollen tube staining with aniline blue. For certain crosses, pre-zygotic barriers could also be assumed, but pollen tube growth was not totally inhibited. Therefore, embryo rescue techniques via ovule culture were established to overcome the post-zygotic barriers. Ovules were isolated from maternal plants 5-7 weeks after pollination in most cases and then cultured in vitro. Overall, 219 hybrids were successfully obtained, of which 16 were derived from inter-sectional crosses. Hybrids were verified by flow cytometry and/or by molecular DNA markers.

Evaluation of reproductive barriers contributes to the development of novel interspecific hybrids in the Kalanchoë genus

“…The formation of viviparous plants, however, which is a typical trait of the Bryophyllum section , was not observed in the hybrids. This agrees with results of intersectional crosses among K. spathulata and K. laxiflora, [ 13 , 43 ] and K. blossfeldiana and K. pubescens [ 12 ] where formation of viviparous plants in hybrids was not detected. The findings suggest that this trait is recessive or quantitative [ 13 ].…”

“…The obtained results revealed a significant correlation ( P ≤ 0.05) between the numbers of pollen tubes on the stigma and parental divergence. This may indicate heterospecific differences in composition of stigmatic exudate, or in size and shape of pollen and stigma morphology (data not shown) as it was suggested in Helleborus species [ 43 ]. As a consequence, the production of seeds and subsequently plants from interspecific crosses of Kalanchoë was more successful within section than inter-sectional crosses (Table 2 ), similarly to results obtained for Helleborus [ 43 ].…”

Factors contributing to the accumulation of reproductive isolation: A mixed model approach

“…Patterns of reproductive isolation inferred from comparative analyses of species willingness or ability to mate (i.e., prezygotic isolation) and produce viable and fertile offspring (i.e., postzygotic isolation) have generated important and influential observations about the evolution of reproductive isolation. These studies have been especially important for making generalizations about comparisons between allopatric and sympatric species pairs, or prezygotic and postzygotic barriers to reproduction (Coyne & Orr, 1989; Funk, Nosil, & Etges, 2006; Malone & Fontenot, 2008; Meiners & Winkelmann, 2012; Mendelson, 2003; Moyle, Olson, & Tiffin, 2004; Presgraves, 2002). These analyses have produced iconic patterns and rules of speciation including that reproductive isolation accumulates more quickly between sympatric species pairs compared to allopatric species pairs and, specifically, that in sympatry prezygotic reproductive isolation appears more quickly than postzygotic reproductive isolation.…”

Factors contributing to the accumulation of reproductive isolation: a mixed model approach

“…Two general modes of reproductive isolation are prezygotic and postzygotic barriers to reproduction. Patterns of reproductive isolation inferred from comparative analyses of crossability (species willingness/ability to mate and produce viable and fertile offspring) have generated important and influential observations about the rate of evolution of reproductive isolation, especially comparing allopatric and sympatric species pairs, or prezygotic and postzygotic barriers to reproduction (Coyne and Orr 1989, Presgraves 2002, Mendelson 2003, Moyle et al 2004, Funk et al 2006, Malone and Fontenot 2008, Meiners and Winkelmann 2012. These analyses have produced iconic patterns/rules of speciation including that reproductive isolation accumulates more quickly between sympatric species pairs compared to allopatric species pairs and, specifically, that in sympatry prezygotic reproductive isolation appears more quickly than postzygotic reproductive isolation.…”