Post-pollination reproductive isolation between diurnally and nocturnally flowering daylilies, Hemerocallis fulva and Hemerocallis citrina

Abstract

To examine whether floral and post-pollination isolation develops independently or not, we conducted a crossing experiment between Hemerocallis fulva and Hemerocallis citrina that shows large floral divergence adapted for diurnal and nocturnal pollinators that have been believed to be fully cross-fertile. Flowers of the two species from sympatric populations were hand-pollinated with conspecific pollen from the same population (control), interspecific pollen from the same area (sympatric cross), and interspecific pollen from the different area (allopatric cross). After capsule dehiscence, the fruit set, seed set per fruit and seed set per flower were determined among three cross categories. The seed sets per flower were 32 and 77% lower in sympatric and allopatric crosses than in the control when H. fulva was the pollen recipient. There was no difference in three reproductive measures among the cross categories when H. citrina was the pollen recipient. This finding indicates that post-pollination isolation does exist between H. fulva and H. citrina, although it is partial, asymmetric, and weakened in sympatry. Our result suggests that floral and post-pollination isolation may develop independently, and reinforcement may not be a general phenomenon in plants.

Reproductive isolation on interspecific backcross of F1 pollen to parental species, Hemerocallis fulva and H. citrina (Hemerocallidaceae)

“…Before considering this question, let us compare our result with the previous result obtained from an F1-producing cross that shows a marked contrast to the current result ; when the parental species are crossed to each other, seed set per flower was reduced to 20-60% if H. fulva was a pollen recipient, but not if H. citrina was a pollen recipient. Yasumoto and Yahara (2006) suggested that the difference in style length and cyto-nuclear interaction could explain the asymmetrical seed set reduction in the F1-producing cross. Can these factors also explain the symmetrical seed set reduction on backcross?…”

“…Hemerocallis fulva and H. citrina were collected in Haifuku (33°14 0 N, 129°24 0 E) and Tsutsumi (33°15 0 N, 129°25 0 E) of Hirado Island (details in Yasumoto and Yahara 2006). From July to August, 2001, reciprocal handpollination was conducted to produce an F1 generation (crosses on 8 H. fulva recipients and 13 H. citrina recipients).…”

“…Selfing and crossing between parental plants and F1 hybrids were always avoided. The stigmas of H. fulva were hand-pollinated in late morning and those of H. citrina were done in early evening following the procedure of Yasumoto and Yahara (2006). Pollen from a single donor was used to pollinate each recipient flower (uncompetitive condition, not mixed).…”

“…For fruit set, all of the denominators of the fruit set data were unity, so that binomial distribution was assigned to the family in glm. For seed set, the family in glm was quasi-binomial for possible overdispersion (details in Yasumoto and Yahara 2006). In all of the logistic regression analyses, we assigned one of the three reproductive success measures to the response variable, the cross category to the independent variable, and the recipient individual and the year to the block.…”

Difference in flowering time can initiate speciation of nocturnally flowering species

“…However, plant species may have isolating mechanisms to prevent hybridization even after pollen deposition by affecting the growth of the pollen tube (differences in style length or pollen tube growth rate), and an establishment of such postmating isolation would increase the probability of the evolution of a nocturnally flowering species. In Hemerocallis species, Yasumoto and Yahara (2006; suggested the existence of such postmating prezygotic isolations. As mentioned by Block and Erhardt (2008), the optimal style length for reproductive success differs between species of pollinators and thus, this trait can be affected by divergent selection.…”