Evidence of post-pollination barriers among three colour morphs of the deceptive orchid Dactylorhiza sambucina (L.) Soó

Abstract

Evidence of post-pollination barriers among three colour morphs of the deceptive orchid Dactylorhiza sambucina (L.) SoóAbstract Floral-colour polymorphism in rewardless orchids has been hypothesized to be maintained by means of naı¨ve insects, which after visiting a flower without reward will tend to fly elsewhere, looking for a flower of a different colour. In this study, levels of male and female reproductive success were monitored in Southern Italy populations of the deceptive orchid Dactylorhiza sambucina, through field observations over 3 years. These populations were characterized by the presence of a rare pink morph which is sympatric with the more frequent yellow and red morphs. In addition, final plant fertility was evaluated through percentages of embryo-containing seeds produced in both natural conditions and hand-pollination experiments. Results showed that pollinator preferences were independent of the morph frequencies and thus do not promote the predicted negative frequency-dependent selection. Although yellow and pink morphs showed significantly higher male reproductive success (RS), we found comparable levels of female RS, which suggest that pollinator behaviour cannot be the main mechanism which maintains this polymorphism. Interestingly, we found different percentages of embryo-containing seeds in fruits set under natural conditions as well as in those obtained from experimental crosses. In particular, pink morph showed a very low intrinsic fertility. Moreover, fertility in intra-was higher than in inter-morph crosses. To our knowledge, this is the first study pointing out the occurrence in the orchid family of post-pollination reproductive barriers. Findings are discussed in light of present hypothesis on the evolution and maintenance of colour polymorphism in deceptive orchids and other angiosperms.

Factors affecting seed set in the crosses between Dendranthema grandiflorum (Ramat.) Kitamura and its wild species

“…Pollination is the primary step in seed set, and events starting at pollination and terminating at fertilization in wide crosses involve complex and harmonious interactions including cell-cell recognition and cellular signaling (Mascarenhas 1993;Wilhelmi and Preuss 1997;Malhó 1998;Franklin-Tong 1999;de Graaf et al 2001). Failures in pollination are thought to be widespread in plants and a common cause of low seed set (Wilcock and Neiland 2002;Marta et al 2004;Pellegrino et al 2005;Hodnett et al 2005;Ram et al 2006;Lee et al 2008;Spielman and Scott 2008). For instance, Mazzucato et al (2003) indicated that a defective pollen-pistil interaction was a main factor leading to low seed set in the parthenocarpic fruit tomato mutant.…”

Pollen competition between morphs in a pollen-color dimorphic herb and the loss of phenotypic polymorphism within populations

“…Studies on species with petal‐color polymorphism also found that pollen from different variants played differential roles in male function, documenting that one morph usually had higher siring success than the others (Schoen and Clegg ; Jones ; Pellegrino et al. ,b). However, if one morph consistently outperforms the others, directional selection would favor a shift from polymorphism to monomorphism within the populations (Schoen and Clegg ; Cooley et al.…”

“…Hand pollination experiments between the three petal color morphs in Dactylorhiza sambucina showed that seed set resulting from intramorph pollination was significantly higher than that from intermorph pollination (Pellegrino et al. ). Such effects of single‐donor pollination treatments on seed production were not observed in the petal color polymorphic Dactylorhiza incarnata (Vallius et al.…”

“…Later studies in Dactylorhiza sambucina suggested that petal color polymorphism could be maintained by postpollination reproductive barriers between morphs rather than negative frequency‐dependent selection due to pollinator preference, in which female reproductive success was significantly higher in intramorph pollination than in intermorph pollination (Pellegrino et al. ; Jersáková et al. ); but it is unclear whether mixtures of pollen from different morphs affect reproductive success.…”

Systematics and evolution of the Dactylorhiza romana/sambucina polyploid complex (Orchidaceae)

“…In addition to the partial morph constancy of bumblebees, other mechanisms have been suggested as being responsible for the maintenance of colour dimorphism (or polymorphism) in D. sambucina . These include negative frequency‐dependent selection (Gigord, Macnair & Smithson, 2001) and partial post‐pollination barriers detected through differential viability and germinability of seeds from experimental crosses (Pellegrino, Bellusci & Musacchio, 2005; Jersákova, Kindlmann & Renner, 2006).…”