Strong post-pollination pre-zygotic isolation between sympatric, food-deceptive Mediterranean orchids

Abstract

Mediterranean orchids that grow in admixed, co-flowering populations, and frequently show hybrid progenies are interesting to use to study the nature and the strength of post-zygotic barriers. However, examination of pre- and post-pollination pre-zygotic isolating mechanisms requires sympatric, co-flowering species pairs that do not produce hybrid swarms. In this study, we analyzed a contact zone between Orchis italica and O. papilionacea, in which hybrid forms have never been reported, although hybridization between members of their groups of appurtenance has been signaled. We investigated pre-pollination barriers observing the floral phenology of both species and identified pollinators by means of molecular analysis of pollinaria collected on the insects captured in the study site. Post-pollination barriers were tested performing manual crosses in order to evaluate pollen germination/pollen tube growth in vivo and fruit and seed formation. Floral phenologies of O. italica and O. papilionacea display nearly overlapping trends, and two common pollinators have been identified by molecular analysis of pollinaria. Thus, pre-pollination barriers are very weak or nonexistent. Bidirectional crosses have shown that the growth of heterospecific pollen tubes is fully blocked in stigmatic cell layers. Since no fruit formation was detected in bidirectional interspecific crosses, we assume that reproductive isolation between the examined species is fully guaranteed by post-pollination pre-zygotic mechanisms acting at stigmatic level. Such condition has been rarely described and may mask the potential action of post-zygotic mechanisms.

Does reproductive isolation reflect the segregation of color forms in Spiranthes sinensis (Pers.) Ames complex (Orchidaceae) in the Chinese Himalayas?

“…() suggested that prezygotic barriers contributed more to reproductive isolation than postzygotic barriers. This hypothesis was supported in other studies (Lowry, Rockwood et al., ; Pellegrino, Bellusci, & Musacchio, ; Sobel & Streisfeld, ; Xu et al., ), but there are important exceptions in some lineages (Borba, Shepherd, & Semir, ; Chen, Luo, & Zhang, ; Costa, Lambert, Borba, & de Queiroz, ; Silva‐Pereira, de Camargo Smidt, & Borba, ). For example, low F 1 germination rates and F 1 pollen sterility, as components of postzygotic isolation, were crucial for reproductive isolation between Mussaenda pubescens var.…”

Pollen competition between two sympatric Orchis species (Orchidaceae): the overtaking of conspecific of heterospecific pollen as a reproductive barrier

“…and Dactylorhiza spp., pollen tubes grow through the stigma surface and into the top of the ovary within 2 or 3 days (Neiland & Wilcock ; Pellegrino et al . ). In our case, heterospecific pollen remains for at least 36–48 h before moving into the ovary and if, in the meantime, conspecific pollen arrives on the stigma, this stops heterospecific pollen tube development; thus, there is an advantage of conspecific pollen fertilising ovules as pollen tubes grow faster than those of heterospecific pollen.…”

Parental Ploidy Strongly Affects Offspring Fitness in Heteroploid Crosses among Three Cytotypes of Autopolyploid Jacobaea carniolica (Asteraceae)

“…The diploid cytotype is genetically more distant from the polyploids than tetraploids and hexaploids are from each other (M. Winkler et al, unpublished data). Therefore, Bateson-Dobzhansky-Muller incompatibilities [75] , pollen-pistil incompatibilities and/or impaired pollen tube growth [76] , or nuclear-cytoplasmic interactions [77] may play a role as well.…”