POLLEN VIGOR AND SEED PRODUCTION IN SYMPATRIC POPULATION OF TWO ORCHID SPECIES AND THEIR HYBRIDS pp. 197-216
Authors: (Elisa Vallius, Department of Biological and Environmental Science, University of Jyväskylä, Finland)
Abstract: Orchids (Orchidaceae) are known to frequently hybridize. Especially in deceptive taxa, pollen flow between species is caused by lack of fidelity in pollinator behaviour, as pollinators tend to move to different-looking plants after an ineffectual visit. Because many of these deceptive species have no post-pollination reproduction barrier between them, production of hybrid seed in sympatric populations is not rare. As a result of interspecific fertilization, either intermediate or non-typical, usually tall plants with large inflorescences appear. These hybrid plants may be polyploids with varying number of chromosomes and they suffer from very low fertility. Many of these hybrids also totally fail to produce pollen, but production of normally structured and totally or partially in-viable pollinia is also possible. In this chapter, the quality of pollen in hybrid plants and its effects on reproductive success of original species are discussed using the hybrids between Dactylorhiza incarnata and D. fuchsii in Irish dune population as an example. Quality of pollen in hybrid plants was examined using fluorescent staining method, and was found to vary from totally in-viable to partially viable. The effects of pollen source on seed production were examined with hand-pollination experiments in parental species and hybrids. Production of F1 seed between D. incarnata and D. fuchsii was comparative to intra-specific fertilization, but only a few seeds were produced following pollination of parental species with hybrid pollen. Seed production in hybrid plants was very low compared to the parental species, but some seed was produced. Asexual reproduction via apomictic seed production in D. incarnata was examined experimentally stimulating stigma chemically and mechanically. Mechanical stimulation did not result in seed production but liquid containing chemicals from orchid pollen did trigger ovule maturation and production of some seed, as did non-related Ranunculus acris pollen. Hybrid pollen in the stigma did not essentially cause interference in seed production of D. incarnata either before or after pollination with intra-specific pollen. Pollen produced by hybrids thus seems not to play an important negative role in the reproduction of the parental species. Some hybrids, however, produce at least partially viable pollen which may lead to back-crossing and lower production of genetically normal progeny. This production of hybrids, possibility of apomictic seed production, and introgression at different levels may be important factors behind the high number of taxa and large intra-specific variation in deceptive terrestrial orchids.