AOBPreview originally published online on November 10, 2008
Annals of Botany 2009 104(3):497-506; doi:10.1093/aob/mcn219
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This article appears in the following Annals of Botany issue: Special Issue: Orchid Biology [View the issue table of contents]
Pollinator convergence and the nature of species' boundaries in sympatric Sardinian Ophrys (Orchidaceae)
1 Dipartimento di Scienze Botaniche, Università degli Studi di Cagliari, Viale S. Ignazio 13, I-09123 Cagliari, Italy
2 Behavioural and Evolutionary Ecology, Free University of Brussels CP 160/12, Av. F. D. Roosevelt 50, B-1050 Brussels, Belgium
3 Institute of Systematic Botany, University of Zürich, Zollikerstrasse 107, CH-8008 Zürich, Switzerland
4 Dipartimento di Biologia Strutturale e Funzionale and Orto Botanico, Università di Napoli Federico II, I-80126 Napoli, Italy
* For correspondence. E-mail cozzolin{at}unina.it
Received: 22 May 2008 Returned for revision: 1 August 2008 Accepted: 30 September 2008 Published electronically: 10 November 2008
Background and Aims: In the sexually deceptive Ophrys genus, species isolation is generally considered ethological and occurs via different, specific pollinators, but there are cases in which Ophrys species can share a common pollinator and differ in pollen placement on the body of the insect. In that condition, species are expected to be reproductively isolated through a pre-mating mechanical barrier. Here, the relative contribution of pre- vs. post-mating barriers to gene flow among two Ophrys species that share a common pollinator and can occur in sympatry is studied.
Methods: A natural hybrid zone on Sardinia between O. iricolor and O. incubacea, sharing Andrena morio as pollinator, was investigated by analysing floral traits involved in pollinator attraction as odour extracts both for non-active and active compounds and for labellum morphology. The genetic architecture of the hybrid zone was also estimated with amplified fragment length polymorphism (AFLP) markers, and pollination fitness and seed set of both parental species and their hybrids in the sympatric zone were estimated by controlled crosses.
Key Results: Although hybrids were intermediate between parental species in labellum morphology and non-active odour compounds, both parental species and hybrids produced a similar odour bouquet for active compounds. However, hybrids produced significantly lower fruit and seed set than parental species, and the genetic architecture of the hybrid zone suggests that they were mostly first-generation hybrids.
Conclusions: The two parental species hybridize in sympatry as a consequence of pollinator overlap and weak mechanical isolation, but post-zygotic barriers reduce hybrid frequency and fitness, and prevent extensive introgression. These results highlight a significant contribution of late post-mating barriers, such as chromosomal divergence, for maintaining reproductive isolation, in an orchid group for which pre-mating barriers are often considered predominant.
Key words: AFLP markers, floral scent variation, hybrid zone, hybrid fitness, Ophrys iricolor, Ophrys incubacea, reproductive isolation, sexual deception
Both authors contributed equally to this work.
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