AOBPreview published online on January 11, 2007
Annals of Botany, doi:10.1093/aob/mcl273
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© The Author 2007. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org
Distribution of Flower Morphs, Ploidy Level and Sexual Reproduction of the Invasive Weed Oxalis pes-caprae in the Western Area of the Mediterranean Region
1 Department of Plant Biology and Soil Sciences, Faculty of Science, University of Vigo, 36200 Vigo, Spain
2 Laboratory of Biotechnology and Cytomics, Department of Biology, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
3 CESAM (Centre for Environmental and Marine Studies), University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
4 LDICOSYB, Department of Biology, Faculty of Sciences of Tétouan, University Abdelmalek Essaâdi, BP 2062, 93002, Tétouan, Morocco
* For correspondence. E-mail lnavarro{at}uvigo.es
Received: 30 September 2006 Returned for revision: 24 October 2006 Accepted: 9 November 2006
BACKGROUND AND AIMS: Oxalis pes-caprae is a widespread invasive weed in regions with a Mediterranean climate. In its native habitat (southern Africa) this species has been reported as heterostylous with trimorphic flowers and a self- and morph-incompatible reproductive system. In most of the areas invaded, only a pentaploid short-styled morphotype that reproduces mainly asexually by bulbils is reported, but this has only been confirmed empirically. This study aims to analyse the floral morph proportions in a wide distribution area, test the sexual female success, and explain the causes of low sexual reproduction of this species in the western area of the Mediterranean Basin.
METHODS: Fifty-five populations of O. pes-caprae were sampled in the Iberian Peninsula and Morocco to evaluate the floral morph ratio and individual fruit set. In plants from a dimorphic population, hand-pollination experiments were performed to evaluate the effect of the pollen source on pollen tube growth through the style. The ploidy level and genome size of individuals of each floral morph were analysed using flow cytometry.
KEY RESULTS: From the populations studied 89·1 % were monomorphic, with most of them containing the short-styled (SS) floral morph, and 10·9 % were dimorphic containing long-styled (LS) and SS morphs. In some of these, isoplethy was verified but no fruit production was observed in any population. A sterile form was also recorded in several populations. Hand-pollination experiments revealed that pollen grains germinated over recipient stigmas. In intermorph crossings, pollen tubes were able to develop and fruit initiation was observed in some cases, while in intramorph pollinations, pollen tube development was sporadic and no fruit initiation was observed. All individuals within each floral form presented the same DNA ploidy level: SS plants were pentaploid and LS and the sterile form were tetraploid.
CONCLUSIONS: The low or null sexual reproduction success of this species in the area of invasion studied seems related with the high frequency of monomorphic populations, the unequal proportion of floral morphs in dimorphic populations and the presence of different ploidy levels between SS and LS morphs. The discovery of the occurrence of an LS floral morph and a sterile form, whose invading capacity in these areas is as yet unknown, will be valuable information for management programmes.
Key words: Flow cytometry, genome size, heterostyly, invasive plant, Oxalis pes-caprae, ploidy level, reproductive biology, weed
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