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Floral miniaturisation and autogamy in boreal-arctic plants are epitomised by Iceland's most frequent orchid, Platanthera hyperborea.

Bateman RM, Sramkó G, Rudall PJ - PeerJ (2015)

Bottom Line: This paper concludes our series of publications comparing island and mainland speciation in European butterfly-orchids, by studying the morphology, phylogenetics and reproductive biology of the controversial circum-arctic species Platanthera (Limnorchis) hyperborea-the most frequent of seven Icelandic orchids.Morphometric data failed to identify any taxonomically meaningful partitions among Icelandic P. hyperborea populations, despite the presence of a distinct and apparently plesiomorphic ribotype at the most glacially influenced habitat sampled.When considered in combination with independently derived lineages of Platanthera on the Azorean and Hawaiian archipelagos also bearing small green flowers, our observations show allometric and paedomorphic reductions in flower size as the primary evolutionary driver, but also indicate strong developmental and functional constraints.

View Article: PubMed Central - HTML - PubMed

Affiliation: Royal Botanic Gardens Kew , Richmond , United Kingdom.

ABSTRACT
Background and Aims. This paper concludes our series of publications comparing island and mainland speciation in European butterfly-orchids, by studying the morphology, phylogenetics and reproductive biology of the controversial circum-arctic species Platanthera (Limnorchis) hyperborea-the most frequent of seven Icelandic orchids. We draw particular attention to its phylogenetic placement, remarkable reproductive biology and morphological convergence on other Platanthera lineages through floral miniaturisation. Methods. Five populations of P. hyperborea in southwest Iceland were measured for 33 morphological characters and subjected to detailed multivariate and univariate analyses, supported by light and scanning electron microscopy of selected flowers. Representative samples from six populations were sequenced for nrITS and placed in a taxonomically broader phylogenetic matrix derived from previous studies. Key Results . Section Limnorchis consists of three distinct ITS-delimited clades based on P. stricta, P. sparsifolia-limosa-aquilonis and P. dilatata-hyperborea. Within the latter group, supposed species boundaries overlap; instead, the data indicate a crude stepwise series of ribotypic transitions extending eastward from North America to Iceland. Morphometric data failed to identify any taxonomically meaningful partitions among Icelandic P. hyperborea populations, despite the presence of a distinct and apparently plesiomorphic ribotype at the most glacially influenced habitat sampled. Microscopic study of the flowers revealed several distinguishing features (some not previously reported), including resupinate lateral sepals, toothed bract margins, club-shaped papillae shared by both the interior of the labellar spur and the stigmatic surface, and an exceptionally adhesive stigma that is reliably covered in disaggregated pollen masses prior to anthesis; auricles are absent. Conclusions. Ribotypes suggest that Icelandic P. hyperborea represents the terminus of a migration route that may have begun in East Asia before passing through North America and presumably Greenland. The incohesive pollinia, rapidly desiccating anther locules, weakly developed rostellum, exceptionally adhesive stigma and the close juxtaposition of compact male and female reproductive organs together conspire to cause routine autogamy and frequent cleistogamy, despite the continued production of substantial nectar reservoirs in the spur and consequent ongoing attraction to the flowers of insects, including mosquitoes. When considered in combination with independently derived lineages of Platanthera on the Azorean and Hawaiian archipelagos also bearing small green flowers, our observations show allometric and paedomorphic reductions in flower size as the primary evolutionary driver, but also indicate strong developmental and functional constraints.

No MeSH data available.


Related in: MedlinePlus

Scanning electron micrographs of flowers and bracts of Icelandic P. hyperborea.(A) Gymnostemium of recently opened flower, showing densely papillate stigmatic surface (s) bearing massulae (m) derived from the pollinaria that formerly occupied the now severely desiccated anther locules (al) bracketing the connective (c) and rostellum (r); also visible are depressions presumably formerly occupied by the viscidia (v), and the labellum (la). (B) Magnified view of (A) to better illustrate the stigmatic features. (C) Gynostemium of another flower that better illustrates the firm attachment of disaggregated pollinium fragments to the adhesive disc secreted by the stigma. (D) Distinctive row of robust, angular cells that characterises the bract margins of this species. Scale bar = 500 µm (A, C), 250 µm (B), 50 µm (D). Images: P Rudall.
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fig-14: Scanning electron micrographs of flowers and bracts of Icelandic P. hyperborea.(A) Gymnostemium of recently opened flower, showing densely papillate stigmatic surface (s) bearing massulae (m) derived from the pollinaria that formerly occupied the now severely desiccated anther locules (al) bracketing the connective (c) and rostellum (r); also visible are depressions presumably formerly occupied by the viscidia (v), and the labellum (la). (B) Magnified view of (A) to better illustrate the stigmatic features. (C) Gynostemium of another flower that better illustrates the firm attachment of disaggregated pollinium fragments to the adhesive disc secreted by the stigma. (D) Distinctive row of robust, angular cells that characterises the bract margins of this species. Scale bar = 500 µm (A, C), 250 µm (B), 50 µm (D). Images: P Rudall.

Mentions: The wide range of gynostemium morphologies exhibited by the genus Platanthera s.l. was surveyed by Efimov (2011). Within Section Platanthera, some details of P. bifolia, P. chlorantha and P. holmboei were illustrated by Claessens & Kleynen (2011) and Bateman, James & Rudall (2012), and later were compared with the three Azorean species plus P. algeriensis by Bateman, Rudall & Moura (2013). In order to adequately describe the gynostemium of P. hyperborea, and to obtain further details of the other floral structures, it proved necessary to examine representative flowers microscopically in the laboratory. Bracts were examined under the light microscope, demonstrating that the finely serrated margin reflects a distinctive row of comparatively large (80–90 µm) and thick-walled triangular cells (Fig. 14D). However, the small size, uniform colouration and translucent texture of the flowers of P. hyperborea meant that details of the floral parts were much more readily discerned through scanning than light microscopy. Flowers from five plants were examined and measured (Figs. 13–15, Table 2).


Floral miniaturisation and autogamy in boreal-arctic plants are epitomised by Iceland's most frequent orchid, Platanthera hyperborea.

Bateman RM, Sramkó G, Rudall PJ - PeerJ (2015)

Scanning electron micrographs of flowers and bracts of Icelandic P. hyperborea.(A) Gymnostemium of recently opened flower, showing densely papillate stigmatic surface (s) bearing massulae (m) derived from the pollinaria that formerly occupied the now severely desiccated anther locules (al) bracketing the connective (c) and rostellum (r); also visible are depressions presumably formerly occupied by the viscidia (v), and the labellum (la). (B) Magnified view of (A) to better illustrate the stigmatic features. (C) Gynostemium of another flower that better illustrates the firm attachment of disaggregated pollinium fragments to the adhesive disc secreted by the stigma. (D) Distinctive row of robust, angular cells that characterises the bract margins of this species. Scale bar = 500 µm (A, C), 250 µm (B), 50 µm (D). Images: P Rudall.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4400879&req=5

fig-14: Scanning electron micrographs of flowers and bracts of Icelandic P. hyperborea.(A) Gymnostemium of recently opened flower, showing densely papillate stigmatic surface (s) bearing massulae (m) derived from the pollinaria that formerly occupied the now severely desiccated anther locules (al) bracketing the connective (c) and rostellum (r); also visible are depressions presumably formerly occupied by the viscidia (v), and the labellum (la). (B) Magnified view of (A) to better illustrate the stigmatic features. (C) Gynostemium of another flower that better illustrates the firm attachment of disaggregated pollinium fragments to the adhesive disc secreted by the stigma. (D) Distinctive row of robust, angular cells that characterises the bract margins of this species. Scale bar = 500 µm (A, C), 250 µm (B), 50 µm (D). Images: P Rudall.
Mentions: The wide range of gynostemium morphologies exhibited by the genus Platanthera s.l. was surveyed by Efimov (2011). Within Section Platanthera, some details of P. bifolia, P. chlorantha and P. holmboei were illustrated by Claessens & Kleynen (2011) and Bateman, James & Rudall (2012), and later were compared with the three Azorean species plus P. algeriensis by Bateman, Rudall & Moura (2013). In order to adequately describe the gynostemium of P. hyperborea, and to obtain further details of the other floral structures, it proved necessary to examine representative flowers microscopically in the laboratory. Bracts were examined under the light microscope, demonstrating that the finely serrated margin reflects a distinctive row of comparatively large (80–90 µm) and thick-walled triangular cells (Fig. 14D). However, the small size, uniform colouration and translucent texture of the flowers of P. hyperborea meant that details of the floral parts were much more readily discerned through scanning than light microscopy. Flowers from five plants were examined and measured (Figs. 13–15, Table 2).

Bottom Line: This paper concludes our series of publications comparing island and mainland speciation in European butterfly-orchids, by studying the morphology, phylogenetics and reproductive biology of the controversial circum-arctic species Platanthera (Limnorchis) hyperborea-the most frequent of seven Icelandic orchids.Morphometric data failed to identify any taxonomically meaningful partitions among Icelandic P. hyperborea populations, despite the presence of a distinct and apparently plesiomorphic ribotype at the most glacially influenced habitat sampled.When considered in combination with independently derived lineages of Platanthera on the Azorean and Hawaiian archipelagos also bearing small green flowers, our observations show allometric and paedomorphic reductions in flower size as the primary evolutionary driver, but also indicate strong developmental and functional constraints.

View Article: PubMed Central - HTML - PubMed

Affiliation: Royal Botanic Gardens Kew , Richmond , United Kingdom.

ABSTRACT
Background and Aims. This paper concludes our series of publications comparing island and mainland speciation in European butterfly-orchids, by studying the morphology, phylogenetics and reproductive biology of the controversial circum-arctic species Platanthera (Limnorchis) hyperborea-the most frequent of seven Icelandic orchids. We draw particular attention to its phylogenetic placement, remarkable reproductive biology and morphological convergence on other Platanthera lineages through floral miniaturisation. Methods. Five populations of P. hyperborea in southwest Iceland were measured for 33 morphological characters and subjected to detailed multivariate and univariate analyses, supported by light and scanning electron microscopy of selected flowers. Representative samples from six populations were sequenced for nrITS and placed in a taxonomically broader phylogenetic matrix derived from previous studies. Key Results . Section Limnorchis consists of three distinct ITS-delimited clades based on P. stricta, P. sparsifolia-limosa-aquilonis and P. dilatata-hyperborea. Within the latter group, supposed species boundaries overlap; instead, the data indicate a crude stepwise series of ribotypic transitions extending eastward from North America to Iceland. Morphometric data failed to identify any taxonomically meaningful partitions among Icelandic P. hyperborea populations, despite the presence of a distinct and apparently plesiomorphic ribotype at the most glacially influenced habitat sampled. Microscopic study of the flowers revealed several distinguishing features (some not previously reported), including resupinate lateral sepals, toothed bract margins, club-shaped papillae shared by both the interior of the labellar spur and the stigmatic surface, and an exceptionally adhesive stigma that is reliably covered in disaggregated pollen masses prior to anthesis; auricles are absent. Conclusions. Ribotypes suggest that Icelandic P. hyperborea represents the terminus of a migration route that may have begun in East Asia before passing through North America and presumably Greenland. The incohesive pollinia, rapidly desiccating anther locules, weakly developed rostellum, exceptionally adhesive stigma and the close juxtaposition of compact male and female reproductive organs together conspire to cause routine autogamy and frequent cleistogamy, despite the continued production of substantial nectar reservoirs in the spur and consequent ongoing attraction to the flowers of insects, including mosquitoes. When considered in combination with independently derived lineages of Platanthera on the Azorean and Hawaiian archipelagos also bearing small green flowers, our observations show allometric and paedomorphic reductions in flower size as the primary evolutionary driver, but also indicate strong developmental and functional constraints.

No MeSH data available.


Related in: MedlinePlus