Stigma form and surface in the tribe Boragineae (Boraginaceae): micromorphological diversity, relationships with pollen, and systematic relevance

Abstract

A comparative analysis of stigma form in 62 species representing 15 genera of the tribe Boragineae by means of light and electron (transmission and scanning) microscopy highlighted the structural diversity, peculiarity, and functional specialization of the receptive surface of the boraginaceous stigma. Cluster analysis identified 13 stigma morphotypes, 6 of which were peculiar to the genera Anchusella, Brunnera, Cynoglottis, Gastrocotyle, Pentaglottis, and Trachystemon. Nonea showed the broadest intrageneric stigma polymorphism, with 6 types differing in terms of stigma form and papillar structure. Stigma and pollen variations were poorly associated in this complex genus, while in other groups, the pollen-stigma typological correspondence indicated the taxonomic distinctiveness of some critical species, such as Gastrocotyle hispida (Forssk.) C.B. Clarke and Anchusella spp. Positive correlations were found between pollen and papillar size, between interpapillar distance and pollen equatorial diameter, and between pollen volume and style length. Qualitative and quantitative pollen-stigma relationships suggested functional implications of an adaptive nature that were interpreted through the recognition of three distinct stages of early interaction, i.e., capture, lodging, and hydration of the grains.Résumé : L'étude comparative de la forme et de la surface réceptive du stigmate dans 62 espèces de 15 genres de la tribu Boragineae, au moyen de la microscopie optique et électronique à transmission et à balayage, a mis en évidence la diversité structurale, la particularité et la spécialisation fonctionnelle de cette surface. L'analyse multivariée a permis de identifier 13 morphotypes, dont 6 exclusifs de genres Anchusella, Brunnera, Cynoglottis, Gastrocotyle, Pentaglottis et Trachystemon. Nonea a montré un polymorphisme intragénérique élevé, avec six types different dans la form du stigmate et la structure des papilles. Dans ce genre, la variation du stigmate est faiblement associée avec la diversité palynologique, tandis que dans des autres groups la correspondance entre la diversité stigmatique et palynologique a confirmé l'originalité taxonomique de quelques espèces critiques, comme Gastrocotyle hispida (Forssk.) C.B. Clarke et Anchusella spp. La dimension du pollen est corrélée avec celle des papilles du stigmate, et le diamètre equatorial du pollen avec leur distance sur la surface réceptive. Le volume du pollen est correlé avec la longuer du style. Les rapports quantitatifs et qualitatifs entre les papilles de la surface réceptive et le pollen suggère des implications fonctionnelles de nature adaptative interprétées en fonction de trois phases d'interaction précoce, c'est-à-dire capture, logement et hydratation des grains de pollen.

Dissection for Floral Micromorphology and Plastid Genome of Valuable Medicinal Borages Arnebia and Lithospermum (Boraginaceae)

“…Floral micromorphological characteristics observed using scanning electron microscopy (SEM) have proven to be useful for taxonomic delimitation and phylogenetic inference of various groups, including Adoxaceae (Konarska, 2017), Crassulaceae (Giuliani et al, 2018), Nymphaeaceae (Coiro and Barone Lumaga, 2018), Lentibulariaceae (Płachno et al, 2019), and Rosaceae (Song et al, 2020). Notably, taxonomic and systematic relevance of floral micromorphology, including that of the corolla, calyx, style, and stigma, has been confirmed within Boraginaceae (Bigazzi et al, 1999;Bacchetta et al, 2000;Bigazzi and Selvi, 2000;Akçin, 2009).…”

Morphological diversity and function of the stigma in Ficus species (Moraceae)

“…Although the function of the stigma is the same for all angiosperms, stigma structure (shape, cellular composition and secretion occurrence) and physiology are very diverse (Edlund et al, 2004;Endress, 1994;Heslop-Harrison, 1992). The shape of the stigma, for example, may be of taxonomic value (Bigazzi and Selvi, 2000;Brown and Gilmartin, 1989;Heslop-Harrison, 1981), in addition to being related to adaptations to pollen grain deposition, such as the plumose stigma in the anemophilous species of Poaceae (Heslop-Harrison, 1992), or requiring the visit/manipulation 3 of an animal to become receptive to pollen germination (Basso-Alves et al, 2011;Sigrist and Sazima, 2004).…”

The morphology of stigma of Asteraceae observed by scanning electron microscopy

“…Stigma morphology was described as previously reported (Erbar & Leins, 2015;Heslop-Harrison, 1981;Liu, 2001), and morphological data were measured as previously reported (Bigazzi & Selvi, 2000;Lord & Heslop-Harrison, 1984). Briefly, 20 stigmas from each sample were selected to measure stigma lobe length and width.…”