Abstract Detail

Macroevolution

Bateman, Richard Mark [1].

Geitonogamy blurs the distinction between autogamy and allogamy: a case study in Epipactis (Orchidaceae).

The North Temperate orchid clade Epipactis section Epipactis has become a model system for the study of speciation through transitions traditionally regarded as passing from allogamy through facultative to obligate autogamy. Several well-researched structural modifications are said to encourage this transition: (1) reduction in the size and/or hydration of the adhesive viscidial disc responsible for attaching the pollinaria to pollinating wasps and bees, such that pollinaria are less likely to be removed; (2) ontogenetic extension of the anther apex such that the gynostemium apex curves downward, bringing the pollinaria into closer proximity with the stigma; (3) decreased cohesion of the viscid threads that link the permanent tetrads of pollen in the pollinaria, allowing the formerly cohesive mass of tens of thousands of pollen grains to dissociate for abiotic dispersal, and particularly; (4) reduction in the size and prominence of the rostellum that separates the pollinaria from the stigma, thus permitting pollinium fragments to fall onto the subjacent stigma through the effects of wind, rain-splash or simply gravity. RAD-seq data were used by Sramkó et al. (2019) to reconstruct the phylogenetic relationships of 108 plants representing 27 named ingroup taxa recognised by traditional taxonomy, which on the basis of these results were recircumscribed to 11 species that collectively spanned the full range of supposed reproductive categories. However, all 11 species yielded F statistics consistent with dominant (but not obligate) autogamy; only the widespread, ecologically tolerant E. helleborine had a somewhat lower mean F value of 0.3 than the more ecologically specialised species, the majority of which evolved relatively recently from within the consequently paraphyletic E. helleborine. This consistently high level of self-fertilisation is attributed to extensive geitonogamy, which is facilitated by the systematic collection of nectar by pollinators from the base to the apex of the many-flowered inflorescence, combined with unusually short caudicles in the genus that are incapable of ensuring the gradual downcurving of pollinaria that characterises most other European orchid genera. In contrast, rare allogamy events are assumed to prevent F values from exceeding 0.85 and thus qualifying as obligately autogamous. The descendant species are more ecologically specialised, and less tolerant of competition from other ground flora, than E. helleborine, leading to speculation that other factors, such as the relationship of Epipactis taxa with their mycorrhizae, may be more important to their speciation than the oft-exaggerated dichotomy between reproductive strategies that have proven considerably less distinct than previously believed.

1 - Royal Botanic Gardens Kew, Jodrell Laboratory, Kew, Richmond, Surrey, TW9 3DS, UK

Keywords:
Allogamy
Autogamy
Epipactis
floral morphology
geitonogamy
Speciation.

Presentation Type: Oral Paper
Session: MACROIII, Macroevolution III
Location: Virtual/Virtual
Date: Friday, July 23rd, 2021
Time: 10:00 AM(EDT)
Number: MACROIII001
Abstract ID:693
Candidate for Awards:None