Whole plastomes are not enough: phylogenomic and morphometric exploration at multiple demographic levels of the bee orchid clade Ophrys sect. Sphegodes

• Published in: Journal of experimental botany 2021-02, Vol.72 (2), p.654-681
• Main Authors: Bateman, Richard M, Rudall, Paula J, Murphy, Alexander R M, Cowan, Robyn S, Devey, Dion S, Peréz-Escobar, Oscar A
• Format: Article
• Language: English
• ISSN: 0022-0957; 1460-2431
• DOI: 10.1093/jxb/eraa467

Bee orchids are widely regarded as a model system for exploring recent adaptive radiation. However, detailed morphometric and plastome studies show that few supposed species have achieved the required levels of reproductive isolation; microevolution does not necessarily equate with macroevolution. Abstract Plastid sequences have long dominated phylogeny reconstruction at all time depths, predicated on a usually untested assumption that they accurately represent the evolutionary histories of phenotypically circumscribed species. We combined detailed in situ morphometrics (124 plants) and whole-plastome sequencing through genome skimming (71 plants) in order to better understand species-level diversity and speciation in arguably the most challenging monophyletic group within the taxonomically controversial, pseudo-copulatory bee orchid genus Ophrys. Using trees and ordinations, we interpreted the data at four nested demographic levels—macrospecies, mesospecies, microspecies, and local population—seeking the optimal level for bona fide species. Neither morphological nor molecular discontinuities are evident at any level below macrospecies, the observed overlap among taxa suggesting that both mesospecies and microspecies reflect arbitrary division of a continuum of variation. Plastomes represent geographic location more strongly than taxonomic assignment and correlate poorly with morphology, suggesting widespread plastid capture and possibly post-glacial expansion from multiple southern refugia. As they are rarely directly involved in the speciation process, plastomes depend on extinction of intermediate lineages to provide phylogenetic signal and so cannot adequately document evolutionary radiations. The popular ‘ethological’ evolutionary model recognizes as numerous ‘ecological species’ (microspecies) lineages perceived as actively diverging as a result of density-dependent selection on very few features that immediately dictate extreme pollinator specificity. However, it is assumed rather than demonstrated that the many microspecies are genuinely diverging. We conversely envisage a complex four-dimensional reticulate network of lineages, generated locally and transiently through a wide spectrum of mechanisms, but each unlikely to maintain an independent evolutionary trajectory long enough to genuinely speciate by escaping ongoing gene flow. The frequent but localized microevolution that characterizes the Ophrys sphegodes complex is often convergent and rare