In situmorphometric survey elucidates the evolutionary systematics of the EurasianHimantoglossumclade (Orchidaceae: Orchidinae)
Authors: Bateman Richard M. , Molnár V. Attila , Sramkó Gábor 
Published: January 2017

Background and Aims

The charismaticHimantoglossum s.l.clade of Eurasian orchids contains an unusually large proportion of taxa that are of controversial circumscriptions and considerable conservation concern. Whereas our previously published study addressed the molecular phylogenetics and phylogeography of every named taxon within the clade, here we use detailed morphometric data obtained from the same populations to compare genotypes with associated phenotypes, in order to better explore taxonomic circumscription and character evolution within the clade.

Methods

Between one and 12 plants found in 25 populations that encompassed the entire distribution of theHimantoglossum s.l.clade were measuredin situfor 51 morphological characters. Results for 45 of those characters were subjected to detailed multivariate and univariate analyses.

Key Results

Multivariate analyses readily separate subgenusBarliaand subgenusComperiafrom subgenusHimantoglossum, and also the early-divergentH. formosumfrom the less divergent remainder of subgenusHimantoglossum. The sequence of divergence of these four lineages is confidently resolved. Our experimental approach to morphometric character analysis demonstrates clearly that phenotypic evolution withinHimantoglossumis unusually multi-dimensional.

Conclusions

Degrees of divergence between taxa shown by morphological analyses approximate those previously shown using molecular analyses.Himantoglossum s.l. is readily divisible into three subgenera. The three sections of subgenusHimantoglossum—hircinum,caprinumandformosum—are arrayed from west to east with only limited geographical overlap. At this taxonomic level, their juxtaposition combines with conflict between contrasting datasets to complicate attempts to distinguish between clinal variation and the discontinuities that by definition separatebona fidespecies. All taxa achieve allogamy via food deceit and have only weak pollinator specificity. Artificial crossing demonstrates that intrinsic sterility barriers are weak. Although we have found evidence of gene flow among and within the three sections of subgenusHimantoglossum, reports of natural hybrids are surprisingly rare, probably because putative parents are sufficiently similar to questionably warrant the status of species. Phenological separation and increased xeromorphy characterise the origin of subgenusBarlia. Several individual morphological characters show evidence of parallel acquisition, and loss of features is especially frequent in floral markings among members of sectioncaprinum. Detailed patterns of gain and loss demonstrate that several different categories of flower markings are inherited independently. Along with the dimensions of labellar lobes, these pigmentation characters have been over-emphasised in previous taxonomic treatments. Increased plant vigour was a crucial element of the origin of the genus, but vegetative characters underwent remarkably little subsequent evolution. Attempts to reconstruct hypothetical ancestors at internal nodes of the phylogeny are weakened by (a) uncertain placement ofSteveniellaas sister toHimantoglossum s.l.and (b) uncertain relationships among subtly different putative species within sectioncaprinum. Nonetheless, heterochronic/allometric trends, ultimately limited by functional constraints, clearly dictate transitions between contrasting flower sizes and complex labellum shapes.