Molecular phylogenetic analyses reveal multiple long-distance dispersal events and extensive cryptic speciation in Nervilia (Orchidaceae), an isolated basal Epidendroid genus

The terrestrial orchid genus Nervilia is diagnosed by its hysteranthous
pattern of emergence but is nested among leafless myco-heterotrophic
lineages in the lower Epidendroideae. Comprising at least 80 species
distributed across Africa, Asia, and Oceania, the genus remains poorly
known and plagued by vague and overlapping species circumscriptions,
especially within each of a series of taxonomically intractable species
complexes. Prior small-scale, exploratory molecular phylogenetic analyses
have revealed the existence of cryptic species, but little is otherwise
understood of its origin, the scale and timing of its biogeographic
spread, or the palaeoclimatic factors that have shaped its ecology and
given rise to contemporary patterns of occurrence. Here, we sample widely
throughout the generic range, including material of equivocal identity and
probable undescribed status, as well as multiple accessions referable to
several widespread ‘macrospecies’, enabling an evaluation of taxonomic
boundaries at both species and sectional levels. Our dated ancestral area
analysis supports an origin in Africa in the Early Oligocene, with spread
eastwards to Asia occurring in the Late Miocene, plausibly via the
Gomphotherium land bridge at a time when it supported woodland and savanna
ecosystems. An ancestral association with seasonally arid landscapes is
inferred. Enormous taxonomic radiation in Asia within the last 8 million
years ties in with the dramatic Himalayan-Tibetan Plateau uplift and
associated intensification of the Asia monsoon. Multiple long-range
migrations appear to have occurred thereafter, as the genus colonised
Malesia and Oceania from the Pliocene onwards, undergoing niche
differentiation to occupy the understorey of closed-canopy forest in the
process. The bulk of contemporary species diversity is thus relatively
recent, potentially explaining the ubiquity of cryptic speciation, which
still leaves numerous species overlooked and unnamed, though widespread
disjunct species pairs also hint at high mobility across continents,
extinction, and a history of climate-induced vicariance. Persistent
taxonomic challenges are highlighted.