The evolution of mechanical barriers leading to pollen placement shifts (i.e. changes of the position of pollen loads on pollinators) are exclusively reported in plants with high floral integration (i.e. coordinated covariance of floral traits) and accuracy (Armbruster and Muchhala 2009), notably in Orchidaceae (e.g. Dressler 1968; Nilsson et al. 1987). Column exsertion is one of the characters associated with the evolution of these shifts, as observed in Stylidiaceae (Armbruster et al. 1994) and Campanulaceae (Muchhala and Potts 2007; Muchhala 2008), but its importance in orchids remains undocumented.

A preliminary review of hawkmoth pollination in angraecoid orchids (Farminhão 2021), identified two clades of African species that possibly illustrate how column exsertion leads to shifts of pollen attachment sites in species sharing large hawkmoths as pollinators. In the Guineo-Congolian region, Aerangis gracillima (Kraenzl.) J.C.Arends & J.Stewart presents a column attaining more than double the length of those of closely-allied Aerangis stelligera Summerh. and A. bouarensis Chiron. Similarly, in eastern and southeastern Africa, and with a more sizeable difference, Ypsilopus schliebenii (Mansf.) D’haijère & Stévart bears a column four to approximately five times longer than sister Y. amaniensis (Kraenzl.) D’haijère & Stévart. The striking resemblance in column elongation of A. gracillima and Y. schliebenii led taxonomists to class both species in the now defunct genus Barombia Schltr. (Schlechter 1914; Cribb 1980), which was revealed to be polyphyletic by molecular phylogenetics (Simo-Droissart et al. 2018; D’haijère et al. 2019; Farminhão et al. 2020).

To better understand this parallelism, we explored the geographical distribution of A. stelligera, A. gracillima, Y. amaniensis, and Y. schliebenii and examined in further detail morphological variation within these taxa, with a focus on the two species of Ypsilopus Summerh.

Ypsilopus is an angraecoid genus (from the cyrtridactyloid clade) confined to eastern and southern Africa, which encompasses 12 species arranged in two sections, comprising species formerly included in Tridactyle Schltr and Rangaeris (Schltr.) Summerh. (D’haijère et al. 2019, 2021; Farminhão et al. 2021). Ypsilopus schliebenii and Y. amaniensis are the only representatives of Ypsilopus sect. Barombiella (Szlach.) D’haijère & Stévart, which were formerly classified within polyphyletic Rangaeris (Farminhão et al. 2020). The well-known Y. amaniensis is widespread in East Africa from Eritrea and Ethiopia south to Tanzania (Cribb 1989; Demissew et al. 2004). It is also reported from Zimbabwe, amongst others, by Ball (1978) and la Croix and Cribb (1998). However, the three collections (viz. Ball 1394, Jackson 56814, Mullin in GHS 25198) we have examined from there differ significantly from the East African ones, enough to consider them to belong to a distinct, albeit closely allied species which we describe here.

We mapped all occurrences of A. gracillima, A. stelligera, Y. amaniensis, and Y. schliebenii vouchered by specimens deposited in BR, BRLU, P, K, and SRGH using QGIS v.3.4.15.

We compiled spur length and column measurements for the same angraecoid species based on a comprehensive literature survey (Cribb 1989; Geerinck 1992; la Croix and Cribb 1998; Szlachetko and Olsweski 2001). Body measurements of Xanthopan morganii (Walker, 1856), the largest hawkmoth from Tropical Africa, were estimated using ImageJ v.1.52d (Schneider et al. 2012) based on forewing length and iconography provided by Minet et al. (2021).

We applied standard herbarium practices to investigate the variability of plants identified as Y. amaniensis kept at BR, K, and SRGH, including reproductions of specimens kept at FI, and all type material (acronyms following Thiers 2023). Based on newly collected data, we update the key to the sections of Ypsilopus, with a focus on Ypsilopus sect Barombiella, presented by D’haijère et al. (2019, 2021). Distribution records and photographs of species in Y. sect. Barombiella were mined from social media, namely iNaturalist, Facebook, and Flickr, and used to complete the description, phenology, and range of the species.

IUCN Red List categories and criteria (IUCN 2022) were applied to evaluate the conservation status of the new species. The number of “locations” (sensu IUCN 2022) was calculated considering the type of threats, such that a single “location” may include more than one adjacent occurrence. The Extent of Occurrence (EOO) and Area of Occupancy (AOO) were calculated using GeoCAT (Bachman et al. 2011) on georeferenced specimen data. The AOO was calculated based on a 2 × 2 km grid cell size.

In Aerangis Rchb.f. and Ypsilopus, the Barombia-type column that has evolved in parallel occurs in the species with the narrowest distribution, which is marginal to the range of the most widespread taxa with shorter columns (Fig. 1). There is at least one contact zone between A. gracillima and A. stelligera in southern Cameroon, while species in Y. sect. Barombiella are separated by large geographical gaps: ca 200 km and 800 km separate Y. schliebenii from the closest populations of Y. amaniensis to the north and south of its range, respectively. The population of Y. zimbabweensis, newly described here, is separated by a gap of more than 1,200 km from the southernmost occurrence of Y. amaniensis in the Rubeho and Uluguru Mountains in Tanzania.

The range of spur and column length in the studied angraecoids is summarised in Table 1.

Ypsilopus zimbabweensis can be morphometrically separated from Y. amaniensis namely according to leaf length, flower number, and peduncle length. Differences are summarised in the taxonomic treatment.

Distribution of A. gracillima, A. stelligera, Y. amaniensis, and Y. schliebenii is consistent with different floristic bioregions of the Afrotropics (Droissart et al. 2018): in the Guineo-Congolian region, A. gracillima is confined to Lower Guinea, while sister A. stelligera is more widespread in Lower Guinea and Congolia; in Eastern Africa, the range of Y. schliebenii is centred in the Southern Rift montane region, whereas Y. amaniensis is more widespread in Central Tanzania and East African montane regions, with the isolated population of Y. zimbabweensis included in the South Zambesian. The geographical pattern of column length distribution suggests that column exsertion may have been reinforced after secondary contact in a scenario of allopatric/peripatric speciation in both angraecoid clades (Farminhão 2021).

Considering spur length (see Johnson et al. 2017) and field observations (Martins and Johnson 2013; Balducci et al. 2019), it is likely that the Aerangis gracillima–A. stelligera clade and Ypsilopus sect. Barombiella share some large sphingid species as their exclusive pollinators, namely Agrius convolvuli (Linnaeus, 1758), Coelonia fulvinotata (Butler, 1875), and/or Xanthopan morganii. It is hypothesised that the pollinia attachment site shifted from the head to the dorsal region of the thorax (Fig. 1), based on the head-thorax length of 1.9 cm in Xanthopan morganii, mirroring the gap in column exsertion (see Table 1). The use of camera traps optimised for studying plant-insect interactions (e.g. Droissart et al. 2021) will be instrumental to test these hypotheses, notably near the contact zones between sister species.

Populations previously identified as Y. amaniensis in Zimbabwe are here recognised as Ypsilopus zimbabweensis sp. nov. The novelty is apparently endemic to the Central Watershed biogeographical area (Mapaura 2002), and it is one of the five angiosperms restricted to the inselbergs of Zimbabwe (Seine et al. 1998; Mapaura 2002), the others being Craterostigma syncerus (Seine, Eb.Fisch. & Barthlott) Eb.Fisch., Schäferh. & Kai Müll., Delosperma steytlerae L.Bolus, Kalanchoe wildii Raym.-Hamet ex R.Fern., and Portulaca rhodesiana R.A.Dyer. The number of species in Y. sect. Barombiella, thus, rises to three, all presenting non-overlapping latitudinal distributions in Tropical Africa.