A foundation monograph of Ipomoea (Convolvulaceae) in the New World

Major clades of Ipomoea
Figure 1 summarises the phylogeny of Ipomoea and shows the genus is divided into two clades of similar size. These are labelled for ease of reference but are not formally recognized. The two clades are dominated by species from the Old World and New World respectively but with many exceptions. The Old World Clade (OWC) consists of species previously placed in Argyreia, Rivea, Stictocardia and Lepistemon as well as many always included in Ipomoea. The New World Clade (NWC) consists of an early diverging grade of Old World elements and a species-rich clade dominated by species from the Americas, but which also includes all species endemic to Australia. The existence of this fundamental split within Ipomoea had been posited by previous research (Miller et al. 1999 Manos et al. 2001) although based on much poorer taxon sampling.
Both NWC and OWC contain elements that were recognized previously as genera and appear as smaller clades within NWC or OWC. The only previously recognized genus that is represented by native species in both NWC and OWC is Turbina, although most of its species are in OWC. Turbina is polyphyletic containing several heterogeneous elements and is consequently rejected. Similarly, we reject the New World genus Exogonium as it was founded on a hypocrateriform corolla adapted for bird pollination and this character is homoplastic occurring in various different clades within NWC. In NWC, species formerly grouped under the names Arborescens, Batatas, Pharbitis, Calonyction and Quamoclit all form small clades which more or less coincide with their traditional circumscription and so are used by us as names for the corresponding clades. In OWC the generic names Argyreia, Astripomoea, Stictocardia and Lepistemon form clades of varying sizes and we continue to use these names for these distinct clades. Unlike the New World clades recognized above, these Old World clades vary considerably in size, Argyreia having around 125 species (including Rivea), Stictocardia around ten and Lepistemon only two so barely meriting recognition. All these nine clades which are assigned traditional names are more or less diagnosable using combinations of morphological characters.
NWC comprises about 450 species. Apart from a few species previously placed in Turbina, all species belong to Echinoconieae subgroup Ipomoeeae in Hallier’s (1893a, b) classification. Most (All?) Australian endemics belong to this clade and it is also well represented in Africa. Within NWC, two very large clades are recognizable. One clade (mostly South American but including Batatas) roughly coincides with subgenus Eriospermum (Hallier f.) Verdc. ex D.F. Austin as defined by Austin and Huáman (1996), although Austin included many elements which do not belong, such as Ipomoea rubens, I. lindenii, I. violacea, I. imperati, I. magnifolia, I. habeliana etc. (Muñoz-Rodríguez et al. 2019). We refer to this as Clade A. There is a second very large, mainly Mexican, clade that has not previously been recognized which includes Austin’s subgenera Ipomoea and Quamoclit as well as some other species. We refer to this as Clade B. Clades C, D, E and F represent smaller clades within NWC, this last essentially African with one New World endemic (I. habeliana).
Our studies have revealed many smaller clades to which a traditional name cannot be readily attached. The two largest are both in Clade A of NWC and we refer to these as Clades A1 and A2. Some of the species in Clade A1 were treated as series Jalapa by Austin and Huáman, but in a very inconsistent way. It is found throughout the neotropics but is most diverse in South America. The Arborescens group form a small clade within A1. Clade A2 is also found throughout the neotropics but is particularly important in the Caribbean, as nearly all the 25 endemic species of that region belong to it. Elements of this clade were referred to as Microsticta by McPherson (1980) and as series Eriospermum by Austin and Huáman (1996). Both Clade A1 and Clade A2 are usually recognizable morphologically, the former by its pubescent corolla and rather soft, flattish sepals and the latter by its usually glabrous corolla and coriaceous, often convex, ovate to elliptic sepals. We note that there are a few exceptions in both these clades and that several of these diagnostic characters are homoplastic in other parts of Ipomoea. Clade A3 is a small clade comprising the Batatas group. Ipomoea cryptica is sister to this clade in the nuclear phylogeny but not in the chloroplast phylogeny.
Apart from Pharbitis, Calonyction and Quamoclit, there are several small clades which are more or less diagnosable morphologically within Clade B. There is a small clade (Species 328–334) of seven species centred on Ipomoea costellata assigned the name Pedatisecta by House (1908b) characterized by digitately divided leaves. These were treated as part of Sect. Leptocallis by McDonald (1995) but the name Leptocallis has to refer to a quite different small clade (Species 280–288) centred on I. capillacea, perhaps characterized by tuberous storage roots. The most distinct small clade in Clade B consists of five species characterized by pinnatifid leaves and centred on I. stans (Species 275–279). These were included in Sect Tyrianthinae by McDonald (2001) but this name cannot be used for this clade as the type, I. orizabensis, belongs to a different clade. Since the six small clades discussed here account for only a small proportion of the species in Clade B, we have avoided any formal recognition of these names.
Clade C also contains a number of small clades which are more or less diagnosable morphologically or geographically, although the best known species, Ipomoea pes-caprae, belongs to a clade of Australian species. A small clade of four species (Species 345–348) centred on I. asarifolia can be recognized by their very unequal, transversely muricate sepals. Another small clade of South American species consisting of perhaps eight species centred on I. maurandioides (Species 356–363) that can be recognized by their glabrous indumentum, unequal sepals and often trailing habit.
The Old World Clade (OWC) contains around 350 species mostly from the palaeotropics. It includes most species treated as Echinoconieae subgroup Argyreieae by Hallier including all species placed in Argyreia, Rivea (which is nested within Argyreia), Stictocardia and some species placed in Turbina. Only a few relatively small clades are composed of neotropical species. Much the largest is the clade of around 12 species centred on I. corymbosa but with morphologically very disparate elements, including I. ochracea, I. regnellii, I. crinicalyx, I. cuscoenesis and I. daturiflora.
There are important practical implications from our molecular results. Since there is no obvious or close correlation between morphological characters and the Ipomoea phylogeny, it is currently impossible to propose an infrageneric classification along traditional lines. Although most clades cannot be defined morphologically, they do have certain morphological tendencies, which we have highlighted and discussed in the notes that precede the description of the species in each clade. As noted above, some of the smaller clades are well-defined and, where this is the case, their distinctive morphological features are indicated. We have also tentatively used molecular results to inform the placement of individual species within clades.
It should be stressed that we have faced a problem that we share with previous botanists working on the classification of Ipomoea. Some species are not available for study or sequencing and so cannot be assigned unequivocally to a clade. In this situation, we have inferred the position of species from their morphology. Most placements will be uncontroversial but in a few cases they are little more than guesses. The notes following each species indicate where placement is particularly uncertain.