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

History
Generic delimitation
Ipomoea as constituted by Linnaeus was based on Ipomoea pes-tigridis L. and contained various elements, including I. quamoclit and I. coccinea (Quamoclit Clade, page 556), I. triloba and I. lacunosa (Batatas Clade, page 387), I. violacea, I. alba and I. carolina as well as species of Merremia Dennst. ex Endl. and Jacquemontia Choisy and even a species of Hydrophyllaceae, I. nyctelea L. (=Ellisia nyctelea (L.) L.). It was not clearly defined and several species since treated as belonging to Ipomoea were placed in Convolvulus L. by Linnaeus including I. purpurea and I. pes-caprae.
Jacquin, Vahl, Willdenow and others of Linnaeus’ successors in the later part of the 18^th century continued placing species of Convolvulaceae rather arbitrarily in either Convolvulus L. or Ipomoea. Only Cavanilles’ placements came close to coinciding with a modern concept of I.omoea. Some authors, like Desrousseaux (1792), maintained a wide concept of Convolvulus that included all species of Ipomoea and it was only in 1810 that a clear distinction between the two genera, based on stigma morphology, was established by Robert Brown (1810: 484). He contrasted the 2–3-lobed, capitate stigma of Ipomoea with the two filiform stigmas of Convolvulus. Brown recognized the ovary of Ipomoea as being 2–3 locular but made no attempt to subdivide the genus based on the number of ovary cells. Although Roemer and Schultes (1819) followed Brown’s classification, a wide circumscription of Convolvulus remained current for some time. Both Kunth (1819) and Sprengel (1824 1827) included Ipomoea within Convolvulus and it was not until the various publications of Choisy (1834, 1838, 1845) that Ipomoea was permanently separated from Convolvulus.
Choisy (1834, 1845) subdivided Ipomoea s.l. into several genera based on a series of ovary and fruit characters. He recognized a tribe Argyreieae Choisy comprising a heterogeneous group of genera including Argyreia, Rivea, Legendrea (=Turbina) and Marcellia on the basis of their having indehiscent fruits. The tribe Convolvuleae Choisy, in contrast, was characterized by having dehiscent fruiting capsules. In this second group, Choisy recognized Quamoclit, Mina, Batatas, Pharbitis, Calonyction, Exogonium and Lepistemon as distinct from but related to Ipomoea based on characters of the ovary and corolla. Quamoclit was recognized as distinct because of the 4-locular ovary, each cell with a single seed. Mina was separated from Quamoclit because of the suburceolate corolla shape. Together these two genera comprise what we recognize as the Quamoclit Clade (page 556). Pharbitis was separated on the basis of having a 3-lobed stigma and 3-locular ovary, each cell with two seeds, this genus constituting the Pharbitis Clade (page 430). Choisy’s Batatas was vaguely defined and is very heterogeneous comprising many extraneous elements besides I. batatas and I. triloba. Calonyction and Exogonium were separated from Ipomoea on the basis of their corolla, large, showy, white or pale lilac in the case of Calonyction but merely tubular in the case of Exogonium. A small clade of species of which I. alba is the best known more or less coincides with Choisy’s Calonyction, which was redefined and extended by Hallier (1897b). Exogonium was accepted by House (1908a) and other authors but is very heterogeneous and therefore polyphyletic, so bearing no clear relationship to the clades recognized in our molecular studies. Lepistemon was separated because of the large scales at the base of the stamens, a character that sometimes appears elsewhere in the genus, for example in some specimens of I. batatoides.
Choisy’s system continued in use until the 1890s when it was essentially reproduced in the account of Convolvulaceae in Die Natürlichen Pflanzenfamilien (Peter 1891). However, acceptance was never universal and Grisebach (1862b) reduced many of Choisy’s genera to sections of Ipomoea, a decision in which he was followed by Meisner (1869), Gray (1878, 1886) and others. Nevertheless, it was only in 1893 that a major generic reorganization was proposed by Hallier. The major innovation in Hallier’s (1893a, b) system was the use of pollen. He divided the Convolvulaceae into two pollen groups based on whether the pollen was smooth or spiny. The spiny pollen group, which included Ipomoea, was itself divided into two subgroups essentially on the basis of the fruit distinction proposed by Choisy for his tribes Argyreieae and Convolvuleae. The first subgroup (Echinoconieae subgroup Ipomoeeae) was composed of species with a dehiscent capsular fruit and comprised Lepistemon, Calonyction and Quamoclit as well as Ipomoea (in which Hallier included Exogonium, Pharbitis, Marcellia and Legendrea). The second subgroup (Echinoconieae subgroup Argyreieae) was characterized by its indehiscent fruit and comprised Argyreia, Rivea, Ipomoea tiliifolia and Blinkworthia. Two new genera in this second group were established: Stictocardia to accommodate I. tiliifolia and a few related species based on the prominent black leaf glands and strongly accrescent sepals and Astrochlaena (=Astripomoea Meeuse) to accommodate a group of mostly erect South African plants with stellate hairs and shortly oblong stigmas (Hallier 1893b: 159). For the first time Merremia Dennst. ex Endl. was clearly distinguished to accommodate species previously placed in Ipomoea but distinct because of their non-spiny pollen and generally white, cream or yellow flowers (Hallier 1893a). In fact, Merremia sensu Hallier represents a heterogeneous collection of species although dividing it up into natural genera is problematic (Simões et al. 2015, Simões and Staples 2017).
Hallier’s system has endured with only a few, relatively minor changes for about 125 years. A handful of new genera were established to include small, morphologically distinct splinter groups from the Ipomoeeae such as Lepistemonopsis with fleshy scales at the base of the filaments and Pentacrostigma with a 5-lobed stigma and 5-locular ovary. On the other hand the genera Calonyction, Mina and Quamoclit, all recognized by Hallier, were gradually abandoned; none was recognized by O’Donell in his various publications (O’Donell 1959a, b) although Mina is still occasionally accepted (Deroin 2001). Within the Argyreieae there has been uncertainty about the limits of various genera, notably Rivea and Turbina, the latter reincorporated in this subgroup and unique for its nearly pantropical distribution. A new genus, Paralepistemon was established to include two African species with thickened filament bases. (Lejoly and Lisowski 1986). In passing, it should be noted that a rather eccentric attempt to reclassify Convolvulaceae by Roberty (1952, 1964) has been universally rejected, like a similar earlier attempt by Rafinesque (1837, 1838a,b).
Recent phylogenetic studies point towards the acceptance of a broad concept of Ipomoea to include all Hallier’s Echinoconieae. Initial studies by Wilkin (1999) and confirmed by our own more extensive sampling (Muñoz-Rodríguez et al. 2019) have shown that even when smaller genera recognized within the tribe Ipomoeeae have strong phylogenetic support, they are nested within Ipomoea. Manos et al. (2001) showed that species with spiny pollen split into two major clades, a result confirmed by Muñoz-Rodríguez et al. (2019). One clade consists of species placed in Stictocardia, Rivea and Argyreia together with a superficially heterogeneous group of species from Ipomoea and Turbina, composed mainly (but not exclusively) of Old World species. The second clade consists of mostly (but not exclusively) New World species but includes Astripomoea, some species hitherto treated as Turbina and all Australian endemics we have sampled. Neither clade can be diagnosed by specific morphological features and it seems that there are multiple origins for many of the characters in Ipomoea including both the capsular and indehiscent fruit types as well as the different number of ovary cells, many characters thus being homoplastic. It is clear from these studies that Ipomoea as hitherto understood is not monophyletic and an expanded circumscription of Ipomoea is required to secure its monophyly (Stefanovic et al. 2003).
Our own extensive studies (Muñoz-Rodríguez et al. 2019) confirm earlier papers and support an Old World origin of Ipomoea s.l. They indicate that the recognition of a broad Ipomoea based on the presence of spiny pollen is the only logical solution that integrates monophyly and diagnosability. The alternative of dividing the whole clade into many small, formally recognized groups is not recommended due to high levels of homoplasy, lack of diagnostic characters and a complex tree model in which it is not obvious to which clades some species, not sampled for molecular data, should be placed. Consequently, we are adopting a wide concept of the genus to include Argyreia, Astripomoea, Blinkworthia, Lepistemon, Lepistemonopsis, Rivea, Stictocardia and Turbina, which are all nested within Ipomoea. However, as the genus is so large, we informally recognize certain diagnosable clades within Ipomoea to facilitate discussion and reflect the phylogenetic history of the genus. These informal clades include some traditionally recognized genera as well as newly discovered groups that contain similar looking plants and are geographically coherent. Where we have recognized informal taxa, we have been as explicit as possible about what species belong to those clades.