Taxonomic revision and distribution of herbaceous Paramollugo (Molluginaceae) in the Eastern Hemisphere

Abstract The genus Paramollugo with the type species Paramollugo nudicaulis (≡Mollugo nudicaulis) has recently been described after molecular investigations. Here we report two new endemic Malagasy species: Paramollugo simulans and Paramollugo elliotii, and transfer a forgotten New Caledonian endemic Mollugo digyna to Paramollugo (Paramollugo digyna). Consequently, the number of Paramollugo species in the Eastern Hemisphere is increased from three to six. Almost all genus representatives (except Paramollugo nudicaulis, which has a wide distribution in Southern Asia, Arabia and tropical Africa) are endemic to Madagascar, Somalia, or New Caledonia. Since the type of seed coat ornamentation is crucial for species delimitation, a diagnostic key with new taxonomically significant carpological characters and other new traits is provided for all the herbaceous Paramollugo. The distribution patterns of Paramollugo nudicaulis s.str., Paramollugo simulans and Paramollugo elliotii are presented.


Introduction
Th e new genus Paramollugo Th ulin has recently been split from Mollugo s.l. after molecular investigations (Th ulin et al. 2016). It comprises fi ve herbaceous species and one small shrub, the Malagasy P. decandra (ex-Mollugo decandra). Herbaceous Paramollugo species clearly diff er from all other Molluginaceae by having rosulate leaves only. Th e infl orescences in herbaceous Paramollugo are bracteose, dichasial or trichasial ('pseudodichotomous'), with long infl orescence stems (Batenburg et al. (1984) sub Mollugo) and numerous sterile branches up to 2.5 cm long, which are rarely seen in Molluginaceae. Th e seeds of some species have only recently been investigated (Hassan et al. 2005 sub Mollugo; Sukhorukov and Kushunina in press), and their morphology and ornamentation support the data obtained from a molecular phylogeny study (Christin et al. 2011), which suggests that Paramollugo angustifolia and P. navassensis are close to P. nudicaulis. Th e following herbaceous species were accepted after establishing the new generic name (Th ulin et al. 2016): Paramollugo nudicaulis (type species) with wide distribution in the Eastern Hemisphere, P. angustifolia known only from the type locality in south-central Somalia (Gilbert and Th ulin (1993) sub Mollugo angustifolia), P. cuneifolia (Grisebach (1866) sub Mollugo nudicaulis var. cuneifolia), P. deltoidea from Cuba (Léon (1950) sub Mollugo deltoidea), and P. navassensis from several Caribbean islands (Ekman (1929) sub Mollugo nudicaulis var. navassensis). Morphologically, these species diff er in life form (annual or perennial), leaf shape and sometimes leaf width (Gilbert and Th ulin 1993;Th ulin et al. 2016).
We have previously shown that seed morphology and ultrasculpture are very useful for the diagnostics and taxonomy of major groups within the former Mollugo s.l., and that the 'Mollugo nudicaulis' group (=Paramollugo) needs further taxonomic studies (Sukhorukov et al. 2016;Sukhorukov and Kushunina in press). Here we provide a closer look at the taxonomy of the herbaceous Paramollugo taxa and discuss new morphological characters essential for delimiting the taxa within this complicated group.

Materials and methods
Th e revision of herbarium material was undertaken in the collections B, BM, BR, E, HUJ, K, LE, M, MHA, MSB, MW, P, PRA, W, and WU (acronyms follow Th iers 2016+). In the herbaria visited in 2015-early 2016, prior to the publication of the new taxonomic revision of Molluginaceae (Th ulin et al. 2016), the specimens belonging to the new Paramollugo species described here were labelled by us as Mollugo (including the holotype specimens). Th e distribution maps of Paramollugo nudicaulis, P. angustifolia, P. elliotii and P. simulans are based upon the specimens examined in the herbaria listed above. Th ese specimens are cited in the Results and Discussion section for the new taxa and combination or in the Appendix for Paramollugo nudicaulis and P. angustifolia. Countries are listed in alphabetical order.
Th e seed surface of Paramollugo was examined using scanning electron microscopy (SEM; JSM-6380, JEOL Ltd., Japan) at 15 kV after sputtercoating with gold-palladium. Fallen fruits (1-2) from the following vouchers were used for SEM:

Results
Six Paramollugo species in the Eastern Hemisphere (P. angustifolia, P. decandra, P. digyna, P. elliotii, P. nudicaulis, P. simulans) are accepted in the present paper, instead of the three previously recognized species (P. angustifolia, P. decandra, and P. nudicaulis). Th e core species, P. nudicaulis s.str., is found to be widely distributed in the tropics of the Eastern Hemisphere as a weed species (see Fig. 1 and Appendix for vouchers), and this conclusion is supported by morphological and carpological (including SEM) comparison of the observed specimens. Paramollugo is best represented in Madagascar, where four species are found: herbaceous P. simulans, P. elliotii, P. nudicaulis, and shrubby P. decandra. Except for P. nudicaulis, all species are endemics of this island. Another enigmatic taxon described as Mollugo caespitosa (Scott Elliot 1891) and included in the list of Madagascan endemics (Clifton 2012) is now synonymized with M. decandra (Sukhorukov et al. 2016). Th e genus Paramollugo in the Eastern Hemisphere is one of the most morphologically diffi cult groups in Molluginaceae. We found that the following reproductive characters are useful for delimiting the Paramollugo species: size of the perianth segments, length of the anthers, number of stylodia and locules in the capsule, and seed shape and ornamentation. Seed surface is either papillate (P. digyna, P. decandra, P. nudicaulis; Fig. 2) or colliculate (P. angustifolia, P. elliotii, P. simulans; Fig. 3), while the secondary ornamentation of the testa cells is cross-striate in P. nudicaulis and warty in P. digyna, or absent in P. angustifolia, P. decandra, P. elliotii, and P. simulans. All six species are united in having small pits in the boundaries between seed coat cells (but the pits are hardly discernible in P. elliotii).

Key to the delimitation of herbaceous Paramollugo species in the Eastern Hemisphere
Paramollugo decandra is not included in the text; it is a small shrub (this character differentiates this species from all other Paramollugo in this region, which are herbs)  (Fig. 4). Diagnosis. Paramollugo simulans is morphologically similar to P. nudicaulis but diff ers by having colliculate seed surface (testa cells slightly convex, not papillae-like) with no striate secondary cell ornamentation. Some forms of P. simulans with linear leaves look like P. angustifolia, but they reliably diff er by the seed size (0.55-0.7 mm and 0.35-0.45 mm, respectively).

Conservation status.
Although a rather limited number of specimens is available, it is clear that P. simulans can occupy disturbed habitats and is therefore unlikely to be at risk of extinction. Following the IUCN (2016) guidelines, this species should be categorized as not threatened.
Etymology. Th e specifi c epithet "simulans" means "imitating" and refers to the morphological similarities of the new species and Mollugo nudicaulis.
Ecology. All examined specimens were collected on the sand dunes near the sea at lower elevations 0-300 m. We assume that P. elliotii is a psammophilous species.
Flowering and fruiting. All examined specimens (collected in January, February, July, September or December) bear both fl owers and fruits. However, the exact phenology has to be investigated more precisely, since it can depend on the geographical latitude and other conditions. General distribution. Endemic to Madagascar (Fig. 5). Conservation status. All except one specimen are nearly a hundred years old or even older, and no information is available on the current extent of the wild populations of this species. Th erefore P. elliotii is given a Data Defi cient (DD) status (IUCN 2016). Neotype (Sukhorukov, selected here). NEW CALEDONIA. West face of Massif de Koniambo, 300 m, iron-serpentine scrub on track through "maquis", 11 October 1963, P.S. Green 1287 (K001045648!).
Discussion. Th is taxon described by X. Montrouzier was completely forgotten and cited as Mollugo nudicaulis in the treatments of the New Caledonian fl ora (e.g., Guillaumin 1948, Rageau 1973. However, P. digyna is easily recognized by unique reniform or even triangular leaf blades (Fig. 7), and by a tendency to become a shortlived perennial (in contrast to annual P. nudicaulis). Unfortunately, many specimens were collected without the basal portion or in their fi rst growing season, and it is still unclear which life form is predominant in P. digyna. Neither perennial habit nor unusual leaf shape were mentioned in the protologue (Montrouzier 1860), but the author indicated the presence of two-valved capsules. We also found that the specimens collected in New Caledonia have two-or rarely three-valved capsules with 8 or fewer seeds, in contrast to other herbaceous Paramollugo, which are characterized by capsules with three locules and up to 30 seeds. Th e outer walls of the testa cells are elongated, almost rectangular, with secondary warty outgrowths. A similar seed surface was observed in the Caribbean P. navassensis and is clearly distinct from P. nudicaulis (Sukhorukov and Kushunina in press, sub Mollugo).
No original material of Paramollugo digyna was traced. Th e herbarium of X. Montrouzier, who described M. digyna, is located in University of Montpellier herbarium (MPU), with some duplicates in the National Museum of Natural History, Paris (P). Only one specimen of P. digyna was found, without a precise location (MPU-310526). No specimens are cited in the protologue (Montrouzier 1860). Th e sheet found in MPU (no. 169) and dated 1866 might have been collected later, and hence cannot be considered as original material. However, the plant clearly originates from New Caledonia due to the writing on the sheet "Stylos binos semper, capsulamque bivalvam vidi!" ("stylodia always two, capsule two-valved"). Since the original material is not found (Art. 9.16 of ICN) here we select a neotype that bears a plant with perennial life form, reniform leaves and two-valved capsules as the most indicative characters of P. digyna.
Paramollugo digyna is known only from New Caledonia where it usually grows on rocky or stony substrates; locally common (Guillaumin 1964